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concurrent-delete-race-problem

Debugging race problems when opening a file that is being deleted (on Mac OS)

Notes

  • There is a python version (go in python and run ./run-concurrent.sh, that on Mac almost always raises the following error:

    OK- 3 os.stat_result(st_mode=33188, st_ino=8674943678, st_dev=16777220, st_nlink=0, st_uid=501, st_gid=20, st_size=7, st_atime=1594646008, st_mtime=1594646008, st_ctime=1594646008)
ER1 3 os.stat_result(st_mode=32768, st_ino=0, st_dev=16777220, st_nlink=1, st_uid=0, st_gid=0, st_size=0, st_atime=0, st_mtime=0, st_ctime=0)
Exists: False
Traceback (most recent call last):
File "test-conc-read.py", line 17, in <module>
    assert content == b'CONTENT', 'FOUND INSTEAD: {}'.format(content)
AssertionError: FOUND INSTEAD: b''

    Example of a run that failed like this

    So it seems that when I get zero bytes, it's because st_ino=0 (pointing to inode 0).

    Actually, very often I get the error:

    Traceback (most recent call last):
  File "test-conc-read.py", line 12, in <module>
    with open(dest_fname, 'rb') as fhandle:
FileNotFoundError: [Errno 2] No such file or directory: 'DEST.txt'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "test-conc-read.py", line 31, in <module>
    fhandle.write(b'CONTENT')
OSError: [Errno 22] Invalid argument

    that is even another race condition that actually shows equally frequently. Example of a run that failed like this

  • I managed to reproduce the problem at least once (but it's hard to reproduce in C) with the C code that seems to be doing the same. Note that on C the unlinks seem to be much faster, so I need to run for much longer I think. I got the output:

    FOUND INSTEAD: ''
ER ST_INO: 0

    Example of a run that failed like this

Conclusions

In both cases, the error comes from the fact that the underlying fopen call returns an fd, but this is associated with st_ino = 0.

Other information

  • I reported this to python

  • After feedback from the python developers, I also submitted the bug directly to Apple via the Feedback Assistant app (Jul 202). I got a feedback number FB8009608.

    • In Jan 22 I received an answer from Apple: 
      After reviewing your feedback, we have some additional information for you, or some additional information, or action is necessary for this issue: 

Please verify this issue with macOS 12.2 Release and update your bug report with your results by logging into https://feedbackassistant.apple.com/ or by using the Feedback Assistant app.

macOS 12.2 Release (21D49)
https://developer.apple.com/download/
Posted Date: Jan. 26th, 2022

If the issue persists, please attach a new sysdiagnose captured in the latest build and attach it to the bug report. Thank you.
      (Note: I didn't verify yet if the problem is now fixed in that version of macOS)