In Modules/_posixsubprocess.c, (the helper module for Lib/subprocess.py) there's a function called _sanity_check_python_fd_sequence which checks, as its comment says, if the fds in the sequence are all positive and sorted. The check to verify if it is sorted is incorrect (missing the update to the prev_fd variable), and also it is missing a test to see if fd's are repeated (which they shouldn't be, so the test should use <= rather than <). The attached patch, written against Python 3.4.3 source code, fixes it.
Haha, yes, that description and patch look correct. Thanks! Fortunately this bug is low impact as this was just a sanity check and the calling code from subprocess.py was already passing the correct data in. An ideal regression test: An explicit unittest that calls the _posixsubprocess API with some bad sequences of values in fds_to_keep and uses assertRaises to check for the appropriate ValueError("bad value(s) in fds_to_keep") coming out of it...
heh, you're right. it's a trivial obvious fix for the mistake in the existing implementation. writing a test and committing. the other option would be to get rid of the sanity check entirely or change it not to use the odd "require a sorted list" code. but i like the sanity check and refactoring it to not require a sorted list within this code path would be complicated.
