Fix race condition in import system with failing imports

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.

Changes:
- Lib/importlib/_bootstrap.py: Add check after fast path in _find_and_load()
- Python/import.c: Add checks in PyImport_GetModule() and
  PyImport_ImportModuleLevelObject()
- Add regression test test_import_failure_race_condition()

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: gpshead

Lib/importlib/_bootstrap.py

@@ -1280,6 +1280,14 @@ def _find_and_load(name, import_):
         # NOTE: because of this, initializing must be set *before*
         # putting the new module in sys.modules.
         _lock_unlock_module(name)
+    else:
+        # Verify the module is still in sys.modules. Another thread may have
+        # removed it (due to import failure) between our sys.modules.get()
+        # above and the _initializing check. 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.
+        if sys.modules.get(name) is not module:
+            return _find_and_load(name, import_)
 
     if module is None:
         message = f'import of {name} halted; None in sys.modules'

Lib/test/test_importlib/test_threaded_import.py

@@ -259,6 +259,71 @@ def test_multiprocessing_pool_circular_import(self, size):
                           'partial', 'pool_in_threads.py')
         script_helper.assert_python_ok(fn)
 
+    def test_import_failure_race_condition(self):
+        # Regression test for 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
+        # 3. Thread 1's import fails, removes module from sys.modules
+        # 4. Thread 2 should NOT return the stale module reference
+        os.mkdir(TESTFN)
+        self.addCleanup(shutil.rmtree, TESTFN)
+        sys.path.insert(0, TESTFN)
+        self.addCleanup(sys.path.remove, TESTFN)
+
+        # Create a module that partially initializes then fails
+        modname = 'failing_import_module'
+        with open(os.path.join(TESTFN, modname + '.py'), 'w') as f:
+            f.write('''
+import time
+PARTIAL_ATTR = 'initialized'
+time.sleep(0.05)  # Widen race window
+raise RuntimeError("Intentional import failure")
+''')
+        self.addCleanup(forget, modname)
+        importlib.invalidate_caches()
+
+        errors = []
+        results = []
+
+        def do_import(delay=0):
+            time.sleep(delay)
+            try:
+                mod = __import__(modname)
+                # If we got a module, verify it's in sys.modules
+                if modname not in sys.modules:
+                    errors.append(
+                        f"Got module {mod!r} but {modname!r} not in sys.modules"
+                    )
+                elif sys.modules[modname] is not mod:
+                    errors.append(
+                        f"Got different module than sys.modules[{modname!r}]"
+                    )
+                else:
+                    results.append(('success', mod))
+            except RuntimeError:
+                results.append(('RuntimeError',))
+            except Exception as e:
+                errors.append(f"Unexpected exception: {e}")
+
+        # Run multiple iterations to increase chance of hitting the race
+        for _ in range(10):
+            errors.clear()
+            results.clear()
+            if modname in sys.modules:
+                del sys.modules[modname]
+
+            t1 = threading.Thread(target=do_import, args=(0,))
+            t2 = threading.Thread(target=do_import, args=(0.01,))
+            t1.start()
+            t2.start()
+            t1.join()
+            t2.join()
+
+            # Neither thread should have errors about stale modules
+            self.assertEqual(errors, [], f"Race condition detected: {errors}")
+
 
 def setUpModule():
     thread_info = threading_helper.threading_setup()

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

@@ -0,0 +1,2 @@
+Fix race condition in import system where a thread could receive a stale
+module reference when another thread's import fails.

Python/import.c

@@ -301,6 +301,18 @@ PyImport_GetModule(PyObject *name)
             remove_importlib_frames(tstate);
             return NULL;
         }
+        /* 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().
+           Unlike the import path, we return NULL here since this function
+           only retrieves existing modules and doesn't trigger new imports. */
+        PyObject *mod_check = import_get_module(tstate, name);
+        if (mod_check != mod) {
+            Py_XDECREF(mod_check);
+            Py_DECREF(mod);
+            return NULL;
+        }
+        Py_DECREF(mod_check);
     }
     return mod;
 }
@@ -3882,6 +3894,24 @@ 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);
+            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);