From dbd6ff685c7503076ff76a3b6c83ed44cef66dac Mon Sep 17 00:00:00 2001
From: Philipp Gesang <philipp.gesang@intra2net.com>
Date: Thu, 31 Aug 2017 10:57:17 +0200
Subject: [PATCH] describe corruption mechanisms and their function in testing

---
 testing/test_recover.py |   66 +++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 64 insertions(+), 2 deletions(-)

diff --git a/testing/test_recover.py b/testing/test_recover.py
index a5016c0..a8ebc55 100644
--- a/testing/test_recover.py
+++ b/testing/test_recover.py
@@ -1,3 +1,65 @@
+"""
+Intra2net 2017
+
+===============================================================================
+               test_recover.py – behavior facing file corruption
+===============================================================================
+
+Corruptors have the signature ``(unittest × string × bool × bool) → void``,
+where the *string* argument is the name of the file to modify, the *booleans*
+specialize the operation for compressed and encrypted data. Issues are
+communicated upward by throwing.
+
+    - corrupt_header ():
+      Modify the first object header where it hurts. With encryption, the tag
+      is corrupted to cause authentication of the decrypted data to fail. For
+      compressed data, the two byte magic is altered, for uncompressed
+      archives, the tar header checksum field.
+
+    - corrupt_ctsize ():
+      Modify the *ctsize* field of a PDTCRYPT header. The goal is to have
+      decryption continue past the end of the object, causing data
+      authentication to fail and file reads to be at odds with the offsets in
+      the index. Only applicable to encrypted archives; will raise
+      *UndefinedTest* otherwise.
+
+    - corrupt_entire_header ():
+      Invert all bits of the first object header (PDTCRYPT, gzip, tar) without
+      affecting the payload. This renders the object unreadable; the file will
+      be resemble one with arbitrary leading data but all the remaining object
+      offsets intact, so the contents can still be extracted with index based
+      recovery.
+
+    - corrupt_payload_start ():
+      For all header variants, skip to the first byte past the header and
+      corrupt it. Encrypted objects will fail to authenticate. Compressed
+      objects will yield a bad CRC32. The Tar layer will take no notice but
+      the extracted object will fail an independent checksum comparison with
+      that of the original file.
+
+    - corrupt_leading_garbage ():
+      Prepend random data to an otherwise valid file. Creates a situation that
+      index based recovery cannot handle by shifting the offsets of all objects
+      in the file. In rescue mode, these objects must be located and extracted
+      regardless.
+
+    - corrupt_trailing_data ():
+      Append data to an otherwise valid file. Both the recovery and rescue
+      modes must be able to retrieve all objects from that file.
+
+    - corrupt_volume ():
+      Zero out an entire backup file. This is interesting for multivolume
+      tests: all files from the affected volume must be missing but objects
+      that span volume bounds will still be partially recoverable.
+
+    - corrupt_hole ():
+      Remove a region from a file. Following the damaged part, no object can be
+      recovered in index mode, but rescue mode will still find those. The
+      object containing the start of the hole will fail checksum tests because
+      of the missing part and the overlap with the subsequent object.
+
+"""
+
 import logging
 import os
 import shutil
@@ -130,7 +192,7 @@ def corrupt_entire_header (_, fname, compress, encrypt):
     """
     if encrypt is True:
         flip_bits (fname, 0, 0xff, crypto.PDTCRYPT_HDR_SIZE)
-    elif compress is True: # invalidate magic
+    elif compress is True:
         flip_bits (fname, 0, 0xff, gz_header_size (fname))
     else:
         flip_bits (fname, 0, 0xff, tarfile.BLOCKSIZE)
@@ -183,7 +245,7 @@ def corrupt_leading_garbage (_, fname, compress, encrypt):
 
 def corrupt_trailing_data (_, fname, compress, encrypt):
     """
-    Modify the byte following the object header structure of the format.
+    Append random data to file.
     """
     junk = os.urandom (42)
     fd = os.open (fname, os.O_WRONLY | os.O_APPEND)
-- 
1.7.1