:param cond: Function to call; code will wait for this to return something
other than `False`
:param interval: Time (in seconds) to sleep between each attempt at `cond`
- :returns: Pair of result and error message if appropriate or :py:value:`None`.
+ :returns: A Pair of result and error message if appropriate or `None`.
:rtype: (run_result, str | None)
"""
llog.debug("cmd_block_till: %r, %d s, %r", cmd, timeout, cond)