bpo-30814: Fixed a race condition when import a submodule from a package. (#2580)

Author: serhiy-storchaka

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

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

@@ -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.
 

Misc/NEWS

@@ -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);