bpo-30814: Fixed a race condition when import a submodule from a package.

Lib/importlib/_bootstrap.py

@@ -956,9 +956,19 @@ def _find_and_load_unlocked(name, import_):
 
 
 def _find_and_load(name, import_):
-    """Find and load the module, and release the import lock."""
-    with _ModuleLockManager(name):
-        return _find_and_load_unlocked(name, import_)
+    """Find and load the module."""
+    _imp.acquire_lock()
+    if name not in sys.modules:
+        with _ModuleLockManager(name):
+            return _find_and_load_unlocked(name, import_)
+    module = sys.modules[name]
+    if module is None:
+        _imp.release_lock()
+        message = ('import of {} halted; '
+                   'None in sys.modules'.format(name))
+        raise ModuleNotFoundError(message, name=name)
+    _lock_unlock_module(name)
+    return module
 
 
 def _gcd_import(name, package=None, level=0):
@@ -973,17 +983,7 @@ def _gcd_import(name, package=None, level=0):
     _sanity_check(name, package, level)
     if level > 0:
         name = _resolve_name(name, package, level)
-    _imp.acquire_lock()
-    if name not in sys.modules:
-        return _find_and_load(name, _gcd_import)
-    module = sys.modules[name]
-    if module is None:
-        _imp.release_lock()
-        message = ('import of {} halted; '
-                   'None in sys.modules'.format(name))
-        raise ModuleNotFoundError(message, name=name)
-    _lock_unlock_module(name)
-    return module
+    return _find_and_load(name, _gcd_import)
 
 
 def _handle_fromlist(module, fromlist, import_):

Lib/test/test_import/__init__.py

@@ -10,6 +10,8 @@
 import random
 import stat
 import sys
+import threading
+import time
 import unittest
 import unittest.mock as mock
 import textwrap
@@ -380,6 +382,32 @@ def __getattr__(self, _):
         self.assertEqual(str(cm.exception),
             "cannot import name 'does_not_exist' from '<unknown module name>' (unknown location)")
 
+    def test_concurrency(self):
+        sys.path.insert(0, os.path.join(os.path.dirname(__file__), 'data'))
+        try:
+            exc = None
+            def run():
+                event.wait()
+                try:
+                    import package
+                except BaseException as e:
+                    nonlocal exc
+                    exc = e
+
+            for i in range(10):
+                event = threading.Event()
+                threads = [threading.Thread(target=run) for x in range(2)]
+                try:
+                    with test.support.start_threads(threads, event.set):
+                        time.sleep(0)
+                finally:
+                    sys.modules.pop('package', None)
+                    sys.modules.pop('package.submodule', None)
+                if exc is not None:
+                    raise exc
+        finally:
+            del sys.path[0]
+
 
 @skip_if_dont_write_bytecode
 class FilePermissionTests(unittest.TestCase):

Lib/test/test_import/data/package/__init__.py

@@ -0,0 +1,2 @@
+import package.submodule
+package.submodule

Misc/NEWS

@@ -10,6 +10,8 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
+- bpo-30814: Fixed a race condition when import a submodule from a package.
+
 - bpo-30736: The internal unicodedata database has been upgraded to Unicode
   10.0.
 

Python/import.c

@@ -1532,18 +1532,7 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
     }
 
     mod = PyDict_GetItem(interp->modules, abs_name);
-    if (mod == Py_None) {
-        PyObject *msg = PyUnicode_FromFormat("import of %R halted; "
-                                             "None in sys.modules", abs_name);
-        if (msg != NULL) {
-            PyErr_SetImportErrorSubclass(PyExc_ModuleNotFoundError, msg,
-                    abs_name, NULL);
-            Py_DECREF(msg);
-        }
-        mod = NULL;
-        goto error;
-    }
-    else if (mod != NULL) {
+    if (mod != NULL && mod != Py_None) {
         _Py_IDENTIFIER(__spec__);
         _Py_IDENTIFIER(_initializing);
         _Py_IDENTIFIER(_lock_unlock_module);
@@ -1584,10 +1573,6 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
         }
     }
     else {
-#ifdef WITH_THREAD
-        _PyImport_AcquireLock();
-#endif
-        /* _bootstrap._find_and_load() releases the import lock */
         mod = _PyObject_CallMethodIdObjArgs(interp->importlib,
                                             &PyId__find_and_load, abs_name,
                                             interp->import_func, NULL);