enable strict IV checking by default during decryption

[python-delta-tar] / deltatar / tarfile.py

#!/usr/bin/env python3
#-------------------------------------------------------------------
# tarfile.py
#-------------------------------------------------------------------
# Copyright (C) 2002 Lars Gustäbel <lars@gustaebel.de>
# All rights reserved.
#
# Permission  is  hereby granted,  free  of charge,  to  any person
# obtaining a  copy of  this software  and associated documentation
# files  (the  "Software"),  to   deal  in  the  Software   without
# restriction,  including  without limitation  the  rights to  use,
# copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies  of  the  Software,  and to  permit  persons  to  whom the
# Software  is  furnished  to  do  so,  subject  to  the  following
# conditions:
#
# The above copyright  notice and this  permission notice shall  be
# included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS  IS", WITHOUT WARRANTY OF ANY  KIND,
# EXPRESS OR IMPLIED, INCLUDING  BUT NOT LIMITED TO  THE WARRANTIES
# OF  MERCHANTABILITY,  FITNESS   FOR  A  PARTICULAR   PURPOSE  AND
# NONINFRINGEMENT.  IN  NO  EVENT SHALL  THE  AUTHORS  OR COPYRIGHT
# HOLDERS  BE LIABLE  FOR ANY  CLAIM, DAMAGES  OR OTHER  LIABILITY,
# WHETHER  IN AN  ACTION OF  CONTRACT, TORT  OR OTHERWISE,  ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
# OTHER DEALINGS IN THE SOFTWARE.
#
"""Read from and write to tar format archives.
"""

__version__ = "$Revision: 85213 $"
# $Source$

version     = "0.9.0"
__author__  = "Lars Gustäbel (lars@gustaebel.de)"
__date__    = "$Date$"
__cvsid__   = "$Id$"
__credits__ = "Gustavo Niemeyer, Niels Gustäbel, Richard Townsend, Eduardo Robles."

#---------
# Imports
#---------
import binascii
import copy
import errno
import functools
import io
import mmap
import operator
import os
import re
import shutil
import stat
import struct
import sys
import time

import traceback # XXX

from . import crypto

try:
    import grp, pwd
except ImportError:
    grp = pwd = None

# os.symlink on Windows prior to 6.0 raises NotImplementedError
symlink_exception = (AttributeError, NotImplementedError)
try:
    # OSError (winerror=1314) will be raised if the caller does not hold the
    # SeCreateSymbolicLinkPrivilege privilege
    symlink_exception += (OSError,)
except NameError:
    pass

# from tarfile import *
__all__ = ["TarFile", "TarInfo", "is_tarfile", "TarError"]

from builtins import open as _open # Since 'open' is TarFile.open

#---------------------------------------------------------
# tar constants
#---------------------------------------------------------
NUL = b"\0"                     # the null character
BLOCKSIZE = 512                 # length of processing blocks
RECORDSIZE = BLOCKSIZE * 20     # length of records
GNU_MAGIC = b"ustar  \0"        # magic gnu tar string
POSIX_MAGIC = b"ustar\x0000"    # magic posix tar string

LENGTH_NAME = 100               # maximum length of a filename
LENGTH_LINK = 100               # maximum length of a linkname
LENGTH_PREFIX = 155             # maximum length of the prefix field

REGTYPE = b"0"                  # regular file
AREGTYPE = b"\0"                # regular file
LNKTYPE = b"1"                  # link (inside tarfile)
SYMTYPE = b"2"                  # symbolic link
CHRTYPE = b"3"                  # character special device
BLKTYPE = b"4"                  # block special device
DIRTYPE = b"5"                  # directory
FIFOTYPE = b"6"                 # fifo special device
CONTTYPE = b"7"                 # contiguous file

GNUTYPE_LONGNAME = b"L"         # GNU tar longname
GNUTYPE_LONGLINK = b"K"         # GNU tar longlink
GNUTYPE_SPARSE = b"S"           # GNU tar sparse file
GNUTYPE_MULTIVOL = b"M"         # GNU tar continuation of a file that began on
                                # another volume

XHDTYPE = b"x"                  # POSIX.1-2001 extended header
XGLTYPE = b"g"                  # POSIX.1-2001 global header
SOLARIS_XHDTYPE = b"X"          # Solaris extended header

USTAR_FORMAT = 0                # POSIX.1-1988 (ustar) format
GNU_FORMAT = 1                  # GNU tar format
PAX_FORMAT = 2                  # POSIX.1-2001 (pax) format
DEFAULT_FORMAT = GNU_FORMAT

GZ_FMT_HEADER        = b"<BBBBLBB"
GZ_HEADER_SIZE       = 10   # not including the name
GZ_MAGIC             = (0x1f, 0x8b) # 0o37, 0o213
GZ_METHOD_DEFLATE    = 0x08 # 0o10
GZ_FLAG_FTEXT        = 1 << 0 # ASCII payload
GZ_FLAG_FHCRC        = 1 << 1 # CRC16
GZ_FLAG_FEXTRA       = 1 << 2 # extra field
GZ_FLAG_FNAME        = 1 << 3 # set by default in gzip
GZ_FLAG_FCOMMENT     = 1 << 4 # NUL-terminated comment
GZ_FLAG_RESERVED     = 7 << 5 # unassigned
GZ_DEFLATE_FLAGS     = 0x00 # 0o00, never read (deflate.c)
GZ_OS_CODE           = 0x03 # 0o03, default in gzip (tailor.h)
GZ_MAGIC_BYTES       = struct.pack ("<BB", GZ_MAGIC [0], GZ_MAGIC [1])
GZ_MAGIC_DEFLATE     = struct.pack ("<BBB", GZ_MAGIC [0], GZ_MAGIC [1],
                                    GZ_METHOD_DEFLATE)

TOLERANCE_STRICT  = 0
TOLERANCE_RECOVER = 1 # rely on offsets in index
TOLERANCE_RESCUE  = 2 # deduce metadata from archive contents

BUFSIZE           = 16 * 1024

#---------------------------------------------------------
# archive handling mode
#---------------------------------------------------------

ARCMODE_PLAIN    = 0
ARCMODE_ENCRYPT  = 1 << 0
ARCMODE_COMPRESS = 1 << 1
ARCMODE_CONCAT   = 1 << 2

def arcmode_fmt (m):
    if m == ARCMODE_PLAIN:
        return "PLAIN"
    first = True
    ret = "["
    def chkappend (b, s):
        nonlocal m
        nonlocal ret
        nonlocal first
        if m & b:
            if first is True: first = False
            else: ret += " |"
            ret += " " + s
    chkappend (ARCMODE_ENCRYPT,  "ENCRYPT")
    chkappend (ARCMODE_COMPRESS, "COMPRESS")
    chkappend (ARCMODE_CONCAT,   "CONCAT")
    return ret + " ]"


def arcmode_set (concat=False, encryption=None, comptype=None, init=ARCMODE_PLAIN):
    ret = init
    if bool (concat) is True:
        ret |= ARCMODE_CONCAT
    if encryption is not None:
        ret |= ARCMODE_ENCRYPT
    if comptype == "gz":
        ret |= ARCMODE_COMPRESS
    return ret

#---------------------------------------------------------
# tarfile constants
#---------------------------------------------------------
# File types that tarfile supports:
SUPPORTED_TYPES = (REGTYPE, AREGTYPE, LNKTYPE,
                   SYMTYPE, DIRTYPE, FIFOTYPE,
                   CONTTYPE, CHRTYPE, BLKTYPE,
                   GNUTYPE_LONGNAME, GNUTYPE_LONGLINK,
                   GNUTYPE_SPARSE, GNUTYPE_MULTIVOL)

# File types that will be treated as a regular file.
REGULAR_TYPES = (REGTYPE, AREGTYPE,
                 CONTTYPE, GNUTYPE_SPARSE)

# File types that are part of the GNU tar format.
GNU_TYPES = (GNUTYPE_LONGNAME, GNUTYPE_LONGLINK,
             GNUTYPE_SPARSE, GNUTYPE_MULTIVOL)

# Fields from a pax header that override a TarInfo attribute.
PAX_FIELDS = ("path", "linkpath", "size", "mtime",
              "uid", "gid", "uname", "gname")

# Fields from a pax header that are affected by hdrcharset.
PAX_NAME_FIELDS = {"path", "linkpath", "uname", "gname"}

# Fields in a pax header that are numbers, all other fields
# are treated as strings.
PAX_NUMBER_FIELDS = {
    "atime": float,
    "ctime": float,
    "mtime": float,
    "uid": int,
    "gid": int,
    "size": int
}

#---------------------------------------------------------
# initialization
#---------------------------------------------------------

if os.name in ("nt", "ce"):
    ENCODING = "utf-8"
else:
    ENCODING = sys.getfilesystemencoding()

#---------------------------------------------------------
# Some useful functions
#---------------------------------------------------------

def stn(s, length, encoding, errors):
    """Convert a string to a null-terminated bytes object.
    """
    s = s.encode(encoding, errors)
    return s[:length] + (length - len(s)) * NUL

def nts(s, encoding, errors):
    """Convert a null-terminated bytes object to a string.
    """
    p = s.find(b"\0")
    if p != -1:
        s = s[:p]
    return s.decode(encoding, errors)

def sbtn(s, length, encoding, errors):
    """Convert a string or a bunch of bytes to a null-terminated bytes object
    of specific size.
    """
    if isinstance(s, str):
        s = s.encode(encoding, errors)
    return s[:length] + (length - len(s)) * NUL

def nti(s):
    """Convert a number field to a python number.
    """
    # There are two possible encodings for a number field, see
    # itn() below.
    if s[0] in (0o200, 0o377):
        n = 0
        for i in range(len(s) - 1):
            n <<= 8
            n += s[i + 1]
        if s[0] == 0o377:
            n = -(256 ** (len(s) - 1) - n)
    else:
        try:
            n = int(nts(s, "ascii", "strict") or "0", 8)
        except ValueError:
            raise InvalidHeaderError("invalid header")
    return n

def itn(n, digits=8, format=DEFAULT_FORMAT):
    """Convert a python number to a number field.
    """
    # POSIX 1003.1-1988 requires numbers to be encoded as a string of
    # octal digits followed by a null-byte, this allows values up to
    # (8**(digits-1))-1. GNU tar allows storing numbers greater than
    # that if necessary. A leading 0o200 or 0o377 byte indicate this
    # particular encoding, the following digits-1 bytes are a big-endian
    # base-256 representation. This allows values up to (256**(digits-1))-1.
    # A 0o200 byte indicates a positive number, a 0o377 byte a negative
    # number.
    if 0 <= n < 8 ** (digits - 1):
        s = bytes("%0*o" % (digits - 1, int(n)), "ascii") + NUL
    elif format == GNU_FORMAT and -256 ** (digits - 1) <= n < 256 ** (digits - 1):
        if n >= 0:
            s = bytearray([0o200])
        else:
            s = bytearray([0o377])
            n = 256 ** digits + n

        for i in range(digits - 1):
            s.insert(1, n & 0o377)
            n >>= 8
    else:
        raise ValueError("overflow in number field")

    return s

def calc_chksums(buf):
    """Calculate the checksum for a member's header by summing up all
       characters except for the chksum field which is treated as if
       it was filled with spaces. According to the GNU tar sources,
       some tars (Sun and NeXT) calculate chksum with signed char,
       which will be different if there are chars in the buffer with
       the high bit set. So we calculate two checksums, unsigned and
       signed.
    """
    unsigned_chksum = 256 + sum(struct.unpack_from("148B8x356B", buf))
    signed_chksum = 256 + sum(struct.unpack_from("148b8x356b", buf))
    return unsigned_chksum, signed_chksum

def copyfileobj(src, dst, length=None):
    """Copy length bytes from fileobj src to fileobj dst.
       If length is None, copy the entire content.
    """
    if length == 0:
        return
    if length is None:
        shutil.copyfileobj(src, dst)
        return

    blocks, remainder = divmod(length, BUFSIZE)
    for b in range(blocks):
        buf = src.read(BUFSIZE)
        dst.write(buf)
        if len(buf) < BUFSIZE:
            raise OSError("end of file reached")
    if remainder != 0:
        buf = src.read(remainder)
        dst.write(buf)
        if len(buf) < remainder:
            raise OSError("end of file reached")


def filemode(mode):
    """Deprecated in this location; use stat.filemode."""
    import warnings
    warnings.warn("deprecated in favor of stat.filemode",
                  DeprecationWarning, 2)
    return stat.filemode(mode)

class TarError(Exception):
    """Base exception."""
    pass
class ExtractError(TarError):
    """General exception for extract errors."""
    pass
class ReadError(TarError):
    """Exception for unreadable tar archives."""
    pass
class CompressionError(TarError):
    """Exception for unavailable compression methods."""
    pass
class StreamError(TarError):
    """Exception for unsupported operations on stream-like TarFiles."""
    pass
class HeaderError(TarError):
    """Base exception for header errors."""
    pass
class EmptyHeaderError(HeaderError):
    """Exception for empty headers."""
    pass
class TruncatedHeaderError(HeaderError):
    """Exception for truncated headers."""
    pass
class EOFHeaderError(HeaderError):
    """Exception for end of file headers."""
    pass
class InvalidHeaderError(HeaderError):
    """Exception for invalid headers."""
    pass
class SubsequentHeaderError(HeaderError):
    """Exception for missing and invalid extended headers."""
    pass
class InvalidEncryptionError(TarError):
    """Exception for undefined crypto modes and combinations."""
    pass
class DecryptionError(TarError):
    """Exception for error during decryption."""
    pass
class EncryptionError(TarError):
    """Exception for error during encryption."""
    pass
class EndOfFile(Exception):
    """Signal end of file condition when they’re not an error."""
    pass

#---------------------------
# internal stream interface
#---------------------------
class _LowLevelFile:
    """Low-level file object. Supports reading and writing.
       It is used instead of a regular file object for streaming
       access.
    """

    def __init__(self, name, mode):
        _mode = {
            "r": os.O_RDONLY,
            "w": os.O_RDWR | os.O_CREAT | os.O_TRUNC,
        }[mode]
        if hasattr(os, "O_BINARY"):
            _mode |= os.O_BINARY                    # pylint: disable=no-member
        self.fd = os.open(name, _mode, 0o666)
        self.offset = 0

    def close(self):
        os.close(self.fd)

    def read(self, size):
        ret = os.read(self.fd, size)
        self.offset += len(ret)
        return ret

    def write(self, s, pos=None):
        if pos is not None:
            p0 = self.offset
            os.lseek (self.fd, pos, os.SEEK_SET)
        n = os.write(self.fd, s)
        if pos is None:
            self.offset += len(s)
        else:
            append = pos + n - p0
            if append > 0:
                self.offset += append
            os.lseek (self.fd, p0, os.SEEK_SET)

    def tell(self):
        return self.offset

    def seek_set (self, pos):
        os.lseek (self.fd, pos, os.SEEK_SET)
        self.offset = pos


def gz_header (name=None):
    timestamp = int(time.time())
    flags     = 0x0

    if name is None:
        name = b""
    else:
        flags |= GZ_FLAG_FNAME
        if type(name) is str:
            name = name.encode("iso-8859-1", "replace")
        if name.endswith(b".pdtcrypt"):
            name = name[:-9]
        if name.endswith(b".gz"):
            name = name[:-3]
        # RFC1952 says we must use ISO-8859-1 for the FNAME field.
        name += NUL

    hdr = struct.pack (GZ_FMT_HEADER,
                       GZ_MAGIC [0], GZ_MAGIC [1],
                       GZ_METHOD_DEFLATE, flags,
                       timestamp,
                       GZ_DEFLATE_FLAGS, GZ_OS_CODE)

    return hdr + name


class _Stream:
    """Class that serves as an adapter between TarFile and
       a stream-like object.  The stream-like object only
       needs to have a read() or write() method and is accessed
       blockwise.  Use of gzip or bzip2 compression is possible.
       A stream-like object could be for example: sys.stdin,
       sys.stdout, a socket, a tape device etc.

       _Stream is intended to be used only internally but is
       nevertherless used externally by Deltatar.

       When encrypting, the ``enccounter`` will be used for
       initializing the first cryptographic context. When
       decrypting, its value will be compared to the decrypted
       object. Decryption fails if the value does not match.
       In effect, this means that a ``_Stream`` whose ctor was
       passed ``enccounter`` can only be used to encrypt or
       decrypt a single object.
    """

    remainder = -1 # track size in encrypted entries
    tolerance = TOLERANCE_STRICT

    def __init__(self, name, mode, comptype, fileobj, bufsize,
                 concat=False, encryption=None, enccounter=None,
                 compresslevel=9, tolerance=TOLERANCE_STRICT):
        """Construct a _Stream object.
        """
        self.arcmode = arcmode_set (concat, encryption, comptype)
        self.tolerance = tolerance

        self._extfileobj = True
        if fileobj is None:
            fileobj = _LowLevelFile(name, mode)
            self._extfileobj = False

        if comptype == '*':
            # Enable transparent compression detection for the
            # stream interface
            fileobj = _StreamProxy(fileobj)
            comptype = fileobj.getcomptype()
        if comptype == '':
            comptype = "tar"

        self.enccounter = None
        if self.arcmode & ARCMODE_ENCRYPT:
            self.enccounter = enccounter

        self.name     = name or ""
        self.mode     = mode
        self.comptype = comptype
        self.cmp      = None
        self.fileobj  = fileobj
        self.bufsize  = bufsize
        self.buf      = b""
        self.pos      = 0
        self.concat_pos = 0
        self.closed   = False
        self.flags    = 0
        self.last_block_offset = 0
        self.dbuf     = b"" # ???
        self.exception = None # communicate decompression failure
        self.compresslevel = compresslevel
        self.bytes_written = 0
        # crypto parameters
        self.encryption = encryption
        self.lasthdr    = None

        if encryption is not None:
            encryption.reset_last_iv ()

        try:
            if comptype == "gz":
                try:
                    import zlib
                except ImportError:
                    raise CompressionError("zlib module is not available")
                self.zlib = zlib
                if mode == "r":
                    self.exception = zlib.error
                    self._init_read_gz()
                elif mode == "w":
                    if not (self.arcmode & ARCMODE_CONCAT):
                        if self.arcmode & ARCMODE_ENCRYPT:
                            self._init_write_encrypt (name)
                        self._init_write_gz ()
                self.crc = zlib.crc32(b"") & 0xFFFFffff

            elif comptype == "bz2":
                if self.arcmode & ARCMODE_ENCRYPT:
                    raise InvalidEncryptionError("encryption not available for "
                                                 "compression “%s”" % comptype)
                try:
                    import bz2
                except ImportError:
                    raise CompressionError("bz2 module is not available")
                if mode == "r":
                    self.dbuf = b""
                    self.cmp = bz2.BZ2Decompressor()
                    self.exception = OSError
                else:
                    self.cmp = bz2.BZ2Compressor()

            elif comptype == 'xz':
                if self.arcmode & ARCMODE_ENCRYPT:
                    raise InvalidEncryptionError("encryption not available for "
                                                 "compression “%s”" % comptype)
                try:
                    import lzma
                except ImportError:
                    raise CompressionError("lzma module is not available")
                if mode == "r":
                    self.dbuf = b""
                    self.cmp = lzma.LZMADecompressor()
                    self.exception = lzma.LZMAError
                else:
                    self.cmp = lzma.LZMACompressor()

            elif comptype == "tar":
                if not (self.arcmode & ARCMODE_CONCAT) \
                        and mode == "w" \
                        and self.arcmode & ARCMODE_ENCRYPT:
                    self._init_write_encrypt (name)

            else:
                if self.arcmode & ARCMODE_ENCRYPT:
                    raise InvalidEncryptionError("encryption not available for "
                                                 "compression “%s”" % comptype)
                raise CompressionError("unknown compression type %r" % comptype)

        except:
            if not self._extfileobj:
                self.fileobj.close()
            self.closed = True
            raise

    def __del__(self):
        if hasattr(self, "closed") and not self.closed:
            try:
                self.close()
            except crypto.InternalError:
                # context already finalized due to abort but close() tried
                # to use it
                pass


    def next (self, name):
        if self.arcmode & ARCMODE_COMPRESS:
            if getattr (self, "cmp", None) is not None:
                self._finalize_write_gz ()
        self.__sync()
        if self.arcmode & ~(ARCMODE_ENCRYPT | ARCMODE_COMPRESS):
            self.last_block_offset = self.fileobj.tell()
        if self.arcmode & ARCMODE_ENCRYPT:
            self._finalize_write_encrypt ()
            self._init_write_encrypt (name, set_last_block_offset=True)
        if self.arcmode & ARCMODE_COMPRESS:
            self._init_write_gz (set_last_block_offset =
                                 not (self.arcmode & ARCMODE_ENCRYPT))
        return self.last_block_offset


    def next_volume (self, name):
        # with non-concat modes, this is taken care by the _Stream
        # ctor as invoked by the newvol handler
        if self.arcmode & ARCMODE_COMPRESS:
            if getattr (self, "cmp", None) is not None:
                # e. g. compressed PAX header written
                self._finalize_write_gz ()
        if self.arcmode & ARCMODE_ENCRYPT:
            self._init_write_encrypt (name)
        if self.arcmode & ARCMODE_COMPRESS:
            self._init_write_gz ()


    def _init_write_encrypt (self, entry=None, set_last_block_offset=False):
        """
        Save position for delayed write of header; fill the header location
        with dummy bytes.
        """
        # first thing, proclaim new object to the encryption context
        # secondly, assemble the header with the updated parameters
        # and commit it directly to the underlying stream, bypassing the
        # encryption layer in .__write().
        dummyhdr = self.encryption.next (entry, counter=self.enccounter)
        if dummyhdr is None:
            raise EncryptionError ("Crypto.next(): bad dummy header") # XXX
        self.lasthdr = self.fileobj.tell()
        self.__write_to_file(dummyhdr)
        if set_last_block_offset is True:
            self.last_block_offset = self.lasthdr


    def _finalize_write_encrypt (self):
        """
        Seek back to header position, read dummy bytes, finalize crypto
        obtaining the actual header, write header, seek back to current
        position.

        Returns the list of IV fixed parts as used during encryption.
        """
        if self.lasthdr is not None:
            pos0 = self.fileobj.tell ()
            self.fileobj.seek_set (self.lasthdr)
            dummy = self.fileobj.read (crypto.PDTCRYPT_HDR_SIZE)
            pos1 = self.fileobj.tell ()
            dpos = pos1 - self.lasthdr
            assert dpos == crypto.PDTCRYPT_HDR_SIZE
            self.fileobj.seek_set (pos0)
            data, hdr, _ = self.encryption.done (dummy)
            self.__write_to_file(hdr, pos=self.lasthdr)
            self.__write_to_file(data) # append remainder of data
            self.lasthdr = -1


    def _finalize_write_gz (self):
        if self.cmp is not None:
            chunk = self.buf + self.cmp.flush()
            if chunk:
                if self.comptype == "gz":
                    # The native zlib crc is an unsigned 32-bit integer, but
                    # the Python wrapper implicitly casts that to a signed C
                    # long.  So, on a 32-bit box self.crc may "look negative",
                    # while the same crc on a 64-bit box may "look positive".
                    # To avoid irksome warnings from the `struct` module, force
                    # it to look positive on all boxes.
                    chunk += struct.pack("<L", self.crc & 0xffffffff)
                    chunk += struct.pack("<L", self.concat_pos & 0xffffFFFF)
                self.__enc_write (chunk)
                self.buf = b""


    def _init_write_gz (self, set_last_block_offset=False):
        '''
        Add a new gzip block, closing last one
        '''
        self.concat_pos = 0
        self.crc = self.zlib.crc32(b"") & 0xFFFFffff
        first = self.cmp is None
        self.cmp = self.zlib.compressobj(self.compresslevel,
                                         self.zlib.DEFLATED,
                                         -self.zlib.MAX_WBITS,
                                         self.zlib.DEF_MEM_LEVEL,
                                         0)

        # if aes, we encrypt after compression
        if set_last_block_offset is True:
            self.last_block_offset = self.fileobj.tell()

        self.__write(gz_header (self.name if first is True else None))


    def write(self, s):
        """Write string s to the stream.
        """
        if self.comptype == "gz":
            self.crc = self.zlib.crc32(s, self.crc) & 0xFFFFffff
        self.pos += len(s)
        self.concat_pos += len(s)
        if self.cmp is not None:
            s = self.cmp.compress(s)
        self.__write(s)

    def __sync(self):
        """Write what’s left in the buffer to the stream."""
        self.__write (b"") # → len (buf) <= bufsiz
        self.__enc_write (self.buf)
        self.buf = b""

    def __write(self, s):
        """Writes (and encodes) string s to the stream blockwise

        will wait with encoding/writing until block is complete
        """
        self.buf += s
        while len(self.buf) > self.bufsize:
            self.__enc_write(self.buf[:self.bufsize])
            self.buf = self.buf[self.bufsize:]


    def __write_to_file(self, s, pos=None):
        '''
        Writes directly to the fileobj; updates self.bytes_written. If “pos” is
        given, the stream will seek to that position first and back afterwards,
        and the total of bytes written is not updated.
        '''
        self.fileobj.write(s, pos)
        if pos is None:
            self.bytes_written += len(s)


    def __enc_write(self, s):
        """
        If encryption is active, the string s is encrypted before being written
        to the file.
        """
        if len (s) == 0:
            return
        if self.arcmode & ARCMODE_ENCRYPT:
            buf = s
            while len (buf) > 0:
                n, ct = self.encryption.process(buf)
                self.__write_to_file(ct)
                buf = buf [n:]
                if len (buf) > 0:
                    # The entire plaintext was not consumed: The size limit
                    # for encrypted objects was reached. Transparently create
                    # a new encrypted object and continue processing the input.
                    self._finalize_write_encrypt ()
                    self._init_write_encrypt ()
        else:
            self.__write_to_file(s)


    def estim_file_size(self):
        """ estimates size of file if closing it now

        The result may differ greatly from the amount of data sent to write()
        due to compression, encryption and buffering.

        In tests the result (before calling close()) was up to 12k smaller than
        the final file size if compression is being used because zlib/bz2
        compressors do not allow inspection of their buffered data :-(

        Still, we add what close() would add: 8 bytes for gz checksum, one
        encryption block size if encryption is used and the size of our own
        buffer
        """
        if self.closed:
            return self.bytes_written

        result = self.bytes_written
        if self.buf:
            result += len(self.buf)
        if self.comptype == 'gz':
            result += 8   # 2 longs = 8 byte (no extra info written for bzip2)
        return result

    def close(self, close_fileobj=True):
        """Close the _Stream object. No operation should be
           done on it afterwards.
        """

        if self.closed:
            return

        if close_fileobj is True:

            if self.mode == "w":
                if self.arcmode & ARCMODE_COMPRESS:
                    self._finalize_write_gz ()
                # end of Tar archive marker (two empty blocks) was written
                # finalize encryption last; no writes may be performed after
                # this point
                self.__sync ()
                if self.arcmode & ARCMODE_ENCRYPT:
                    self._finalize_write_encrypt ()

            if not self._extfileobj:
                self.fileobj.close()
        else:
            # read the zlib crc and length and check them
            if self.mode == "r" and self.comptype == "gz":
                read_crc = self.__read(4)
                read_length = self.__read(4)
                calculated_crc = self.crc
                if struct.unpack("<L", read_crc)[0] != calculated_crc:
                    raise CompressionError("bad gzip crc")
        self.closed = True


    def _init_read_gz(self):
        """Initialize for reading a gzip compressed fileobj.
        """
        self.cmp = self.zlib.decompressobj(-self.zlib.MAX_WBITS)

        read2 = self.__read(2)
        if read2 == b"":
            raise EndOfFile ("_init_read_gz(): read returned zero bytes at pos "
                             "%d" % self.fileobj.tell())
        # taken from gzip.GzipFile with some alterations
        if read2 != GZ_MAGIC_BYTES:
            raise ReadError("not a gzip file")

        read1 = self.__read(1)
        if read1 == b"":
            raise EndOfFile ("_init_read_gz(): read returned zero bytes inside "
                             "gzip header at pos %d" % self.fileobj.tell())
        if ord (read1) != GZ_METHOD_DEFLATE:
            raise CompressionError("unsupported compression method")

        self.flags = flag = ord(self.__read(1))
        self.__read(6) # discard timestamp[4], deflate flags, os code

        if flag & GZ_FLAG_FEXTRA:
            xlen = ord(self.__read(1)) + 256 * ord(self.__read(1))
            self.read(xlen)
        if flag & GZ_FLAG_FNAME:
            while True:
                s = self.__read(1)
                if not s or s == NUL:
                    break
        if flag & GZ_FLAG_FCOMMENT:
            while True:
                s = self.__read(1)
                if not s or s == NUL:
                    break
        if flag & GZ_FLAG_FHCRC:
            self.__read(2)

    def _init_read_encrypt (self):
        """Initialize encryption for next entry in archive. Read a header and
        notify the crypto context."""
        if self.arcmode & ARCMODE_ENCRYPT:
            lasthdr = self.fileobj.tell ()
            try:
                hdr = crypto.hdr_read_stream (self.fileobj)
            except crypto.EndOfFile:
                return False
            except crypto.InvalidHeader as exn:
                raise DecryptionError ("Crypto.hdr_read_stream(): error “%s” "
                                       "processing %r at pos %d"
                                       % (exn, self.fileobj, lasthdr)) \
                      from exn
            if self.enccounter is not None:
                # enforce that the iv counter in the header matches an
                # explicitly requested one
                iv = crypto.hdr_iv_counter (hdr)
                if iv != self.enccounter:
                    raise DecryptionError ("expected IV counter %d, got %d"
                                           % (self.enccounter, iv))
            self.lasthdr   = lasthdr
            self.remainder = hdr ["ctsize"] # distance to next header
            try:
                self.encryption.next (hdr)
            except crypto.InvalidParameter as exn:
                raise DecryptionError ("Crypto.next(): error “%s” "
                                       "processing %r at pos %d"
                                       % (exn, self.fileobj, lasthdr)) \
                      from exn

        return True


    def _read_encrypt (self, buf):
        """
        Demote a program error to a decryption error in tolerant mode. This
        allows recovery from corrupted headers and invalid data.
        """
        try:
            return self.encryption.process (buf)
        except RuntimeError as exn:
            if self.tolerance != TOLERANCE_STRICT:
                raise DecryptionError (exn)
            raise


    def _finalize_read_encrypt (self):
        """
        Finalize decryption.
        """
        if      self.arcmode & ARCMODE_ENCRYPT \
            and self.lasthdr is not None :
            assert self.remainder >= 0
            if self.remainder > 0:
                self.remainder = 0
            try:
                data = self.encryption.done ()
            except crypto.InvalidGCMTag as exn:
                raise DecryptionError ("decryption failed: %s" % exn)
            return data


    def tell(self):
        """Return the stream's file pointer position.
        """
        return self.pos

    def seek(self, pos=0):
        """Set the stream's file pointer to pos. Negative seeking
           is forbidden.
        """
        if pos == self.pos:
            pass # nothing to do
        elif pos - self.pos >= 0:
            blocks, remainder = divmod(pos - self.pos, self.bufsize)
            if self.encryption is not None:
                # IV succession is only preserved between successive objects.
                self.encryption.reset_last_iv ()
            for i in range(blocks):
                self.read(self.bufsize)
            self.read(remainder)
        else:
            raise StreamError("seeking backwards is not allowed")
        return self.pos

    def read(self, size=None):
        """Return the next size number of bytes from the stream.
           If size is not defined, return all bytes of the stream
           up to EOF.
        """
        if size is None:
            t = []
            while True:
                buf = self._read(self.bufsize)
                if not buf:
                    break
                t.append(buf)
            buf = b"".join(t)
        else:
            buf = self._read(size)
        self.pos += len(buf)
        return buf

    def readline(self):
        """Reads just one line, new line character included
        """
        # if \n in dbuf, no read neads to be done
        if b'\n' in self.dbuf:
            pos = self.dbuf.index(b'\n') + 1
            ret = self.dbuf[:pos]
            self.dbuf = self.dbuf[pos:]
            return ret

        buf = []
        while True:
            chunk = self._read(self.bufsize)

            # nothing more to read, so return the buffer
            if not chunk:
                return b''.join(buf)

            buf.append(chunk)

            # if \n found, return the new line
            if b'\n' in chunk:
                dbuf = b''.join(buf)
                pos = dbuf.index(b'\n') + 1
                self.dbuf = dbuf[pos:] + self.dbuf
                return dbuf[:pos]

    def _read(self, size):
        """Return size bytes from the stream.
        """
        c = len(self.dbuf)
        t = [self.dbuf]

        while c < size:
            buf = self.__read(self.bufsize)
            if not buf:
                break

            if self.cmp is not None:
                try:
                    buf = self.cmp.decompress(buf)
                except self.exception as exn:
                    raise ReadError("invalid compressed data (%r)" % exn)
                except Exception as e:
                    # happens at the end of the file
                    # _init_read_gz failed in the previous iteration so
                    # self.cmp.decompress fails here
                    if self.arcmode & ARCMODE_CONCAT:
                        pass
                    else:
                        raise ReadError("invalid compressed data")
                if self.arcmode & ARCMODE_COMPRESS and hasattr(self, "crc"):
                    self.crc = self.zlib.crc32(buf, self.crc) & 0xFFFFffff
                if self.arcmode & ARCMODE_CONCAT \
                        and len(self.cmp.unused_data) != 0:
                    self.buf = self.cmp.unused_data + self.buf
                    self.close(close_fileobj=False)
                    try:
                        self._init_read_gz()
                    except DecryptionError:
                        if self.tolerance != TOLERANCE_STRICT:
                            # return whatever data was processed successfully
                            if len (buf) > 0:
                                t.append (buf)
                            if len (t) > 0:
                                break
                        raise
                    except ReadError: # gzip troubles
                        if self.tolerance == TOLERANCE_RESCUE:
                            if len (buf) > 0:
                                t.append (buf)
                            if len (t) > 0:
                                break
                        raise
                    except EndOfFile:
                        # happens at the end of the file
                        pass
                    self.crc = self.zlib.crc32(b"") & 0xFFFFffff
                    self.closed = False
            t.append(buf)
            c += len(buf)
        t = b"".join(t)
        self.dbuf = t[size:]
        return t[:size]


    def __read(self, size):
        """
        Return size bytes from stream. If internal buffer is empty, read
        another block from the stream.

        The function returns up to size bytes of data. When an error occurs
        during decryption, everything until the end of the last successfully
        finalized object is returned.
        """
        c = len(self.buf)
        t = [self.buf] if c > 0 else []
        good_crypto = len (t)

        while c < size:
            todo = size
            try:
                if self.arcmode & ARCMODE_ENCRYPT:
                    if self.remainder <= 0:
                        # prepare next object
                        if self._init_read_encrypt () is False: # EOF
                            buf = None
                            break # while

                    # only read up to the end of the encrypted object
                    todo = min (size, self.remainder)
                buf = self.fileobj.read(todo)
                if self.arcmode & ARCMODE_ENCRYPT:
                    # decrypt the thing
                    buf = self._read_encrypt (buf)
                    if todo == self.remainder:
                        # at the end of a crypto object; finalization will fail if
                        # the GCM tag does not match
                        trailing = self._finalize_read_encrypt ()
                        good_crypto = len (t) + 1
                        if len (trailing) > 0:
                            buf += trailing
                        self.remainder = 0
                    else:
                        self.remainder -= todo
            except DecryptionError:
                if self.tolerance == TOLERANCE_STRICT:
                    raise
                self.encryption.drop ()
                if self.tolerance == TOLERANCE_RECOVER:
                    if good_crypto == 0:
                        raise
                    # this may occur at any of the three crypto operations above.
                    # some objects did validate; discard all data after it; next
                    # call will start with the bad object and error out immediately
                    self.buf = b"".join (t [good_crypto:])
                    return b"".join (t [:good_crypto])
                elif self.tolerance == TOLERANCE_RESCUE:
                    # keep what we have so far despite the finalization issue
                    t.append (buf)
                    c += len (buf)
                    break
                else:
                    raise RuntimeError("internal error: bad tolerance level")

            if not buf: ## XXX stream terminated prematurely; this should be an error
                break

            t.append(buf)
            c += len(buf)
        t = b"".join(t)
        self.buf = t[size:]

        return t[:size]


class _StreamProxy(object):
    """Small proxy class that enables transparent compression
       detection for the Stream interface (mode 'r|*').
    """

    def __init__(self, fileobj):
        self.fileobj = fileobj
        self.buf = self.fileobj.read(BLOCKSIZE)

    def read(self, size):                       # pylint: disable=method-hidden
        self.read = self.fileobj.read
        return self.buf

    def getcomptype(self):
        if self.buf.startswith(GZ_MAGIC_DEFLATE):
            return "gz"
        elif self.buf[0:3] == b"BZh" and self.buf[4:10] == b"1AY&SY":
            return "bz2"
        elif self.buf.startswith((b"\x5d\x00\x00\x80", b"\xfd7zXZ")):
            return "xz"
        else:
            return "tar"

    def close(self):
        self.fileobj.close()
# class StreamProxy

#------------------------
# Extraction file object
#------------------------
class _FileInFile(object):
    """A thin wrapper around an existing file object that
       provides a part of its data as an individual file
       object.
    """

    def __init__(self, fileobj, offset, size, blockinfo=None):
        self.fileobj = fileobj
        self.offset = offset
        self.size = size
        self.position = 0
        self.name = getattr(fileobj, "name", None)
        self.closed = False

        if blockinfo is None:
            blockinfo = [(0, size)]

        # Construct a map with data and zero blocks.
        self.map_index = 0
        self.map = []
        lastpos = 0
        realpos = self.offset
        for offset, size in blockinfo:
            if offset > lastpos:
                self.map.append((False, lastpos, offset, None))
            self.map.append((True, offset, offset + size, realpos))
            realpos += size
            lastpos = offset + size
        if lastpos < self.size:
            self.map.append((False, lastpos, self.size, None))

    def flush(self):
        pass

    def readable(self):
        return True

    def writable(self):
        return False

    def seekable(self):
        return self.fileobj.seekable()

    def tell(self):
        """Return the current file position.
        """
        return self.position

    def seek(self, position, whence=io.SEEK_SET):
        """Seek to a position in the file.
        """
        if whence == io.SEEK_SET:
            self.position = min(max(position, 0), self.size)
        elif whence == io.SEEK_CUR:
            if position < 0:
                self.position = max(self.position + position, 0)
            else:
                self.position = min(self.position + position, self.size)
        elif whence == io.SEEK_END:
            self.position = max(min(self.size + position, self.size), 0)
        else:
            raise ValueError("Invalid argument")
        return self.position

    def read(self, size=None):
        """Read data from the file.
        """
        if size is None:
            size = self.size - self.position
        else:
            size = min(size, self.size - self.position)

        buf = b""
        while size > 0:
            while True:
                data, start, stop, offset = self.map[self.map_index]
                if start <= self.position < stop:
                    break
                else:
                    self.map_index += 1
                    if self.map_index == len(self.map):
                        self.map_index = 0
            length = min(size, stop - self.position)
            if data:
                self.fileobj.seek(offset + (self.position - start))
                buf += self.fileobj.read(length)
            else:
                buf += NUL * length
            size -= length
            self.position += length
        return buf

    def readinto(self, b):
        buf = self.read(len(b))
        b[:len(buf)] = buf
        return len(buf)

    def close(self):
        self.closed = True
#class _FileInFile


class ExFileObject(io.BufferedReader):

    def __init__(self, tarfile, tarinfo):
        fileobj = _FileInFile(tarfile.fileobj, tarinfo.offset_data,
                tarinfo.size, tarinfo.sparse)
        super().__init__(fileobj)
#class ExFileObject

#------------------
# Exported Classes
#------------------
class TarInfo(object):
    """Informational class which holds the details about an
       archive member given by a tar header block.
       TarInfo objects are returned by TarFile.getmember(),
       TarFile.getmembers() and TarFile.gettarinfo() and are
       usually created internally.
    """

    __slots__ = ("name", "mode", "uid", "gid", "size", "mtime",
                 "chksum", "type", "linkname", "uname", "gname",
                 "devmajor", "devminor", "volume_offset",
                 "offset", "offset_data", "pax_headers", "sparse",
                 "tarfile", "_sparse_structs", "_link_target")

    def __init__(self, name=""):
        """Construct a TarInfo object. name is the optional name
           of the member.
        """
        self.name = name        # member name
        self.mode = 0o644       # file permissions
        self.uid = 0            # user id
        self.gid = 0            # group id
        self.size = 0           # file size
        self.mtime = 0          # modification time
        self.chksum = 0         # header checksum
        self.type = REGTYPE     # member type
        self.linkname = ""      # link name
        self.uname = ""         # user name
        self.gname = ""         # group name
        self.devmajor = 0       # device major number
        self.devminor = 0       # device minor number

        self.offset = 0         # the tar header starts here
        self.offset_data = 0    # the file's data starts here
        self.volume_offset = 0  # the file's data corresponds with the data
                                # starting at this position

        self.sparse = None      # sparse member information
        self.pax_headers = {}   # pax header information

    # In pax headers the "name" and "linkname" field are called
    # "path" and "linkpath".
    def _getpath(self):
        return self.name
    def _setpath(self, name):
        self.name = name
    path = property(_getpath, _setpath)

    def _getlinkpath(self):
        return self.linkname
    def _setlinkpath(self, linkname):
        self.linkname = linkname
    linkpath = property(_getlinkpath, _setlinkpath)

    def __repr__(self):
        return "<%s %r at %#x>" % (self.__class__.__name__,self.name,id(self))

    def get_info(self, encoding=None, errors=None):
        """Return the TarInfo's attributes as a dictionary.
        """
        info = {
            "name":     self.name,
            "mode":     self.mode & 0o7777,
            "uid":      self.uid,
            "gid":      self.gid,
            "size":     self.size,
            "mtime":    self.mtime,
            "chksum":   self.chksum,
            "type":     self.type,
            "linkname": self.linkname,
            "uname":    self.uname,
            "gname":    self.gname,
            "devmajor": self.devmajor,
            "devminor": self.devminor,
            "offset_data": self.offset_data,
            "volume_offset": self.volume_offset
        }

        if info["type"] == DIRTYPE and not info["name"].endswith("/"):
            info["name"] += "/"

        return info

    def tobuf(self, format=DEFAULT_FORMAT, encoding=ENCODING,
              errors="surrogateescape"):
        """Return a tar header as a string of 512 byte blocks.
        """
        info = self.get_info(encoding, errors)

        if format == USTAR_FORMAT:
            return self.create_ustar_header(info, encoding, errors)
        elif format == GNU_FORMAT:
            return self.create_gnu_header(info, encoding, errors)
        elif format == PAX_FORMAT:
            return self.create_pax_header(info, encoding, errors)
        else:
            raise ValueError("invalid format")

    def create_ustar_header(self, info, encoding, errors):
        """Return the object as a ustar header block.
        """
        info["magic"] = POSIX_MAGIC

        if len(info["linkname"]) > LENGTH_LINK:
            raise ValueError("linkname is too long")

        if len(info["name"]) > LENGTH_NAME:
            info["prefix"], info["name"] = self._posix_split_name(info["name"])

        return self._create_header(info, USTAR_FORMAT, encoding, errors)

    def create_gnu_header(self, info, encoding, errors):
        """Return the object as a GNU header block sequence.
        """
        info["magic"] = GNU_MAGIC

        if self.ismultivol():
            prefix = [
                itn(info.get("atime", 0), 12, GNU_FORMAT),
                itn(info.get("ctime", 0), 12, GNU_FORMAT),
                itn(self.volume_offset, 12, GNU_FORMAT),
                itn(0, 119, GNU_FORMAT), # stuff unused in this tar implementation, set to zero
            ]
            info['prefix'] = b"".join(prefix)
            info['size'] = info['size'] - self.volume_offset

        buf = b""
        if len(info["linkname"]) > LENGTH_LINK:
            buf += self._create_gnu_long_header(info["linkname"],
                GNUTYPE_LONGLINK, encoding, errors)

        if len(info["name"]) > LENGTH_NAME:
            buf += self._create_gnu_long_header(info["name"], GNUTYPE_LONGNAME,
                                                encoding, errors)

        return buf + self._create_header(info, GNU_FORMAT, encoding, errors)

    def create_pax_header(self, info, encoding, errors):
        """Return the object as a ustar header block. If it cannot be
           represented this way, prepend a pax extended header sequence
           with supplement information.
        """
        info["magic"] = POSIX_MAGIC
        pax_headers = self.pax_headers.copy()
        if self.ismultivol():
            info['size'] = info['size'] - self.volume_offset

        # Test string fields for values that exceed the field length or cannot
        # be represented in ASCII encoding.
        for name, hname, length in (
                ("name", "path", LENGTH_NAME),
                ("linkname", "linkpath", LENGTH_LINK),
                ("uname", "uname", 32),
                ("gname", "gname", 32)):

            if hname in pax_headers:
                # The pax header has priority.
                continue

            # Try to encode the string as ASCII.
            try:
                info[name].encode("ascii", "strict")
            except UnicodeEncodeError:
                pax_headers[hname] = info[name]
                continue

            if len(info[name]) > length:
                pax_headers[hname] = info[name]

        # Test number fields for values that exceed the field limit or values
        # that like to be stored as float.
        for name, digits in (("uid", 8), ("gid", 8), ("size", 12), ("mtime", 12)):
            if name in pax_headers:
                # The pax header has priority. Avoid overflow.
                info[name] = 0
                continue

            val = info[name]
            if not 0 <= val < 8 ** (digits - 1) or isinstance(val, float):
                pax_headers[name] = str(val)
                info[name] = 0

        # Create a pax extended header if necessary.
        if pax_headers:
            buf = self._create_pax_generic_header(pax_headers, XHDTYPE, encoding)
        else:
            buf = b""

        return buf + self._create_header(info, USTAR_FORMAT, "ascii", "replace")

    @classmethod
    def create_pax_global_header(cls, pax_headers):
        """Return the object as a pax global header block sequence.
        """
        return cls._create_pax_generic_header(pax_headers, XGLTYPE, "utf-8")

    def _posix_split_name(self, name):
        """Split a name longer than 100 chars into a prefix
           and a name part.
        """
        prefix = name[:LENGTH_PREFIX + 1]
        while prefix and prefix[-1] != "/":
            prefix = prefix[:-1]

        name = name[len(prefix):]
        prefix = prefix[:-1]

        if not prefix or len(name) > LENGTH_NAME:
            raise ValueError("name is too long")
        return prefix, name

    @staticmethod
    def _create_header(info, format, encoding, errors):
        """Return a header block. info is a dictionary with file
           information, format must be one of the *_FORMAT constants.
        """
        parts = [
            stn(info.get("name", ""), 100, encoding, errors),
            itn(info.get("mode", 0) & 0o7777, 8, format),
            itn(info.get("uid", 0), 8, format),
            itn(info.get("gid", 0), 8, format),
            itn(info.get("size", 0), 12, format),
            itn(info.get("mtime", 0), 12, format),
            b"        ", # checksum field
            info.get("type", REGTYPE),
            stn(info.get("linkname", ""), 100, encoding, errors),
            info.get("magic", POSIX_MAGIC),
            stn(info.get("uname", ""), 32, encoding, errors),
            stn(info.get("gname", ""), 32, encoding, errors),
            itn(info.get("devmajor", 0), 8, format),
            itn(info.get("devminor", 0), 8, format),
            sbtn(info.get("prefix", ""), 155, encoding, errors)
        ]

        buf = struct.pack("%ds" % BLOCKSIZE, b"".join(parts))
        chksum = calc_chksums(buf[-BLOCKSIZE:])[0]
        buf = buf[:-364] + bytes("%06o\0" % chksum, "ascii") + buf[-357:]
        return buf

    @staticmethod
    def _create_payload(payload):
        """Return the string payload filled with zero bytes
           up to the next 512 byte border.
        """
        blocks, remainder = divmod(len(payload), BLOCKSIZE)
        if remainder > 0:
            payload += (BLOCKSIZE - remainder) * NUL
        return payload

    @classmethod
    def _create_gnu_long_header(cls, name, type, encoding, errors):
        """Return a GNUTYPE_LONGNAME or GNUTYPE_LONGLINK sequence
           for name.
        """
        name = name.encode(encoding, errors) + NUL

        info = {}
        info["name"] = "././@LongLink"
        info["type"] = type
        info["size"] = len(name)
        info["magic"] = GNU_MAGIC

        # create extended header + name blocks.
        return cls._create_header(info, USTAR_FORMAT, encoding, errors) + \
                cls._create_payload(name)

    @classmethod
    def _create_pax_generic_header(cls, pax_headers, type, encoding):
        """Return a POSIX.1-2008 extended or global header sequence
           that contains a list of keyword, value pairs. The values
           must be strings.
        """
        # Check if one of the fields contains surrogate characters and thereby
        # forces hdrcharset=BINARY, see _proc_pax() for more information.
        binary = False
        for keyword, value in pax_headers.items():
            try:
                value.encode("utf-8", "strict")
            except UnicodeEncodeError:
                binary = True
                break

        records = b""
        if binary:
            # Put the hdrcharset field at the beginning of the header.
            records += b"21 hdrcharset=BINARY\n"

        for keyword, value in pax_headers.items():
            keyword = keyword.encode("utf-8")
            if binary:
                # Try to restore the original byte representation of `value'.
                # Needless to say, that the encoding must match the string.
                value = value.encode(encoding, "surrogateescape")
            else:
                value = value.encode("utf-8")

            l = len(keyword) + len(value) + 3   # ' ' + '=' + '\n'
            n = p = 0
            while True:
                n = l + len(str(p))
                if n == p:
                    break
                p = n
            records += bytes(str(p), "ascii") + b" " + keyword + b"=" + value + b"\n"

        # We use a hardcoded "././@PaxHeader" name like star does
        # instead of the one that POSIX recommends.
        info = {}
        info["name"] = "././@PaxHeader"
        info["type"] = type
        info["size"] = len(records)
        info["magic"] = POSIX_MAGIC

        # Create pax header + record blocks.
        return cls._create_header(info, USTAR_FORMAT, "ascii", "replace") + \
                cls._create_payload(records)

    @classmethod
    def frombuf(cls, buf, encoding, errors):
        """Construct a TarInfo object from a 512 byte bytes object.
        """
        if len(buf) == 0:
            raise EmptyHeaderError("empty header")
        if len(buf) != BLOCKSIZE:
            raise TruncatedHeaderError("truncated header")
        if buf.count(NUL) == BLOCKSIZE:
            raise EOFHeaderError("end of file header")

        chksum = nti(buf[148:156])
        if chksum not in calc_chksums(buf):
            raise InvalidHeaderError("bad checksum")

        obj = cls()
        obj.name = nts(buf[0:100], encoding, errors)
        obj.mode = nti(buf[100:108])
        obj.uid = nti(buf[108:116])
        obj.gid = nti(buf[116:124])
        obj.size = nti(buf[124:136])
        obj.mtime = nti(buf[136:148])
        obj.chksum = chksum
        obj.type = buf[156:157]
        obj.linkname = nts(buf[157:257], encoding, errors)
        obj.uname = nts(buf[265:297], encoding, errors)
        obj.gname = nts(buf[297:329], encoding, errors)
        obj.devmajor = nti(buf[329:337])
        obj.devminor = nti(buf[337:345])
        prefix = nts(buf[345:500], encoding, errors)

        # The old GNU sparse format occupies some of the unused
        # space in the buffer for up to 4 sparse structures.
        # Save the them for later processing in _proc_sparse().
        if obj.type == GNUTYPE_SPARSE:
            pos = 386
            structs = []
            for i in range(4):
                try:
                    offset = nti(buf[pos:pos + 12])
                    numbytes = nti(buf[pos + 12:pos + 24])
                except ValueError:
                    break
                structs.append((offset, numbytes))
                pos += 24
            isextended = bool(buf[482])
            origsize = nti(buf[483:495])
            obj._sparse_structs = (structs, isextended, origsize)

        # Old V7 tar format represents a directory as a regular
        # file with a trailing slash.
        if obj.type == AREGTYPE and obj.name.endswith("/"):
            obj.type = DIRTYPE

        # Remove redundant slashes from directories.
        if obj.isdir():
            obj.name = obj.name.rstrip("/")

        # Reconstruct a ustar longname.
        if prefix and obj.type not in GNU_TYPES:
            obj.name = prefix + "/" + obj.name
        else:
            obj.offset_data = nti(buf[369:381])
        return obj

    @classmethod
    def fromtarfile(cls, tarfile):
        """Return the next TarInfo object from TarFile object
           tarfile.
        """
        buf = tarfile.fileobj.read(BLOCKSIZE)
        obj = cls.frombuf(buf, tarfile.encoding, tarfile.errors)
        obj.offset = tarfile.fileobj.tell() - BLOCKSIZE
        return obj._proc_member(tarfile)

    #--------------------------------------------------------------------------
    # The following are methods that are called depending on the type of a
    # member. The entry point is _proc_member() which can be overridden in a
    # subclass to add custom _proc_*() methods. A _proc_*() method MUST
    # implement the following
    # operations:
    # 1. Set self.offset_data to the position where the data blocks begin,
    #    if there is data that follows.
    # 2. Set tarfile.offset to the position where the next member's header will
    #    begin.
    # 3. Return self or another valid TarInfo object.
    def _proc_member(self, tarfile):
        """Choose the right processing method depending on
           the type and call it.
        """
        if self.type in (GNUTYPE_LONGNAME, GNUTYPE_LONGLINK):
            return self._proc_gnulong(tarfile)
        elif self.type == GNUTYPE_SPARSE:
            return self._proc_sparse(tarfile)
        elif self.type in (XHDTYPE, XGLTYPE, SOLARIS_XHDTYPE):
            return self._proc_pax(tarfile)
        else:
            return self._proc_builtin(tarfile)

    def _proc_builtin(self, tarfile):
        """Process a builtin type or an unknown type which
           will be treated as a regular file.
        """
        self.offset_data = tarfile.fileobj.tell()
        offset = self.offset_data
        if self.isreg() or self.ismultivol() or self.type not in SUPPORTED_TYPES:
            # Skip the following data blocks.
            offset += self._block(self.size)
        tarfile.offset = offset

        # Patch the TarInfo object with saved global
        # header information.
        self._apply_pax_info(tarfile.pax_headers, tarfile.encoding, tarfile.errors)

        return self

    def _proc_gnulong(self, tarfile):
        """Process the blocks that hold a GNU longname
           or longlink member.
        """
        buf = tarfile.fileobj.read(self._block(self.size))

        # Fetch the next header and process it.
        try:
            next = self.fromtarfile(tarfile)
        except HeaderError:
            raise SubsequentHeaderError("missing or bad subsequent header")

        # Patch the TarInfo object from the next header with
        # the longname information.
        next.offset = self.offset
        if self.type == GNUTYPE_LONGNAME:
            next.name = nts(buf, tarfile.encoding, tarfile.errors)
        elif self.type == GNUTYPE_LONGLINK:
            next.linkname = nts(buf, tarfile.encoding, tarfile.errors)

        return next

    def _proc_sparse(self, tarfile):
        """Process a GNU sparse header plus extra headers.
        """
        # We already collected some sparse structures in frombuf().
        structs, isextended, origsize = self._sparse_structs
        del self._sparse_structs

        # Collect sparse structures from extended header blocks.
        while isextended:
            buf = tarfile.fileobj.read(BLOCKSIZE)
            pos = 0
            for i in range(21):
                try:
                    offset = nti(buf[pos:pos + 12])
                    numbytes = nti(buf[pos + 12:pos + 24])
                except ValueError:
                    break
                if offset and numbytes:
                    structs.append((offset, numbytes))
                pos += 24
            isextended = bool(buf[504])
        self.sparse = structs

        self.offset_data = tarfile.fileobj.tell()
        tarfile.offset = self.offset_data + self._block(self.size)
        self.size = origsize
        return self

    def _proc_pax(self, tarfile):
        """Process an extended or global header as described in
           POSIX.1-2008.
        """
        # Read the header information.
        buf = tarfile.fileobj.read(self._block(self.size))

        # A pax header stores supplemental information for either
        # the following file (extended) or all following files
        # (global).
        if self.type == XGLTYPE:
            pax_headers = tarfile.pax_headers
        else:
            pax_headers = tarfile.pax_headers.copy()

        # Check if the pax header contains a hdrcharset field. This tells us
        # the encoding of the path, linkpath, uname and gname fields. Normally,
        # these fields are UTF-8 encoded but since POSIX.1-2008 tar
        # implementations are allowed to store them as raw binary strings if
        # the translation to UTF-8 fails.
        match = re.search(br"\d+ hdrcharset=([^\n]+)\n", buf)
        if match is not None:
            pax_headers["hdrcharset"] = match.group(1).decode("utf-8")

        # For the time being, we don't care about anything other than "BINARY".
        # The only other value that is currently allowed by the standard is
        # "ISO-IR 10646 2000 UTF-8" in other words UTF-8.
        hdrcharset = pax_headers.get("hdrcharset")
        if hdrcharset == "BINARY":
            encoding = tarfile.encoding
        else:
            encoding = "utf-8"

        # Parse pax header information. A record looks like that:
        # "%d %s=%s\n" % (length, keyword, value). length is the size
        # of the complete record including the length field itself and
        # the newline. keyword and value are both UTF-8 encoded strings.
        regex = re.compile(br"(\d+) ([^=]+)=")
        pos = 0
        while True:
            match = regex.match(buf, pos)
            if not match:
                break

            length, keyword = match.groups()
            length = int(length)
            value = buf[match.end(2) + 1:match.start(1) + length - 1]

            # Normally, we could just use "utf-8" as the encoding and "strict"
            # as the error handler, but we better not take the risk. For
            # example, GNU tar <= 1.23 is known to store filenames it cannot
            # translate to UTF-8 as raw strings (unfortunately without a
            # hdrcharset=BINARY header).
            # We first try the strict standard encoding, and if that fails we
            # fall back on the user's encoding and error handler.
            keyword = self._decode_pax_field(keyword, "utf-8", "utf-8",
                    tarfile.errors)
            if keyword in PAX_NAME_FIELDS:
                value = self._decode_pax_field(value, encoding, tarfile.encoding,
                        tarfile.errors)
            else:
                value = self._decode_pax_field(value, "utf-8", "utf-8",
                        tarfile.errors)

            pax_headers[keyword] = value
            pos += length


        # Fetch the next header.
        try:
            next = self.fromtarfile(tarfile)
        except HeaderError:
            raise SubsequentHeaderError("missing or bad subsequent header")

        # Process GNU sparse information.
        if "GNU.sparse.map" in pax_headers:
            # GNU extended sparse format version 0.1.
            self._proc_gnusparse_01(next, pax_headers)

        elif "GNU.sparse.size" in pax_headers:
            # GNU extended sparse format version 0.0.
            self._proc_gnusparse_00(next, pax_headers, buf)

        elif pax_headers.get("GNU.sparse.major") == "1" and pax_headers.get("GNU.sparse.minor") == "0":
            # GNU extended sparse format version 1.0.
            self._proc_gnusparse_10(next, pax_headers, tarfile)

        if self.type in (XHDTYPE, SOLARIS_XHDTYPE):
            # Patch the TarInfo object with the extended header info.
            next._apply_pax_info(pax_headers, tarfile.encoding, tarfile.errors)
            next.offset = self.offset

            if "size" in pax_headers:
                # If the extended header replaces the size field,
                # we need to recalculate the offset where the next
                # header starts.
                offset = next.offset_data
                if next.isreg() or next.type not in SUPPORTED_TYPES:
                    offset += next._block(next.size)
                tarfile.offset = offset

        if next is not None:
            if "GNU.volume.filename" in pax_headers:
                if pax_headers["GNU.volume.filename"] == next.name:
                    if "GNU.volume.size" in pax_headers:
                        next.size = int(pax_headers["GNU.volume.size"])
                    if "GNU.volume.offset" in pax_headers:
                        next.volume_offset = int(pax_headers["GNU.volume.offset"])

                for key in pax_headers.keys():
                    if key.startswith("GNU.volume"):
                        del tarfile.pax_headers[key]

        return next

    def _proc_gnusparse_00(self, next, pax_headers, buf):
        """Process a GNU tar extended sparse header, version 0.0.
        """
        offsets = []
        for match in re.finditer(br"\d+ GNU.sparse.offset=(\d+)\n", buf):
            offsets.append(int(match.group(1)))
        numbytes = []
        for match in re.finditer(br"\d+ GNU.sparse.numbytes=(\d+)\n", buf):
            numbytes.append(int(match.group(1)))
        next.sparse = list(zip(offsets, numbytes))

    def _proc_gnusparse_01(self, next, pax_headers):
        """Process a GNU tar extended sparse header, version 0.1.
        """
        sparse = [int(x) for x in pax_headers["GNU.sparse.map"].split(",")]
        next.sparse = list(zip(sparse[::2], sparse[1::2]))

    def _proc_gnusparse_10(self, next, pax_headers, tarfile):
        """Process a GNU tar extended sparse header, version 1.0.
        """
        fields = None
        sparse = []
        buf = tarfile.fileobj.read(BLOCKSIZE)
        fields, buf = buf.split(b"\n", 1)
        fields = int(fields)
        while len(sparse) < fields * 2:
            if b"\n" not in buf:
                buf += tarfile.fileobj.read(BLOCKSIZE)
            number, buf = buf.split(b"\n", 1)
            sparse.append(int(number))
        next.offset_data = tarfile.fileobj.tell()
        next.sparse = list(zip(sparse[::2], sparse[1::2]))

    def _apply_pax_info(self, pax_headers, encoding, errors):
        """Replace fields with supplemental information from a previous
           pax extended or global header.
        """
        for keyword, value in pax_headers.items():
            if keyword == "GNU.sparse.name":
                setattr(self, "path", value)
            elif keyword == "GNU.sparse.size":
                setattr(self, "size", int(value))
            elif keyword == "GNU.sparse.realsize":
                setattr(self, "size", int(value))
            elif keyword in PAX_FIELDS:
                if keyword in PAX_NUMBER_FIELDS:
                    try:
                        value = PAX_NUMBER_FIELDS[keyword](value)
                    except ValueError:
                        value = 0
                if keyword == "path":
                    value = value.rstrip("/")       # pylint: disable=no-member
                setattr(self, keyword, value)

        self.pax_headers = pax_headers.copy()

    def _decode_pax_field(self, value, encoding, fallback_encoding, fallback_errors):
        """Decode a single field from a pax record.
        """
        try:
            return value.decode(encoding, "strict")
        except UnicodeDecodeError:
            return value.decode(fallback_encoding, fallback_errors)

    def _block(self, count):
        """Round up a byte count by BLOCKSIZE and return it,
           e.g. _block(834) => 1024.
        """
        blocks, remainder = divmod(count, BLOCKSIZE)
        if remainder:
            blocks += 1
        return blocks * BLOCKSIZE

    def isreg(self):
        return self.type in REGULAR_TYPES
    def isfile(self):
        return self.isreg()
    def isdir(self):
        return self.type == DIRTYPE
    def issym(self):
        return self.type == SYMTYPE
    def islnk(self):
        return self.type == LNKTYPE
    def ischr(self):
        return self.type == CHRTYPE
    def isblk(self):
        return self.type == BLKTYPE
    def isfifo(self):
        return self.type == FIFOTYPE
    def issparse(self):
        return self.sparse is not None
    def isdev(self):
        return self.type in (CHRTYPE, BLKTYPE, FIFOTYPE)
    def ismultivol(self):
        return self.type == GNUTYPE_MULTIVOL or self.volume_offset > 0 or\
            "GNU.volume.offset" in self.pax_headers
# class TarInfo

class TarFile(object):
    """The TarFile Class provides an interface to tar archives.
    """

    debug = 0                   # May be set from 0 (no msgs) to 3 (all msgs)

    dereference = False         # If true, add content of linked file to the
                                # tar file, else the link.

    ignore_zeros = False        # If true, skips empty or invalid blocks and
                                # continues processing.

    max_volume_size = None      # If different from None, establishes maximum
                                # size of tar volumes

    new_volume_handler = None   # function handler to be executed before when
                                # a new volume is needed

    volume_number = 0           # current volume number, used for multi volume
                                # support

    errorlevel = 1              # If 0, fatal errors only appear in debug
                                # messages (if debug >= 0). If > 0, errors
                                # are passed to the caller as exceptions.

    format = DEFAULT_FORMAT     # The format to use when creating an archive.

    encoding = ENCODING         # Encoding for 8-bit character strings.

    errors = None               # Error handler for unicode conversion.

    tarinfo = TarInfo           # The default TarInfo class to use.

    fileobject = ExFileObject   # The file-object for extractfile().

    arcmode = ARCMODE_PLAIN     # Object processing mode (“concat”, encryption,
                                # compression)

    save_to_members = True      # If new members are saved. This can be disabled
                                # if you manage lots of files and don't want
                                # to have high memory usage

    cache_uid2user = {}         # cache to avoid getpwuid calls. It always parses /etc/passwd.
    cache_gid2group = {}        # same cache for groups

    def __init__(self, name=None, mode="r", fileobj=None, format=None,
            tarinfo=None, dereference=None, ignore_zeros=None, encoding=None,
            errors="surrogateescape", pax_headers=None, debug=None,
            errorlevel=None, max_volume_size=None, new_volume_handler=None,
            concat=False, nacl=None,
            save_to_members=True):
        """Open an (uncompressed) tar archive `name'. `mode' is either 'r' to
           read from an existing archive, 'a' to append data to an existing
           file or 'w' to create a new file overwriting an existing one. `mode'
           defaults to 'r'.
           If `fileobj' is given, it is used for reading or writing data. If it
           can be determined, `mode' is overridden by `fileobj's mode.
           `fileobj' is not closed, when TarFile is closed.
        """
        if len(mode) > 1 or mode not in "raw":
            raise ValueError("mode must be 'r', 'a' or 'w'")
        self.mode = mode
        self.arcmode = arcmode_set (concat)
        self.nacl = nacl
        self._mode = {"r": "rb", "a": "r+b", "w": "wb"}[mode]

        if not fileobj:
            if self.mode == "a" and not os.path.exists(name):
                # Create nonexistent files in append mode.
                self.mode = "w"
                self._mode = "wb"
            fileobj = bltn_open(name, self._mode)
            self._extfileobj = False
        else:
            if name is None and hasattr(fileobj, "name"):
                name = fileobj.name
            # when fileobj is a gzip.GzipFile, fileobj.mode is an int (not valid for us)
            if hasattr(fileobj, "mode") and isinstance(fileobj.mode, str):
                self._mode = fileobj.mode
            self._extfileobj = True
        self.name = os.path.abspath(name) if name else None
        self.base_name = self.name = os.path.abspath(name) if name else None
        self.fileobj = fileobj

        # Init attributes.
        if format is not None:
            self.format = format
        if tarinfo is not None:
            self.tarinfo = tarinfo
        if dereference is not None:
            self.dereference = dereference
        if ignore_zeros is not None:
            self.ignore_zeros = ignore_zeros
        if encoding is not None:
            self.encoding = encoding

        self.errors = errors

        if pax_headers is not None and self.format == PAX_FORMAT:
            self.pax_headers = pax_headers
        else:
            self.pax_headers = {}

        if debug is not None:
            self.debug = debug
        if errorlevel is not None:
            self.errorlevel = errorlevel

        # Init datastructures.
        if max_volume_size and max_volume_size < 3*BLOCKSIZE:
            raise ValueError("max_volume_size needs to be at least %d" % (3*BLOCKSIZE))
        if max_volume_size and not callable(new_volume_handler):
            raise ValueError("new_volume_handler needs to be set and be callable for multivolume support")
        if max_volume_size:
            self.max_volume_size = int(max_volume_size)
        else:
            self.max_volume_size = None

        self.save_to_members = save_to_members
        self.new_volume_handler = new_volume_handler
        self.closed = False
        self.members = []       # list of members as TarInfo objects
        self._loaded = False    # flag if all members have been read
        self.offset = self.fileobj.tell()
                                # current position in the archive file
        self.inodes = {}        # dictionary caching the inodes of
                                # archive members already added

        try:
            if self.mode == "r":
                self.firstmember = None
                self.firstmember = self.next()

            if self.mode == "a":
                # Move to the end of the archive,
                # before the first empty block.
                while True:
                    self.fileobj.seek(self.offset)
                    try:
                        tarinfo = self.tarinfo.fromtarfile(self)
                        self.members.append(tarinfo)
                    except EOFHeaderError:
                        self.fileobj.seek(self.offset)
                        break
                    except HeaderError as e:
                        raise ReadError(str(e))

            if self.mode in "aw":
                self._loaded = True

                if self.pax_headers:
                    buf = self.tarinfo.create_pax_global_header(self.pax_headers.copy())
                    self.fileobj.write(buf)
                    self.offset += len(buf)
        except:
            if not self._extfileobj:
                self.fileobj.close()
            self.closed = True
            raise

    #--------------------------------------------------------------------------
    # Below are the classmethods which act as alternate constructors to the
    # TarFile class. The open() method is the only one that is needed for
    # public use; it is the "super"-constructor and is able to select an
    # adequate "sub"-constructor for a particular compression using the mapping
    # from OPEN_METH.
    #
    # This concept allows one to subclass TarFile without losing the comfort of
    # the super-constructor. A sub-constructor is registered and made available
    # by adding it to the mapping in OPEN_METH.

    @classmethod
    def open(cls, name=None, mode="r", fileobj=None, bufsize=RECORDSIZE,
             encryption=None, compresslevel=9, tolerance=TOLERANCE_STRICT,
             **kwargs):
        """Open a tar archive for reading, writing or appending. Return
           an appropriate TarFile class.

           mode:
           'r' or 'r:*' open for reading with transparent compression
           'r:'         open for reading exclusively uncompressed
           'r:gz'       open for reading with gzip compression
           'r:bz2'      open for reading with bzip2 compression
           'r:xz'       open for reading with lzma compression
           'a' or 'a:'  open for appending, creating the file if necessary
           'w' or 'w:'  open for writing without compression
           'w:gz'       open for writing with gzip compression
           'w:bz2'      open for writing with bzip2 compression
           'w:xz'       open for writing with lzma compression

           'r|*'        open a stream of tar blocks with transparent compression
           'r|'         open an uncompressed stream of tar blocks for reading
           'r|gz'       open a gzip compressed stream of tar blocks
           'r|bz2'      open a bzip2 compressed stream of tar blocks
           'r|xz'       open an lzma compressed stream of tar blocks
           'w|'         open an uncompressed stream for writing
           'w|gz'       open a gzip compressed stream for writing
           'w|bz2'      open a bzip2 compressed stream for writing
           'w|xz'       open an lzma compressed stream for writing

           'r#gz'       open a stream of gzip compressed tar blocks for reading
           'w#gz'       open a stream of gzip compressed tar blocks for writing
        """
        if not name and not fileobj:
            raise ValueError("nothing to open")

        if mode in ("r", "r:*"):
            # Find out which *open() is appropriate for opening the file.
            for comptype in cls.OPEN_METH:
                func = getattr(cls, cls.OPEN_METH[comptype])
                if fileobj is not None:
                    saved_pos = fileobj.tell()
                try:
                    return func(name, "r", fileobj, **kwargs)
                except (ReadError, CompressionError) as e:
                    # usually nothing exceptional but sometimes is
                    if fileobj is not None:
                        fileobj.seek(saved_pos)
                    continue
            raise ReadError("file could not be opened successfully")

        elif ":" in mode:
            filemode, comptype = mode.split(":", 1)
            filemode = filemode or "r"
            comptype = comptype or "tar"

            # Select the *open() function according to
            # given compression.
            if comptype in cls.OPEN_METH:
                func = getattr(cls, cls.OPEN_METH[comptype])
            else:
                raise CompressionError("unknown compression type %r" % comptype)

            # Pass on compression level for gzip / bzip2.
            if comptype == 'gz' or comptype == 'bz2':
                kwargs['compresslevel'] = compresslevel

            if 'max_volume_size' in kwargs:
                if comptype != 'tar' and filemode in 'wa' \
                        and kwargs['max_volume_size']:
                    import warnings
                    warnings.warn('Only the first volume will be compressed '
                                  'for modes with "w:"!')

            return func(name, filemode, fileobj, **kwargs)

        elif "|" in mode:
            filemode, comptype = mode.split("|", 1)
            filemode = filemode or "r"
            comptype = comptype or "tar"

            if filemode not in "rw":
                raise ValueError("mode must be 'r' or 'w'")

            t = cls(name, filemode,
                    _Stream(name, filemode, comptype, fileobj, bufsize,
                            compresslevel=compresslevel),
                    **kwargs)
            t._extfileobj = False
            return t

        elif "#" in mode:
            filemode, comptype = mode.split("#", 1)
            filemode = filemode or "r"

            if filemode not in "rw":
                raise ValueError ("mode %s not compatible with concat "
                                  "archive; must be 'r' or 'w'" % mode)

            stream = _Stream(name, filemode, comptype, fileobj, bufsize,
                             concat=True, encryption=encryption,
                             compresslevel=compresslevel, tolerance=tolerance)
            kwargs ["concat"] = True
            try:
                t = cls(name, filemode, stream, **kwargs)
            except: # XXX except what?
                stream.close()
                raise # XXX raise what?
            t._extfileobj = False
            return t

        elif mode in "aw":
            return cls.taropen(name, mode, fileobj, **kwargs)

        raise ValueError("undiscernible mode %r" % mode)


    @classmethod
    def open_at_offset(cls, offset, *a, **kwa):
        """
        Same as ``.open()``, but start reading at the given offset. Assumes a
        seekable file object. Returns *None* if opening failed due to a read
        problem.
        """
        fileobj = kwa.get ("fileobj")
        if fileobj is not None:
            fileobj.seek (offset)

        return cls.open (*a, **kwa)


    @classmethod
    def taropen(cls, name, mode="r", fileobj=None, **kwargs):
        """Open uncompressed tar archive name for reading or writing.
        """
        if len(mode) > 1 or mode not in "raw":
            raise ValueError("mode must be 'r', 'a' or 'w'")
        return cls(name, mode, fileobj, **kwargs)

    @classmethod
    def gzopen(cls, name, mode="r", fileobj=None, compresslevel=9, **kwargs):
        """Open gzip compressed tar archive name for reading or writing.
           Appending is not allowed.
        """
        if len(mode) > 1 or mode not in "rw":
            raise ValueError("mode must be 'r' or 'w'")

        try:
            import gzip
            gzip.GzipFile
        except (ImportError, AttributeError):
            raise CompressionError("gzip module is not available")

        extfileobj = fileobj is not None
        try:
            fileobj = gzip.GzipFile(name, mode + "b", compresslevel, fileobj)
            t = cls.taropen(name, mode, fileobj, **kwargs)
        except OSError:
            if not extfileobj and fileobj is not None:
                fileobj.close()
            if fileobj is None:
                raise
            raise ReadError("not a gzip file")
        except:
            if not extfileobj and fileobj is not None:
                fileobj.close()
            raise
        t._extfileobj = extfileobj
        return t

    @classmethod
    def bz2open(cls, name, mode="r", fileobj=None, compresslevel=9, **kwargs):
        """Open bzip2 compressed tar archive name for reading or writing.
           Appending is not allowed.
        """
        if len(mode) > 1 or mode not in "rw":
            raise ValueError("mode must be 'r' or 'w'.")

        try:
            import bz2
        except ImportError:
            raise CompressionError("bz2 module is not available")

        fileobj = bz2.BZ2File(fileobj or name, mode,
                              compresslevel=compresslevel)

        try:
            t = cls.taropen(name, mode, fileobj, **kwargs)
        except (OSError, EOFError):
            fileobj.close()
            raise ReadError("not a bzip2 file")
        t._extfileobj = False
        return t

    @classmethod
    def xzopen(cls, name, mode="r", fileobj=None, preset=None, **kwargs):
        """Open lzma compressed tar archive name for reading or writing.
           Appending is not allowed.
        """
        if mode not in ("r", "w"):
            raise ValueError("mode must be 'r' or 'w'")

        try:
            import lzma
        except ImportError:
            raise CompressionError("lzma module is not available")

        fileobj = lzma.LZMAFile(fileobj or name, mode, preset=preset)

        try:
            t = cls.taropen(name, mode, fileobj, **kwargs)
        except (lzma.LZMAError, EOFError):
            fileobj.close()
            raise ReadError("not an lzma file")
        t._extfileobj = False
        return t

    # All *open() methods are registered here.
    OPEN_METH = {
        "tar": "taropen",   # uncompressed tar
        "gz":  "gzopen",    # gzip compressed tar
        "bz2": "bz2open",   # bzip2 compressed tar
        "xz":  "xzopen"     # lzma compressed tar
    }

    #--------------------------------------------------------------------------
    # The public methods which TarFile provides:

    def close(self):
        """Close the TarFile. In write-mode, two finishing zero blocks are
           appended to the archive. A special case are empty archives which are
           initialized accordingly so the two mandatory blocks of zeros are
           written abiding by the requested encryption and compression settings.
        """
        if self.closed:
            return

        if self.mode in "aw":
            if self.arcmode & ARCMODE_CONCAT and self.fileobj.tell () == 0:
                self.fileobj.next ("")
            self.fileobj.write(NUL * (BLOCKSIZE * 2))
            self.offset += (BLOCKSIZE * 2)
            # fill up the end with zero-blocks
            # (like option -b20 for tar does)
            blocks, remainder = divmod(self.offset, RECORDSIZE)
            if remainder > 0:
                self.fileobj.write(NUL * (RECORDSIZE - remainder))
        if not self._extfileobj:
            self.fileobj.close()
        self.closed = True

    def getmember(self, name):
        """Return a TarInfo object for member `name'. If `name' can not be
           found in the archive, KeyError is raised. If a member occurs more
           than once in the archive, its last occurrence is assumed to be the
           most up-to-date version.
        """
        tarinfo = self._getmember(name)
        if tarinfo is None:
            raise KeyError("filename %r not found" % name)
        return tarinfo

    def getmembers(self):
        """Return the members of the archive as a list of TarInfo objects. The
           list has the same order as the members in the archive.
        """
        self._check()
        if not self._loaded:    # if we want to obtain a list of
            self._load()        # all members, we first have to
                                # scan the whole archive.
        return self.members

    def get_last_member_offset(self):
        """Return the last member offset. Usually this is self.fileobj.tell(),
           but when there's encryption or concat compression going on it's more
           complicated than that.
        """
        return self.last_block_offset

    def getnames(self):
        """Return the members of the archive as a list of their names. It has
           the same order as the list returned by getmembers().
        """
        return [tarinfo.name for tarinfo in self.getmembers()]

    def gettarinfo(self, name=None, arcname=None, fileobj=None):
        """Create a TarInfo object for either the file `name' or the file
           object `fileobj' (using os.fstat on its file descriptor). You can
           modify some of the TarInfo's attributes before you add it using
           addfile(). If given, `arcname' specifies an alternative name for the
           file in the archive.
        """
        self._check("aw")

        # When fileobj is given, replace name by
        # fileobj's real name.
        if fileobj is not None:
            name = fileobj.name

        # Building the name of the member in the archive.
        # Backward slashes are converted to forward slashes,
        # Absolute paths are turned to relative paths.
        if arcname is None:
            arcname = name
        drv, arcname = os.path.splitdrive(arcname)
        arcname = arcname.replace(os.sep, "/")
        arcname = arcname.lstrip("/")

        # Now, fill the TarInfo object with
        # information specific for the file.
        tarinfo = self.tarinfo()
        tarinfo.tarfile = self

        # Use os.stat or os.lstat, depending on platform
        # and if symlinks shall be resolved.
        if fileobj is None:
            if hasattr(os, "lstat") and not self.dereference:
                statres = os.lstat(name)
            else:
                statres = os.stat(name)
        else:
            statres = os.fstat(fileobj.fileno())
        linkname = ""

        stmd = statres.st_mode
        if stat.S_ISREG(stmd):
            inode = (statres.st_ino, statres.st_dev)
            if not self.dereference and statres.st_nlink > 1 and \
                    inode in self.inodes and arcname != self.inodes[inode]:
                # Is it a hardlink to an already
                # archived file?
                type = LNKTYPE
                linkname = self.inodes[inode]
            else:
                # The inode is added only if its valid.
                # For win32 it is always 0.
                type = REGTYPE
                if inode[0] and self.save_to_members:
                    self.inodes[inode] = arcname
        elif stat.S_ISDIR(stmd):
            type = DIRTYPE
        elif stat.S_ISFIFO(stmd):
            type = FIFOTYPE
        elif stat.S_ISLNK(stmd):
            type = SYMTYPE
            linkname = os.readlink(name)
        elif stat.S_ISCHR(stmd):
            type = CHRTYPE
        elif stat.S_ISBLK(stmd):
            type = BLKTYPE
        else:
            return None

        # Fill the TarInfo object with all
        # information we can get.
        tarinfo.name = arcname
        tarinfo.mode = stmd
        tarinfo.uid = statres.st_uid
        tarinfo.gid = statres.st_gid
        if type == REGTYPE:
            tarinfo.size = statres.st_size
        else:
            tarinfo.size = 0
        tarinfo.mtime = statres.st_mtime
        tarinfo.type = type
        tarinfo.linkname = linkname
        if pwd:
            if tarinfo.uid in self.cache_uid2user:
                tarinfo.uname = self.cache_uid2user[tarinfo.uid]
            else:
                try:
                    tarinfo.uname = pwd.getpwuid(tarinfo.uid)[0]
                    self.cache_uid2user[tarinfo.uid] = tarinfo.uname
                except KeyError:
                    # remember user does not exist:
                    # same default value as in tarinfo class
                    self.cache_uid2user[tarinfo.uid] = ""
        if grp:
            if tarinfo.gid in self.cache_gid2group:
                tarinfo.gname = self.cache_gid2group[tarinfo.gid]
            else:
                try:
                    tarinfo.gname = grp.getgrgid(tarinfo.gid)[0]
                    self.cache_gid2group[tarinfo.gid] = tarinfo.gname
                except KeyError:
                    # remember group does not exist:
                    # same default value as in tarinfo class
                    self.cache_gid2group[tarinfo.gid] = ""

        if type in (CHRTYPE, BLKTYPE):
            if hasattr(os, "major") and hasattr(os, "minor"):
                tarinfo.devmajor = os.major(statres.st_rdev)
                tarinfo.devminor = os.minor(statres.st_rdev)
        return tarinfo

    def list(self, verbose=True):
        """Print a table of contents to sys.stdout. If `verbose' is False, only
           the names of the members are printed. If it is True, an `ls -l'-like
           output is produced.
        """
        self._check()

        for tarinfo in self:
            if verbose:
                print(stat.filemode(tarinfo.mode), end=' ')
                print("%s/%s" % (tarinfo.uname or tarinfo.uid,
                                 tarinfo.gname or tarinfo.gid), end=' ')
                if tarinfo.ischr() or tarinfo.isblk():
                    print("%10s" % ("%d,%d" \
                                    % (tarinfo.devmajor, tarinfo.devminor)), end=' ')
                else:
                    print("%10d" % tarinfo.size, end=' ')
                print("%d-%02d-%02d %02d:%02d:%02d" \
                      % time.localtime(tarinfo.mtime)[:6], end=' ')

            print(tarinfo.name + ("/" if tarinfo.isdir() else ""), end=' ')

            if verbose:
                if tarinfo.issym():
                    print("->", tarinfo.linkname, end=' ')
                if tarinfo.islnk():
                    print("link to", tarinfo.linkname, end=' ')
            print()

    def add(self, name, arcname=None, recursive=True, exclude=None, *, filter=None):
        """Add the file `name' to the archive. `name' may be any type of file
           (directory, fifo, symbolic link, etc.). If given, `arcname'
           specifies an alternative name for the file in the archive.
           Directories are added recursively by default. This can be avoided by
           setting `recursive' to False. `exclude' is a function that should
           return True for each filename to be excluded. `filter' is a function
           that expects a TarInfo object argument and returns the changed
           TarInfo object, if it returns None the TarInfo object will be
           excluded from the archive.
        """
        self._check("aw")

        if arcname is None:
            arcname = name

        # Exclude pathnames.
        if exclude is not None:
            import warnings
            warnings.warn("use the filter argument instead",
                    DeprecationWarning, 2)
            if exclude(name):
                self._dbg(2, "tarfile: Excluded %r" % name)
                return

        # Skip if somebody tries to archive the archive...
        if self.name is not None and os.path.abspath(name) == self.name:
            self._dbg(2, "tarfile: Skipped %r" % name)
            return

        self._dbg(1, name)

        # Create a TarInfo object from the file.
        tarinfo = self.gettarinfo(name, arcname)

        if tarinfo is None:
            self._dbg(1, "tarfile: Unsupported type %r" % name)
            return

        # Change or exclude the TarInfo object.
        if filter is not None:
            tarinfo = filter(tarinfo)
            if tarinfo is None:
                self._dbg(2, "tarfile: Excluded %r" % name)
                return

        # Append the tar header and data to the archive.
        if tarinfo.isreg():
            with bltn_open(name, "rb") as f:
                self.addfile(tarinfo, f)

        elif tarinfo.isdir():
            self.addfile(tarinfo)
            if recursive:
                for f in os.listdir(name):
                    self.add(os.path.join(name, f), os.path.join(arcname, f),
                            recursive, exclude, filter=filter)

        else:
            self.addfile(tarinfo)

    def _size_left_file(self):
        """Calculates size left in a volume with a maximum volume size.

        Assumes self.max_volume_size is set.
        If using compression through a _Stream, use _size_left_stream instead
        """
        # left-over size = max_size - offset - 2 zero-blocks written in close
        size_left = self.max_volume_size - 2*BLOCKSIZE - self.offset
        # limit size left to a discrete number of blocks, because we won't
        # write only half a block when writting the end of a volume
        # and filling with zeros
        return BLOCKSIZE * (size_left // BLOCKSIZE)

    def _size_left_stream(self):
        """ Calculates size left in a volume if using comression/encryption

        Assumes self.max_volume_size is set and self.fileobj is a _Stream
        (otherwise use _size_left_file)
        """
        # left-over size = max_size - bytes written - 2 zero-blocks (close)
        size_left = self.max_volume_size - self.fileobj.estim_file_size() \
            - 2*BLOCKSIZE
        return BLOCKSIZE * (size_left // BLOCKSIZE)

    def addfile(self, tarinfo, fileobj=None):
        """Add the TarInfo object `tarinfo' to the archive. If `fileobj' is
           given, tarinfo.size bytes are read from it and added to the archive.
           You can create TarInfo objects using gettarinfo().
           On Windows platforms, `fileobj' should always be opened with mode
           'rb' to avoid irritation about the file size.
        """
        self._check("aw")

        tarinfo = copy.copy(tarinfo)

        if self.arcmode & ARCMODE_CONCAT:
            self.last_block_offset = self.fileobj.next (tarinfo.name)
        else:
            self.last_block_offset = self.fileobj.tell()

        buf = tarinfo.tobuf(self.format, self.encoding, self.errors)
        self.fileobj.write(buf)
        self.offset += len(buf)

        if self.max_volume_size:
            if isinstance(self.fileobj, _Stream):
                _size_left = self._size_left_stream
            else:
                _size_left = self._size_left_file
        else:
            _size_left = lambda: tarinfo.size

        # If there's no data to follow, finish
        if not fileobj:
            if self.save_to_members:
                self.members.append(tarinfo)
            return

        target_size_left = _size_left()
        source_size_left = tarinfo.size
        assert tarinfo.volume_offset == 0

        # we only split volumes in the middle of a file, that means we have
        # to write at least one block
        if target_size_left < BLOCKSIZE:
            target_size_left = BLOCKSIZE

        # loop over multiple volumes
        while source_size_left > 0:

            # Write as much data as possble from source into target.
            # When compressing data, we cannot easily predict how much data we
            # can write until target_size_left == 0 --> need to iterate
            size_can_write = min(target_size_left, source_size_left)

            while size_can_write > 0:
                copyfileobj(fileobj, self.fileobj, size_can_write)
                self.offset += size_can_write
                source_size_left -= size_can_write
                target_size_left = _size_left()
                size_can_write = min(target_size_left, source_size_left)

            # now target_size_left == 0 or source_size_left == 0

            # if there is data left to write, we need to create a new volume
            if source_size_left > 0:
                # Only finalize the crypto entry here if we’re continuing with
                # another one; otherwise, the encryption must include the block
                # padding below.
                tarinfo.type = GNUTYPE_MULTIVOL

                if not self.new_volume_handler or\
                    not callable(self.new_volume_handler):
                    raise Exception("We need to create a new volume and you "
                                    "didn't supply a new_volume_handler")


                # the new volume handler should do everything needed to
                # start working in a new volume. usually, the handler calls
                # to self.open_volume
                self.volume_number += 1

                # set to be used by open_volume, because in the case of a PAX
                # tar it needs to write information about the volume and offset
                # in the global header
                tarinfo.volume_offset = tarinfo.size - source_size_left
                self.volume_tarinfo = tarinfo

                # the “new_volume_handler” is supposed to call .close() on the
                # “fileobj” _Stream
                self.new_volume_handler(self, self.base_name, self.volume_number)

                self.volume_tarinfo = None

                if self.arcmode & ARCMODE_CONCAT:
                    self.fileobj.next_volume (tarinfo.name)

                # write new volume header
                buf = tarinfo.tobuf(self.format, self.encoding, self.errors)
                self.fileobj.write(buf)
                self.offset += len(buf)

                # adjust variables; open_volume should have reset self.offset
                # --> _size_left should be big again
                target_size_left = _size_left()
                size_can_write = min(target_size_left, source_size_left)
                self._dbg(3, 'new volume')

        # now, all data has been written. We may have to fill up the rest of
        # the block in target with 0s
        remainder = (tarinfo.size - tarinfo.volume_offset) % BLOCKSIZE
        if remainder > 0:
            self.fileobj.write(NUL * (BLOCKSIZE - remainder))
            self.offset += BLOCKSIZE - remainder

        if self.save_to_members:
            self.members.append(tarinfo)

    def open_volume(self, name="", fileobj=None, encryption=None):
        '''
        Called by the user to change this tar file to point to a new volume.
        '''

        # open the file using either fileobj or name
        if not fileobj:
            if self.mode == "a" and not os.path.exists(name):
                # Create nonexistent files in append mode.
                self.mode = "w"
                self._mode = "wb"
            self._extfileobj = False

            if isinstance(self.fileobj, _Stream):
                self._dbg(3, 'open_volume: create a _Stream')
                fileobj = _Stream(name=name,
                            mode=self.fileobj.mode,
                            comptype=self.fileobj.comptype,
                            fileobj=None,
                            bufsize=self.fileobj.bufsize,
                            encryption=encryption or self.fileobj.encryption,
                            concat=self.fileobj.arcmode & ARCMODE_CONCAT,
                            tolerance=self.fileobj.tolerance)
            else:
                # here, we lose information about compression/encryption!
                self._dbg(3, 'open_volume: builtin open')
                fileobj = bltn_open(name, self._mode)
        else:
            if name is None and hasattr(fileobj, "name"):
                name = fileobj.name
            if hasattr(fileobj, "mode"):
                self._mode = fileobj.mode
            self._extfileobj = True
            self._dbg(3, 'open_volume: using external fileobj {}', fileobj)
        self.name = os.path.abspath(name) if name else None
        self.fileobj.close()
        self.fileobj = fileobj

        # init data structures
        self.closed = False
        self.members = []       # list of members as TarInfo objects
        self._loaded = False    # flag if all members have been read
        self.offset = self.fileobj.tell()
                                # current position in the archive file
        self.inodes = {}        # dictionary caching the inodes of
                                # archive members already added

        try:
            if self.mode == "r":
                self.firstmember = None
                self.firstmember = self.next()

            if self.mode == "a":
                # Move to the end of the archive,
                # before the first empty block.
                while True:
                    self.fileobj.seek(self.offset)
                    try:
                        tarinfo = self.tarinfo.fromtarfile(self)
                        self.members.append(tarinfo)
                    except EOFHeaderError:
                        self.fileobj.seek(self.offset)
                        break
                    except HeaderError as e:
                        raise ReadError(str(e))

            if self.mode in "aw":
                self._loaded = True

                if  self.format == PAX_FORMAT:
                    volume_info = {
                        "GNU.volume.filename": str(self.volume_tarinfo.name),
                        "GNU.volume.size": str(self.volume_tarinfo.size - self.volume_tarinfo.volume_offset),
                        "GNU.volume.offset": str(self.volume_tarinfo.volume_offset),
                    }

                    self.pax_headers.update(volume_info)

                    if isinstance(self.fileobj, _Stream):
                        self.fileobj._init_write_gz ()
                    buf = self.tarinfo.create_pax_global_header(volume_info.copy())
                    self.fileobj.write(buf)
                    self.offset += len(buf)
        except Exception as exn:
            if not self._extfileobj:
                self.fileobj.close()
            self.closed = True
            raise

    def extractall(self, path=".", members=None, filter=None, unlink=False):
        """Extract all members from the archive to the current working
           directory and set owner, modification time and permissions on
           directories afterwards. `path' specifies a different directory
           to extract to. `members' is optional and must be a subset of the
           list returned by getmembers().
        """
        directories = []

        if members is None:
            members = self

        for tarinfo in members:
            if self.volume_number > 0 and tarinfo.ismultivol():
                continue

            if filter and not filter(tarinfo):
                continue

            if tarinfo.isdir():
                # Extract directories with a safe mode.
                directories.append(tarinfo)
                tarinfo = copy.copy(tarinfo)
                tarinfo.mode = 0o0700
            # Do not set_attrs directories, as we will do that further down
            self.extract(tarinfo, path, set_attrs=not tarinfo.isdir(), unlink=unlink)

        # Reverse sort directories.
        directories.sort(key=lambda a: a.name)
        directories.reverse()

        # Set correct owner, mtime and filemode on directories.
        for tarinfo in directories:
            dirpath = os.path.join(path, tarinfo.name)
            try:
                self.chown(tarinfo, dirpath)
                self.utime(tarinfo, dirpath)
                self.chmod(tarinfo, dirpath)
            except ExtractError as e:
                if self.errorlevel > 1:
                    raise
                else:
                    self._dbg(1, "tarfile: %s" % e)

    def extract(self, member, path="", set_attrs=True, symlink_cb=None,
                unlink=False):
        """Extract a member from the archive to the current working directory,
           using its full name. Its file information is extracted as accurately
           as possible. `member' may be a filename or a TarInfo object. You can
           specify a different directory using `path'. File attributes (owner,
           mtime, mode) are set unless `set_attrs' is False.
           ``symlink_cb`` is a hook accepting a function that is passed the
           ``member``, ``path``, and ``set_attrs`` arguments if the tarinfo for
           ``member`` indicates a symlink in which case only the callback
           passed will be applied, skipping the actual extraction. In case the
           callback is invoked, its return value is passed on to the caller.
        """
        self._check("r")

        if isinstance(member, str):
            tarinfo = self.getmember(member)
        else:
            tarinfo = member

        # Prepare the link target for makelink().
        if tarinfo.islnk():
            tarinfo._link_target = os.path.join(path, tarinfo.linkname)

        if symlink_cb is not None and tarinfo.issym():
            return symlink_cb(member, path, set_attrs)

        try:
            self._extract_member(tarinfo, os.path.join(path, tarinfo.name),
                                 set_attrs=set_attrs, unlink=unlink)
        except EnvironmentError as e:
            if self.errorlevel > 0:
                raise
            else:
                if e.filename is None:
                    self._dbg(1, "tarfile: %s" % e.strerror)
                else:
                    self._dbg(1, "tarfile: %s %r" % (e.strerror, e.filename))
        except ExtractError as e:
            if self.errorlevel > 1:
                raise
            else:
                self._dbg(1, "tarfile: %s" % e)

    def extractfile(self, member):
        """Extract a member from the archive as a file object. `member' may be
           a filename or a TarInfo object. If `member' is a regular file or a
           link, an io.BufferedReader object is returned. Otherwise, None is
           returned.
        """
        self._check("r")

        if isinstance(member, str):
            tarinfo = self.getmember(member)
        else:
            tarinfo = member

        if tarinfo.isreg() or tarinfo.ismultivol() or\
            tarinfo.type not in SUPPORTED_TYPES:
            # If a member's type is unknown, it is treated as a
            # regular file.
            return self.fileobject(self, tarinfo)

        elif tarinfo.islnk() or tarinfo.issym():
            if isinstance(self.fileobj, _Stream):
                # A small but ugly workaround for the case that someone tries
                # to extract a (sym)link as a file-object from a non-seekable
                # stream of tar blocks.
                raise StreamError("cannot extract (sym)link as file object")
            else:
                # A (sym)link's file object is its target's file object.
                return self.extractfile(self._find_link_target(tarinfo))
        else:
            # If there's no data associated with the member (directory, chrdev,
            # blkdev, etc.), return None instead of a file object.
            return None

    def _extract_member(self, tarinfo, targetpath, set_attrs=True, unlink=False):
        """Extract the TarInfo object tarinfo to a physical
           file called targetpath.
        """
        # Fetch the TarInfo object for the given name
        # and build the destination pathname, replacing
        # forward slashes to platform specific separators.
        targetpath = targetpath.rstrip("/")
        targetpath = targetpath.replace("/", os.sep)

        # Create all upper directories.
        upperdirs = os.path.dirname(targetpath)
        if upperdirs and not os.path.exists(upperdirs):
            # Create directories that are not part of the archive with
            # default permissions.
            os.makedirs(upperdirs)

        if tarinfo.islnk() or tarinfo.issym():
            self._dbg(1, "%s -> %s" % (tarinfo.name, tarinfo.linkname))
        else:
            self._dbg(1, tarinfo.name)

        if unlink is True:
            _unlinkfirst(targetpath)

        if tarinfo.isreg():
            self.makefile(tarinfo, targetpath)
        elif tarinfo.isdir():
            self.makedir(tarinfo, targetpath)
        elif tarinfo.isfifo():
            self.makefifo(tarinfo, targetpath)
        elif tarinfo.ischr() or tarinfo.isblk():
            self.makedev(tarinfo, targetpath)
        elif tarinfo.islnk() or tarinfo.issym():
            self.makelink(tarinfo, targetpath)
        elif tarinfo.type not in SUPPORTED_TYPES:
            self.makeunknown(tarinfo, targetpath)
        else:
            self.makefile(tarinfo, targetpath)

        if set_attrs:
            self.chown(tarinfo, targetpath)
            if not tarinfo.issym():
                self.chmod(tarinfo, targetpath)
                self.utime(tarinfo, targetpath)

    #--------------------------------------------------------------------------
    # Below are the different file methods. They are called via
    # _extract_member() when extract() is called. They can be replaced in a
    # subclass to implement other functionality.

    def makedir(self, tarinfo, targetpath):
        """Make a directory called targetpath.
        """
        try:
            # Use a safe mode for the directory, the real mode is set
            # later in _extract_member().
            os.mkdir(targetpath, 0o0700)
        except FileExistsError:
            pass

    def makefile(self, tarinfo, targetpath):
        """Make a file called targetpath.
        """
        source = self.fileobj
        source.seek(tarinfo.offset_data)
        decrypt = False
        iterate = True
        target = bltn_open(targetpath, "wb")

        if tarinfo.sparse is not None:
            try:
                for offset, size in tarinfo.sparse:
                    target.seek(offset)
                    copyfileobj(source, target, size)
                target.seek(tarinfo.size)
                target.truncate()
            finally:
                target.close()
                return

        while iterate:
            iterate = False
            try:
                copyfileobj(source, target, tarinfo.size)
            except OSError:
                source.close()
                # only if we are extracting a multivolume this can be treated
                if not self.new_volume_handler:
                    raise Exception("We need to read a new volume and you"
                        " didn't supply a new_volume_handler")

                # the new volume handler should do everything needed to
                # start working in a new volume. usually, the handler calls
                # to self.open_volume
                self.volume_number += 1
                self.new_volume_handler(self, self.base_name, self.volume_number)
                tarinfo = self.firstmember
                source = self.fileobj
                iterate = True
            finally:
                if iterate is False: target.close()


    def makeunknown(self, tarinfo, targetpath):
        """Make a file from a TarInfo object with an unknown type
           at targetpath.
        """
        self.makefile(tarinfo, targetpath)
        self._dbg(1, "tarfile: Unknown file type %r, " \
                     "extracted as regular file." % tarinfo.type)

    def makefifo(self, tarinfo, targetpath):
        """Make a fifo called targetpath.
        """
        if hasattr(os, "mkfifo"):
            os.mkfifo(targetpath)
        else:
            raise ExtractError("fifo not supported by system")

    def makedev(self, tarinfo, targetpath):
        """Make a character or block device called targetpath.
        """
        if not hasattr(os, "mknod") or not hasattr(os, "makedev"):
            raise ExtractError("special devices not supported by system")

        mode = tarinfo.mode
        if tarinfo.isblk():
            mode |= stat.S_IFBLK
        else:
            mode |= stat.S_IFCHR

        os.mknod(targetpath, mode,
                 os.makedev(tarinfo.devmajor, tarinfo.devminor))

    def makelink(self, tarinfo, targetpath):
        """Make a (symbolic) link called targetpath. If it cannot be created
          (platform limitation), we try to make a copy of the referenced file
          instead of a link.
        """
        try:
            # For systems that support symbolic and hard links.
            if tarinfo.issym():
                os.symlink(tarinfo.linkname, targetpath)
            else:
                # See extract().
                if os.path.exists(tarinfo._link_target):
                    os.link(tarinfo._link_target, targetpath)
                else:
                    self._extract_member(self._find_link_target(tarinfo),
                                         targetpath)
        except symlink_exception:
            try:
                self._extract_member(self._find_link_target(tarinfo),
                                     targetpath)
            except KeyError:
                raise ExtractError("unable to resolve link inside archive")

    def chown(self, tarinfo, targetpath):
        """Set owner of targetpath according to tarinfo.
        """
        if pwd and hasattr(os, "geteuid") and os.geteuid() == 0:
            # We have to be root to do so.
            try:
                g = grp.getgrnam(tarinfo.gname)[2]
            except KeyError:
                g = tarinfo.gid
            try:
                u = pwd.getpwnam(tarinfo.uname)[2]
            except KeyError:
                u = tarinfo.uid
            try:
                if tarinfo.issym() and hasattr(os, "lchown"):
                    os.lchown(targetpath, u, g)
                else:
                    os.chown(targetpath, u, g)
            except OSError as e:
                raise ExtractError("could not change owner")

    def chmod(self, tarinfo, targetpath):
        """Set file permissions of targetpath according to tarinfo.
        """
        if hasattr(os, 'chmod'):
            try:
                os.chmod(targetpath, tarinfo.mode)
            except OSError as e:
                raise ExtractError("could not change mode")

    def utime(self, tarinfo, targetpath):
        """Set modification time of targetpath according to tarinfo.
        """
        if not hasattr(os, 'utime'):
            return
        try:
            os.utime(targetpath, (tarinfo.mtime, tarinfo.mtime))
        except OSError as e:
            raise ExtractError("could not change modification time")

    #--------------------------------------------------------------------------
    def next(self):
        """Return the next member of the archive as a TarInfo object, when
           TarFile is opened for reading. Return None if there is no more
           available.
        """
        self._check("ra")
        if self.firstmember is not None:
            m = self.firstmember
            self.firstmember = None
            return m

        # Read the next block.
        self.fileobj.seek(self.offset)
        tarinfo = None
        while True:
            try:
                tarinfo = self.tarinfo.fromtarfile(self)
            except EOFHeaderError as e:
                if self.ignore_zeros:
                    self._dbg(2, "0x%X: %s" % (self.offset, e))
                    self.offset += BLOCKSIZE
                    continue
            except InvalidHeaderError as e:
                if self.ignore_zeros:
                    self._dbg(2, "0x%X: %s" % (self.offset, e))
                    self.offset += BLOCKSIZE
                    continue
                elif self.offset == 0:
                    raise ReadError(str(e))
            except EmptyHeaderError:
                if self.offset == 0:
                    raise ReadError("empty file")
            except TruncatedHeaderError as e:
                if self.offset == 0:
                    raise ReadError(str(e))
            except SubsequentHeaderError as e:
                raise ReadError(str(e))
            break

        if tarinfo is not None:
            if self.save_to_members:
                self.members.append(tarinfo)
        else:
            self._loaded = True

        return tarinfo

    #--------------------------------------------------------------------------
    # Little helper methods:

    def _getmember(self, name, tarinfo=None, normalize=False):
        """Find an archive member by name from bottom to top.
           If tarinfo is given, it is used as the starting point.
        """
        # Ensure that all members have been loaded.
        members = self.getmembers()

        # Limit the member search list up to tarinfo.
        if tarinfo is not None:
            members = members[:members.index(tarinfo)]

        if normalize:
            name = os.path.normpath(name)

        for member in reversed(members):
            if normalize:
                member_name = os.path.normpath(member.name)
            else:
                member_name = member.name

            if name == member_name:
                return member

    def _load(self):
        """Read through the entire archive file and look for readable
           members.
        """
        while True:
            tarinfo = self.next()
            if tarinfo is None:
                break
        self._loaded = True

    def _check(self, mode=None):
        """Check if TarFile is still open, and if the operation's mode
           corresponds to TarFile's mode.
        """
        if self.closed:
            raise OSError("%s is closed" % self.__class__.__name__)
        if mode is not None and self.mode not in mode:
            raise OSError("bad operation for mode %r" % self.mode)

    def _find_link_target(self, tarinfo):
        """Find the target member of a symlink or hardlink member in the
           archive.
        """
        if tarinfo.issym():
            # Always search the entire archive.
            linkname = "/".join(filter(None, (os.path.dirname(tarinfo.name), tarinfo.linkname)))
            limit = None
        else:
            # Search the archive before the link, because a hard link is
            # just a reference to an already archived file.
            linkname = tarinfo.linkname
            limit = tarinfo

        member = self._getmember(linkname, tarinfo=limit, normalize=True)
        if member is None:
            raise KeyError("linkname %r not found" % linkname)
        return member

    def __iter__(self):
        """Provide an iterator object.
        """
        if self._loaded:
            return iter(self.members)
        else:
            return TarIter(self)

    def _dbg(self, level, msg, *args):
        """Write debugging output to sys.stderr.
        """
        if level <= self.debug:
            print(msg.format(*args), file=sys.stderr)

    def __enter__(self):
        self._check()
        return self

    def __exit__(self, type, value, traceback):
        if type is None:
            self.close()
        else:
            # An exception occurred. We must not call close() because
            # it would try to write end-of-archive blocks and padding.
            if not self._extfileobj:
                self.fileobj.close()
            self.closed = True

def _unlinkfirst(targetpath):
    try:
        os.unlink(targetpath)
    except OSError as e:
        if e.errno == errno.ENOENT or e.errno == errno.EISDIR:
            pass


# class TarFile

class TarIter:
    """Iterator Class.

       for tarinfo in TarFile(...):
           suite...
    """

    def __init__(self, tarfile):
        """Construct a TarIter object.
        """
        self.tarfile = tarfile
        self.index = 0
    def __iter__(self):
        """Return iterator object.
        """
        return self
    def __next__(self):
        """Return the next item using TarFile's next() method.
           When all members have been read, set TarFile as _loaded.
        """
        # Fix for SF #1100429: Under rare circumstances it can
        # happen that getmembers() is called during iteration,
        # which will cause TarIter to stop prematurely.

        if self.index == 0 and self.tarfile.firstmember is not None:
            tarinfo = self.tarfile.next()
        elif self.index < len(self.tarfile.members):
            tarinfo = self.tarfile.members[self.index]
        elif not self.tarfile._loaded:
            tarinfo = self.tarfile.next()
            if not tarinfo:
                self.tarfile._loaded = True
                raise StopIteration
        else:
            raise StopIteration
        self.index += 1

        return tarinfo

#---------------------------------------------------------
# support functionality for rescue mode
#---------------------------------------------------------

TAR_FMT_HDR = (# See tar(5):
    "<"
    "100s" # ← char name[100];          /* 100 */
      "8s" # ← char mode[8];            /* 108 */
      "8s" # ← char uid[8];             /* 116 */
      "8s" # ← char gid[8];             /* 124 */
     "12s" # ← char size[12];           /* 136 */
     "12s" # ← char mtime[12];          /* 148 */
      "8s" # ← char checksum[8];        /* 156 */
       "B" # ← char typeflag[1];        /* 157 */
    "100s" # ← char linkname[100];      /* 257 */
      "6s" # ← char magic[6];           /* 263 */
      "2s" # ← char version[2];         /* 265 */
     "32s" # ← char uname[32];          /* 297 */
     "32s" # ← char gname[32];          /* 329 */
      "8s" # ← char devmajor[8];        /* 337 */
      "8s" # ← char devminor[8];        /* 345 */
     "12s" # ← char atime[12];          /* 357 */
     "12s" # ← char ctime[12];          /* 369 */
     "12s" # ← char offset[12];         /* 381 */
      "4s" # ← char longnames[4];       /* 385 */
       "B" # ← char unused[1];          /* 386 */
        "" #   struct {
     "12s" # ←       char offset[12];
     "12s" # ←       char numbytes[12];
     "12s" # ←       char offset[12];
     "12s" # ←       char numbytes[12];
     "12s" # ←       char offset[12];
     "12s" # ←       char numbytes[12];
     "12s" # ←       char offset[12];
     "12s" # ←       char numbytes[12];
        "" #   } sparse[4];             /* 482 */
       "B" # ← char isextended[1];      /* 483 */
     "12s" # ← char realsize[12];       /* 495 */
     "17s" # ← char pad[17];            /* 512 */
)

# The “magic” and “version” fields are special:
#
# tar(5)
#    magic   The magic field holds the five characters “ustar” followed by a
#            space.  Note that POSIX ustar archives have a trailing null.
#
# however, “tar.h”:
#
#   /* OLDGNU_MAGIC uses both magic and version fields, which are contiguous.
#      Found in an archive, it indicates an old GNU header format, which will be
#      hopefully become obsolescent.  With OLDGNU_MAGIC, uname and gname are
#      valid, though the header is not truly POSIX conforming.  */
#
#
TAR_HDR_OFF_MAGIC    = 257
TAR_FMT_OLDGNU_MAGIC = b"ustar "

def read_gnu_tar_hdr (data):
    if len (data) != BLOCKSIZE: # header requires one complete block
        return None

    try:
        name, mode, \
            uid, gid, \
            size, mtime, \
            checksum, \
            typeflag, \
            linkname, \
            magic, \
            version, \
            uname, \
            gname, \
            devmajor, \
            devminor, \
            atime, \
            ctime, \
            offset, \
            longnames, \
            unused, \
            offset1, numbytes1, \
            offset2, numbytes2, \
            offset3, numbytes3, \
            offset4, numbytes4, \
            isextended, \
            realsize, \
            pad = struct.unpack (TAR_FMT_HDR, data)
    except struct.error:
        return None

    if magic != TAR_FMT_OLDGNU_MAGIC:
        return None

    # return all except “unused” and “pad”
    return \
        { "name"        : name,     "mode"        : mode
        , "uid"         : uid ,     "gid"         : gid
        , "size"        : size,     "mtime"       : mtime
        , "checksum"    : checksum
        , "typeflag"    : typeflag
        , "linkname"    : linkname
        , "magic"       : magic
        , "version"     : version
        , "uname"       : uname,    "gname"       : gname
        , "devmajor"    : devmajor, "devminor"    : devminor
        , "atime"       : atime,    "ctime"       : ctime
        , "offset"      : offset
        , "longnames"   : longnames
        , "offset1"     : offset1,  "numbytes1"   : numbytes1
        , "offset2"     : offset2,  "numbytes2"   : numbytes2
        , "offset3"     : offset3,  "numbytes3"   : numbytes3
        , "offset4"     : offset4,  "numbytes4"   : numbytes4
        , "isextended"  : isextended
        , "realsize"    : realsize
        }


def tar_hdr_check_chksum (data):
    hdr = read_gnu_tar_hdr (data)
    if hdr is None:
        return False
    s = calc_chksums (data)
    return nti (hdr ["checksum"]) in s


def readable_tar_objects_offsets (ifd):
    """
    Traverse blocks in file, trying to extract tar headers.
    """
    pos     = 0
    offsets = []

    mm = mmap.mmap(ifd, 0, mmap.MAP_SHARED, mmap.PROT_READ)
    pos = TAR_HDR_OFF_MAGIC

    while True:
        pos = mm.find (TAR_FMT_OLDGNU_MAGIC, pos)
        if pos == -1:
            break
        off = pos - TAR_HDR_OFF_MAGIC
        mm.seek (off)
        blk = mm.read (BLOCKSIZE)
        if tar_hdr_check_chksum (blk) is True:
            offsets.append (off)
        pos += 1

    return offsets


def locate_gz_hdr_candidates (fd):
    """
    Walk over instances of the GZ magic in the payload, collecting their
    positions. If the offset of the first found instance is not zero, the file
    begins with leading garbage.

    Note that since the GZ magic consists of only two bytes, we expect a lot of
    false positives inside binary data.

    :return:    The list of offsets in the file.
    """
    pos   = 0
    cands = []
    mm    = mmap.mmap(fd, 0, mmap.MAP_SHARED, mmap.PROT_READ)

    while True:
        pos = mm.find (GZ_MAGIC_BYTES, pos)
        if pos == -1:
            break
        cands.append (pos)
        pos += len (GZ_MAGIC_BYTES)

    return cands


HDR_CAND_GOOD       = 0 # header marks begin of valid object
HDR_CAND_FISHY      = 1 # inconclusive
HDR_CAND_JUNK       = 2 # not a header / object unreadable


def read_cstring (fd, max=-1, encoding=None):
    """
    Read one NUL-terminated string from *fd* into a Python string. If *max* is
    non-negative, reading will terminate after the specified number of bytes.

    Optionally, an *encoding* may be specified to interpret the data as.

    :returns: *None* if parsing failed or the maximum number of bytes has been
              exceeded; a Python string with the data otherwise.
    """
    buf = b""
    l = 0

    while True:
        c = os.read (fd, 1)
        if c == NUL:
            break
        if max >= 0 and l > max:
            return None
        buf += c
        l += 1
    if encoding is not None:
        buf = buf.decode (encoding)

    return buf


def inspect_gz_hdr (fd, off):
    """
    Attempt to parse a Gzip header in *fd* at position *off*. The format is
    documented as RFC1952.

    Returns a verdict about the quality of that header plus the parsed header
    when readable. Problematic sizes such as fields running past the EOF are
    treated as garbage. Properties in which the header merely doesn’t conform
    to the spec (garbage flag bits, bogus timestamp) are considered “fishy”. No
    validation is possible on embedded strings because they are single-byte
    encoded.
    """
    fname   = None
    flags   = 0x00
    dflags  = 0x00
    mtime   = 0x00000000
    oscode  = 0x00
    verdict = HDR_CAND_GOOD

    os.lseek (fd, off, os.SEEK_SET)
    if os.lseek (fd, 0, os.SEEK_CUR) != off:
        return HDR_CAND_JUNK, None

    raw = os.read (fd, GZ_HEADER_SIZE)
    if len (raw) != GZ_HEADER_SIZE:
        return HDR_CAND_JUNK, None

    flags = 0x0
    try:
        _m1, _m2, meth, flags, mtime, dflags, oscode = \
            struct.unpack (GZ_FMT_HEADER, raw)
        if meth != GZ_METHOD_DEFLATE: # only deflate is supported
            return HDR_CAND_JUNK, None
    except struct.error as exn:
        return HDR_CAND_JUNK, None

    if mtime > int (time.time ()):
        verdict = HDR_CAND_FISHY

    if dflags != GZ_DEFLATE_FLAGS:
        verdict = HDR_CAND_FISHY

    if oscode != GZ_OS_CODE:
        verdict = HDR_CAND_FISHY

    if flags & GZ_FLAG_FTEXT: # created by some contrarian
        verdict = HDR_CAND_FISHY
    if flags & GZ_FLAG_FEXTRA:
        xlen = struct.unpack ("<H", os.read (fd, 2))[0]
        xtra = os.read (fd, xlen)
        if len (xtra) != xlen: # eof inside header
            return HDR_CAND_JUNK, None
    if flags & GZ_FLAG_FNAME:
        # read up to the next NUL byte, not exceeding the maximum path length
        # allowed by tar(5)
        fname = read_cstring (fd, max=(LENGTH_NAME + LENGTH_PREFIX),
                              encoding="iso-8859-1")
        if fname is None:
            return HDR_CAND_JUNK, None
    if flags & GZ_FLAG_FCOMMENT:
        fname = read_cstring (fd, max=(LENGTH_NAME + LENGTH_PREFIX),
                              encoding="iso-8859-1")
        if fname is None:
            return HDR_CAND_JUNK, None
    if flags & GZ_FLAG_FHCRC: # half a CRC32; discarded
        crc16 = os.read (fd, 2)
        if len (crc16) != 2: # eof inside header
            return HDR_CAND_JUNK, None
    if flags & GZ_FLAG_RESERVED:
        # according to the RFC, these must not be set
        verdict = HDR_CAND_FISHY

    hlen = os.lseek (fd, 0, os.SEEK_CUR) - off

    return verdict, \
           { "fname"  : fname
           , "flags"  : flags
           , "dflags" : dflags
           , "mtime"  : mtime
           , "oscode" : oscode
           , "hlen"   : hlen
           }


def try_decompress (ifd, off, hdr):
    """
    Attempt to process the object starting at *off* with gzip.

    :returns:   A pair containing the values of the decompressed data and
                the length of the input consumed. Note that the latter value
                may exceed the length of the compressed data because the
                *zlib* module does not provide a means to query how much
                of the input it processed before the end of an object.
    """
    import zlib
    decmp = zlib.decompressobj (-zlib.MAX_WBITS)
    pos   = off
    dlen  = 0 # size of decompressed data

    os.lseek (ifd, pos, os.SEEK_SET)
    while True:
        cnk = os.read (ifd, BUFSIZE)
        pos += len (cnk)
        try:
            data = decmp.decompress (cnk)
        except zlib.error as exn: # probably CRC32 mismatch; terminate softly
            break # fishy
        dlen += len (data)
        if decmp.eof is True:
            break
        if len (cnk) != BUFSIZE: # eof, but not end of decompressed object!
            break # fishy

    return dlen, pos - off

def readable_gz_objects_offsets (ifd, cands):
    """
    Inspect header candidates for parseable *ifd* gzipped objects.
    """
    good = []
    nobj = 0

    for cand in cands:
        nobj += 1
        vdt, hdr = inspect_gz_hdr (ifd, cand)
        if vdt == HDR_CAND_JUNK:
            pass # ignore unreadable ones
        elif vdt in [HDR_CAND_GOOD, HDR_CAND_FISHY]:
            off0 = cand + hdr ["hlen"]
            dlen, clen = try_decompress (ifd, off0, hdr)
            if dlen > 0 and clen > 0:
                good.append (cand)

    return good


def reconstruct_offsets_gz (fname):
    """
    From the given file, retrieve all GZ header-like offsets (“candidates”).
    Then check each of those locations whether they can be processed as
    compressed data.
    """
    ifd = os.open (fname, os.O_RDONLY)

    try:
        cands = locate_gz_hdr_candidates (ifd)
        return readable_gz_objects_offsets (ifd, cands)
    finally:
        os.close (ifd)


def reconstruct_offsets_tar (fname):
    """
    From the given file, retrieve all tar header-like offsets (“candidates”).
    Then check each of those locations whether they can be processed as tar
    data.
    """
    ifd = os.open (fname, os.O_RDONLY)

    try:
        return readable_tar_objects_offsets (ifd)
    finally:
        os.close (ifd)


def read_tarobj_at_offset (fileobj, offset, mode, secret=None,
                           strict_validation=True):
    """
    :type  strict_validation:   bool
    :param strict_validation:   Enable strict IV checking in the crypto
                                layer. Should be disabled when dealing with
                                potentially corrupted data.
    """
    decr = None

    if secret is not None:
        ks   = secret [0]

        if ks == crypto.PDTCRYPT_SECRET_PW:
            decr = crypto.Decrypt (password=secret [1],
                                   strict_ivs=strict_validation)
        elif ks == crypto.PDTCRYPT_SECRET_KEY:
            key = binascii.unhexlify (secret [1])
            decr = crypto.Decrypt (key=key,
                                   strict_ivs=strict_validation)
        else:
            raise RuntimeError

    try:
        tarobj = \
            TarFile.open_at_offset (offset,
                                    mode=mode,
                                    fileobj=fileobj,
                                    format=GNU_FORMAT,
                                    concat='#' in mode,
                                    encryption=decr,
                                    save_to_members=False,
                                    tolerance=TOLERANCE_RESCUE)
    except (ReadError, EndOfFile):
        return None

    return tarobj.next ()


def idxent_of_tarinfo (tarinfo):
    """
    Scrape the information relevant for the index from a *TarInfo* object.
    Keys like the inode number that lack a corresponding field in a TarInfo
    will be set to some neutral value.
    Example output:

        { "inode"  : 0
        , "uid"    : 0
        , "path"   : "snapshot://annotations.db"
        , "offset" : 0
        , "volume" : 0
        , "mode"   : 33152
        , "ctime"  : 1502798115
        , "mtime"  : 1502196423
        , "size"   : 144
        , "type"   : "file"
        , "gid"    : 0
        }

    """

    return \
        { "inode"  : 0            # ignored when reading the index
        , "uid"    : tarinfo.uid
        , "gid"    : tarinfo.gid
        , "path"   : tarinfo.name # keeping URI scheme
        , "offset" : 0            # to be added by the caller
        , "volume" : tarinfo.volume_offset
        , "mode"   : tarinfo.mode
        , "ctime"  : tarinfo.mtime
        , "mtime"  : tarinfo.mtime
        , "size"   : tarinfo.size
        , "type"   : tarinfo.type
        }


def gen_rescue_index (gen_volume_name, mode, maxvol=None, password=None, key=None):
    infos   = []
    psidx   = [] # pseudo index, return value
    offsets = None
    secret  = crypto.make_secret (password=password, key=key)

    nvol = 0

    while True:
        vpath = gen_volume_name (nvol)
        try:
            if secret is not None:
                offsets = crypto.reconstruct_offsets (vpath, secret)
            elif mode == "#gz":
                offsets = reconstruct_offsets_gz (vpath)
            elif mode == "#":
                offsets = reconstruct_offsets_tar (vpath)
            else:
                raise TarError ("no rescue handling for mode “%s”" % mode)
        except FileNotFoundError as exn:
            # volume does not exist
            if maxvol is not None and nvol < maxvol:
                continue # explicit volume number specified, ignore missing ones
            else:
                break

        fileobj = bltn_open (vpath, "rb")

        def aux (acc, off):
            obj = read_tarobj_at_offset (fileobj, off, mode, secret=secret,
                                         strict_validation=False)
            if obj is not None:
                acc.append ((off, nvol, obj))
            return acc
        infos += functools.reduce (aux, offsets, [])

        fileobj.close()

        nvol += 1

    def aux (o, nvol, ti):
        ie = idxent_of_tarinfo (ti)
        ie ["offset"] = o
        ie ["volume"] = nvol
        return ie

    psidx   = [ aux (o, nvol, ti) for o, nvol, ti in infos ]

    return psidx

#--------------------
# exported functions
#--------------------
def is_tarfile(name):
    """Return True if name points to a tar archive that we
       are able to handle, else return False.
    """
    try:
        t = open(name)
        t.close()
        return True
    except TarError:
        return False

bltn_open = open
open = TarFile.open