Issue #15781: Fix two small race conditions in import's module locking.

Lib/importlib/_bootstrap.py

@@ -268,8 +268,10 @@ def _get_module_lock(name):
 
     Should only be called with the import lock taken."""
     lock = None
-    if name in _module_locks:
+    try:
         lock = _module_locks[name]()
+    except KeyError:
+        pass
     if lock is None:
         if _thread is None:
             lock = _DummyModuleLock(name)
@@ -543,6 +545,9 @@ def module_for_loader(fxn):
             # implicitly imports 'locale' and would otherwise trigger an
             # infinite loop.
             module = new_module(fullname)
+            # This must be done before putting the module in sys.modules
+            # (otherwise an optimization shortcut in import.c becomes wrong)
+            module.__initializing__ = True
             sys.modules[fullname] = module
             module.__loader__ = self
             try:
@@ -554,8 +559,9 @@ def module_for_loader(fxn):
                     module.__package__ = fullname
                 else:
                     module.__package__ = fullname.rpartition('.')[0]
-        try:
+        else:
             module.__initializing__ = True
+        try:
             # If __package__ was not set above, __import__() will do it later.
             return fxn(self, module, *args, **kwargs)
         except:

Lib/test/test_threaded_import.py

@@ -224,7 +224,17 @@ class ThreadedImportTests(unittest.TestCase):
 
 @reap_threads
 def test_main():
-    run_unittest(ThreadedImportTests)
+    old_switchinterval = None
+    try:
+        old_switchinterval = sys.getswitchinterval()
+        sys.setswitchinterval(0.00000001)
+    except AttributeError:
+        pass
+    try:
+        run_unittest(ThreadedImportTests)
+    finally:
+        if old_switchinterval is not None:
+            sys.setswitchinterval(old_switchinterval)
 
 if __name__ == "__main__":
     test_main()

Misc/NEWS

@@ -13,6 +13,8 @@ Core and Builtins
 - Issue #15784: Modify OSError.__str__() to better distinguish between
   errno error numbers and Windows error numbers.
 
+- Issue #15781: Fix two small race conditions in import's module locking.
+
 Library
 -------
 

Python/import.c

@@ -1408,7 +1408,11 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *given_globals,
         int initializing = 0;
 
         Py_INCREF(mod);
-        /* Only call _bootstrap._lock_unlock_module() if __initializing__ is true. */
+        /* Optimization: only call _bootstrap._lock_unlock_module() if
+           __initializing__ is true.
+           NOTE: because of this, __initializing__ must be set *before*
+           stuffing the new module in sys.modules.
+         */
         value = _PyObject_GetAttrId(mod, &PyId___initializing__);
         if (value == NULL)
             PyErr_Clear();