realpath has a race condition which might cause it to throw spurious exceptions

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#97,566 2 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
42/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
operating-systems

Research direction

Start with the realpath reproduction linked in the issue and trace the islink/readlink sequence described there. Confirm the failure when a symlink is deleted between those operations; done means realpath no longer raises a spurious exception in that race, with regression coverage for the behavior.

Written by the indexing model from the issue text.

Description

3.10 3.11 3.12 extension-modules type-bug

Bug report

realpath tests if a file is a symlink using islink, which just returns False if the file does not exist. But then uses readlink a bit later to test the source of the link, which throws an exception if the file does not exists. If the file is a symlink and is deleted between islink and readlink, we get a spurious exception.

This reproduces the error using code copy-pasted from Python 3.9: https://gist.github.com/bitonic/e0977aaa674523a9ffe0e77a96cd6cc7 .

Your environment

  • CPython 3.9
  • ArchLinux x86_64
Dominant language
Python
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Avg merge
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Merged PRs (30d)
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