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gh-143650: Fix importlib race condition on import failure (GH-143651)

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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:

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>
This commit is contained in:
Gregory P. Smith 2026-02-10 05:08:33 -08:00 • committed by GitHub
parent b121dc4347
commit ac8b5b6890
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4 changed files with 117 additions and 1 deletions
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43
Python/import.c
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@ -297,12 +297,32 @@ PyImport_GetModule(PyObject *name)
mod = import_get_module(tstate, name);
if (mod != NULL && mod != Py_None) {
if (import_ensure_initialized(tstate->interp, mod, name) < 0) {
goto error;
}
/* Verify the module is still in sys.modules. Another thread may have
removed it (due to import failure) between our import_get_module()
call and the _initializing check in import_ensure_initialized(). */
PyObject *mod_check = import_get_module(tstate, name);
if (mod_check != mod) {
Py_XDECREF(mod_check);
if (_PyErr_Occurred(tstate)) {
goto error;
}
/* The module was removed or replaced. Return NULL to report
"not found" rather than trying to keep up with racing
modifications to sys.modules; returning the new value would
require looping to redo the ensure_initialized check. */
Py_DECREF(mod);
remove_importlib_frames(tstate);
return NULL;
}
Py_DECREF(mod_check);
}
return mod;
error:
Py_DECREF(mod);
remove_importlib_frames(tstate);
return NULL;
}
/* Get the module object corresponding to a module name.
@ -3897,6 +3917,27 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
if (import_ensure_initialized(tstate->interp, mod, abs_name) < 0) {
goto error;
}
/* Verify the module is still in sys.modules. Another thread may have
removed it (due to import failure) between our import_get_module()
call and the _initializing check in import_ensure_initialized().
If removed, we retry the import to preserve normal semantics: the
caller gets the exception from the actual import failure rather
than a synthetic error. */
PyObject *mod_check = import_get_module(tstate, abs_name);
if (mod_check != mod) {
Py_XDECREF(mod_check);
if (_PyErr_Occurred(tstate)) {
goto error;
}
Py_DECREF(mod);
mod = import_find_and_load(tstate, abs_name);
if (mod == NULL) {
goto error;
}
}
else {
Py_DECREF(mod_check);
}
}
else {
Py_XDECREF(mod);