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GeezerVison is showing github.com — gh-153962: Re-raise unexpected `OSError` in subprocess `Popen._internal_poll` by fedonman · Pull Request #153964 · python/cpython

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@fedonman

@fedonman fedonman commented Jul 18, 2026 •
edited by bedevere-app Bot
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On POSIX, Popen.poll() caught every OSError from os.waitpid() but only did something useful with two of them: ECHILD, and the case where the call comes from __del__. Any other error, like EINVAL, was caught and dropped, which left returncode as None and made poll() look like the child was still running.

This adds an else: raise so those unexpected errors reach the caller instead of vanishing.

The new raise only runs when the call isn't from __del__ (that case is handled first, since it passes a deadstate value), so __del__ still never raises. The ECHILD behavior is unchanged.

I added a test that patches subprocess._del_safe.waitpid to raise EINVAL and checks that poll() re-raises it. It fails without the change and passes with it, and the rest of test_subprocess still passes. The patch goes through subprocess._del_safe because that's the copy of os.waitpid this code actually calls.

Found as item 8 in devdanzin's standard library audit: https://gist.github.com/devdanzin/3198710e3c0128fda5e0a7b4e0768e5f

Fixes #153962.

  • Issue: subprocess: Popen._internal_poll silently swallows unexpected OSError from waitpid() #153962

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@fedonman
pythongh-153962: Re-raise unexpected OSError in subprocess Popen._int…
ba1f9aa 
…ernal_poll

On POSIX, Popen._internal_poll() caught every OSError from os.waitpid()
and, unless it was ECHILD or the call came from __del__ (a non-None
_deadstate), silently discarded it, leaving returncode as None and
masking a real failure.

Add an else branch that re-raises such unexpected errors. The re-raise is
only reachable when _deadstate is None; __del__ and _cleanup() always
pass a non-None _deadstate, so __del__ still never raises. ECHILD
handling is unchanged.
@fedonman
fedonman requested a review from gpshead as a code owner July 18, 2026 13:36
@bedevere-app bedevere-app Bot mentioned this pull request Jul 18, 2026
subprocess: Popen._internal_poll silently swallows unexpected OSError from waitpid() #153962
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@bedevere-app bedevere-app Bot added the awaiting review label Jul 18, 2026
@fedonman
Merge branch 'main' into fix-pythongh-153962-internal-poll-oserror
268c11a
@fedonman

fedonman commented Jul 19, 2026

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@zware part of EuroPython sprint.
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@zware zware added the sprint label Jul 19, 2026
@github-project-automation github-project-automation Bot added this to Sprint Jul 19, 2026
@github-project-automation github-project-automation Bot moved this to Todo in Sprint Jul 19, 2026
fedonman added 2 commits July 21, 2026 15:32
@fedonman
Merge branch 'main' into fix-pythongh-153962-internal-poll-oserror
b37dd6e
@fedonman
Merge branch 'main' into fix-pythongh-153962-internal-poll-oserror
9e11f26
@zware

zware commented Jul 27, 2026

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Note: Please don't do these periodic Update branch merges if they're not fixing conflicts. It's impossible to stay perfectly up to date, and it's a drain on CI resources to re-test the same code repeatedly with no other action. If necessary, a maintainer can do a branch update just before merge for a final check.
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@fedonman

fedonman commented Jul 27, 2026

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Noted!
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serhiy-storchaka
serhiy-storchaka reviewed Aug 16, 2026
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@serhiy-storchaka serhiy-storchaka left a comment

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See also @vstinner's objection on the issue.

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Comment thread Lib/subprocess.py
Comment on lines +2195 to +2200
# An unexpected error (e.g. EINVAL) must not be
# silently swallowed, leaving returncode as None:
# surface it to the caller. This branch is never
# reached from __del__, which always passes a
# non-None _deadstate.
raise

@serhiy-storchaka serhiy-storchaka Aug 16, 2026

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It harms readability if the comment is 5 times large than the actual code, especially if the code is presumably unreachable. I suggest either to leave a minimal one-line comment or to remove it.

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@vstinner

vstinner commented Aug 31, 2026

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I suggest closing the PR and its issue.

I'm not sure that it's worth it to fix a theoretical issue.

The PR issue says:

Found while going through devdanzin's audit of the standard library, item 8: https://gist.github.com/devdanzin/3198710e3c0128fda5e0a7b4e0768e5f

So only error fault injection can trigger the issue.
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@fedonman

fedonman commented Sep 2, 2026

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Closing after @vstinner suggestion.
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@fedonman fedonman closed this Sep 2, 2026
@github-project-automation github-project-automation Bot moved this from Todo to Done in Sprint Sep 2, 2026
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@serhiy-storchaka serhiy-storchaka serhiy-storchaka left review comments
@gpshead gpshead Awaiting requested review from gpshead gpshead is a code owner

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@fedonman @zware @vstinner @serhiy-storchaka
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