[3.13] gh-143650: Fix importlib race condition on import failure (GH-143651) #144697
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…onGH-143651) Fix a race condition where a thread could receive a partially-initialized module when another thread's import fails. The race occurs when: 1. Thread 1 starts importing, adds module to sys.modules 2. Thread 2 sees the module in sys.modules via the fast path 3. Thread 1's import fails, removes module from sys.modules 4. Thread 2 returns a stale module reference not in sys.modules The fix adds verification after the "skip lock" optimization in both Python and C code paths to check if the module is still in sys.modules. If the module was removed (due to import failure), we retry the import so the caller receives the actual exception from the import failure rather than a stale module reference. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com> cherry picked from ac8b5b6
gpshead
requested review from
brettcannon,
ericsnowcurrently,
kumaraditya303,
ncoghlan and
warsaw
as code owners
This was referenced Feb 11, 2026
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Fix a race condition where a thread could receive a partially-initialized module when another thread's import fails. The race occurs when:
The fix adds verification after the "skip lock" optimization in both Python and C code paths to check if the module is still in sys.modules. If the module was removed (due to import failure), we retry the import so the caller receives the actual exception from the import failure rather than a stale module reference.
cherry picked from ac8b5b6
Co-Authored-By: Claude Opus 4.5 noreply@anthropic.com
Fixes #143650