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GeezerVison is showing github.com — Fix race condition in import system with failing imports · python/cpython@6393e97

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  • Lib
    • importlib
      • _bootstrap.py
    • test/test_importlib
      • test_threaded_import.py
  • Misc/NEWS.d/next/Core_and_Builtins
    • 2026-01-10-10-58-36.gh-issue-143650.k8mR4x.rst
  • Python
    • import.c
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‎Lib/importlib/_bootstrap.py‎

Copy file name to clipboardExpand all lines: Lib/importlib/_bootstrap.py
+8Lines changed: 8 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1280,6 +1280,14 @@ def _find_and_load(name, import_):
12801280
# NOTE: because of this, initializing must be set *before*
12811281
# putting the new module in sys.modules.
12821282
_lock_unlock_module(name)
1283+
else:
1284+
# Verify the module is still in sys.modules. Another thread may have
1285+
# removed it (due to import failure) between our sys.modules.get()
1286+
# above and the _initializing check. If removed, we retry the import
1287+
# to preserve normal semantics: the caller gets the exception from
1288+
# the actual import failure rather than a synthetic error.
1289+
if sys.modules.get(name) is not module:
1290+
return _find_and_load(name, import_)
12831291

12841292
if module is None:
12851293
message = f'import of {name} halted; None in sys.modules'
Collapse file

‎Lib/test/test_importlib/test_threaded_import.py‎

Copy file name to clipboardExpand all lines: Lib/test/test_importlib/test_threaded_import.py
+65Lines changed: 65 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -259,6 +259,71 @@ def test_multiprocessing_pool_circular_import(self, size):
259259
'partial', 'pool_in_threads.py')
260260
script_helper.assert_python_ok(fn)
261261

262+
def test_import_failure_race_condition(self):
263+
# Regression test for race condition where a thread could receive
264+
# a partially-initialized module when another thread's import fails.
265+
# The race occurs when:
266+
# 1. Thread 1 starts importing, adds module to sys.modules
267+
# 2. Thread 2 sees the module in sys.modules
268+
# 3. Thread 1's import fails, removes module from sys.modules
269+
# 4. Thread 2 should NOT return the stale module reference
270+
os.mkdir(TESTFN)
271+
self.addCleanup(shutil.rmtree, TESTFN)
272+
sys.path.insert(0, TESTFN)
273+
self.addCleanup(sys.path.remove, TESTFN)
274+
275+
# Create a module that partially initializes then fails
276+
modname = 'failing_import_module'
277+
with open(os.path.join(TESTFN, modname + '.py'), 'w') as f:
278+
f.write('''
279+
import time
280+
PARTIAL_ATTR = 'initialized'
281+
time.sleep(0.05) # Widen race window
282+
raise RuntimeError("Intentional import failure")
283+
''')
284+
self.addCleanup(forget, modname)
285+
importlib.invalidate_caches()
286+
287+
errors = []
288+
results = []
289+
290+
def do_import(delay=0):
291+
time.sleep(delay)
292+
try:
293+
mod = __import__(modname)
294+
# If we got a module, verify it's in sys.modules
295+
if modname not in sys.modules:
296+
errors.append(
297+
f"Got module {mod!r} but {modname!r} not in sys.modules"
298+
)
299+
elif sys.modules[modname] is not mod:
300+
errors.append(
301+
f"Got different module than sys.modules[{modname!r}]"
302+
)
303+
else:
304+
results.append(('success', mod))
305+
except RuntimeError:
306+
results.append(('RuntimeError',))
307+
except Exception as e:
308+
errors.append(f"Unexpected exception: {e}")
309+
310+
# Run multiple iterations to increase chance of hitting the race
311+
for _ in range(10):
312+
errors.clear()
313+
results.clear()
314+
if modname in sys.modules:
315+
del sys.modules[modname]
316+
317+
t1 = threading.Thread(target=do_import, args=(0,))
318+
t2 = threading.Thread(target=do_import, args=(0.01,))
319+
t1.start()
320+
t2.start()
321+
t1.join()
322+
t2.join()
323+
324+
# Neither thread should have errors about stale modules
325+
self.assertEqual(errors, [], f"Race condition detected: {errors}")
326+
262327

263328
def setUpModule():
264329
thread_info = threading_helper.threading_setup()
Collapse file

‎Misc/NEWS.d/next/Core_and_Builtins/2026-01-10-10-58-36.gh-issue-143650.k8mR4x.rst‎

Copy file name to clipboard
+2Lines changed: 2 additions & 0 deletions
  • Display the source diff
  • Display the rich diff
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,2 @@
1+
Fix race condition in :mod:`importlib` where a thread could receive a stale
2+
module reference when another thread's import fails.
Collapse file

‎Python/import.c‎

Copy file name to clipboardExpand all lines: Python/import.c
+30Lines changed: 30 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -301,6 +301,18 @@ PyImport_GetModule(PyObject *name)
301301
remove_importlib_frames(tstate);
302302
return NULL;
303303
}
304+
/* Verify the module is still in sys.modules. Another thread may have
305+
removed it (due to import failure) between our import_get_module()
306+
call and the _initializing check in import_ensure_initialized().
307+
Unlike the import path, we return NULL here since this function
308+
only retrieves existing modules and doesn't trigger new imports. */
309+
PyObject *mod_check = import_get_module(tstate, name);
310+
if (mod_check != mod) {
311+
Py_XDECREF(mod_check);
312+
Py_DECREF(mod);
313+
return NULL;
314+
}
315+
Py_DECREF(mod_check);
304316
}
305317
return mod;
306318
}
@@ -3882,6 +3894,24 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
38823894
if (import_ensure_initialized(tstate->interp, mod, abs_name) < 0) {
38833895
goto error;
38843896
}
3897+
/* Verify the module is still in sys.modules. Another thread may have
3898+
removed it (due to import failure) between our import_get_module()
3899+
call and the _initializing check in import_ensure_initialized().
3900+
If removed, we retry the import to preserve normal semantics: the
3901+
caller gets the exception from the actual import failure rather
3902+
than a synthetic error. */
3903+
PyObject *mod_check = import_get_module(tstate, abs_name);
3904+
if (mod_check != mod) {
3905+
Py_XDECREF(mod_check);
3906+
Py_DECREF(mod);
3907+
mod = import_find_and_load(tstate, abs_name);
3908+
if (mod == NULL) {
3909+
goto error;
3910+
}
3911+
}
3912+
else {
3913+
Py_DECREF(mod_check);
3914+
}
38853915
}
38863916
else {
38873917
Py_XDECREF(mod);

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