gh-90549: Fix leak of global named resources using multiprocessing spawn

Lib/multiprocessing/context.py

@@ -223,6 +223,10 @@ class Process(process.BaseProcess):
  def _Popen(process_obj):
  return _default_context.get_context().Process._Popen(process_obj)
 
+ @staticmethod
+ def _after_fork():
+ return _default_context.get_context().Process._after_fork()
+
 class DefaultContext(BaseContext):
  Process = Process
 
@@ -283,6 +287,11 @@ def _Popen(process_obj):
  from .popen_spawn_posix import Popen
  return Popen(process_obj)
 
+ @staticmethod
+ def _after_fork():
+ # process is spawned, nothing to do
+ pass
+
  class ForkServerProcess(process.BaseProcess):
  _start_method = 'forkserver'
  @staticmethod
@@ -326,6 +335,11 @@ def _Popen(process_obj):
  from .popen_spawn_win32 import Popen
  return Popen(process_obj)
 
+ @staticmethod
+ def _after_fork():
+ # process is spawned, nothing to do
+ pass
+
  class SpawnContext(BaseContext):
  _name = 'spawn'
  Process = SpawnProcess

Lib/multiprocessing/process.py

@@ -304,8 +304,7 @@ def _bootstrap(self, parent_sentinel=None):
  if threading._HAVE_THREAD_NATIVE_ID:
  threading.main_thread()._set_native_id()
  try:
- util._finalizer_registry.clear()
- util._run_after_forkers()
+ self._after_fork()
  finally:
  # delay finalization of the old process object until after
  # _run_after_forkers() is executed
@@ -336,6 +335,13 @@ def _bootstrap(self, parent_sentinel=None):
 
  return exitcode
 
+ @staticmethod
+ def _after_fork():
+ from . import util
+ util._finalizer_registry.clear()
+ util._run_after_forkers()
+
+
 #
 # We subclass bytes to avoid accidental transmission of auth keys over network
 #

Lib/test/_test_multiprocessing.py

@@ -5,6 +5,7 @@
 import unittest
 import unittest.mock
 import queue as pyqueue
+import textwrap
 import time
 import io
 import itertools
@@ -5760,6 +5761,35 @@ def test_namespace(self):
  self.run_worker(self._test_namespace, o)
 
 
+class TestNamedResource(unittest.TestCase):
+ def test_global_named_resource_spawn(self):
+ #
+ # gh-90549: Check that global named resources in main module
+ # will not leak by a subprocess, in spawn context.
+ #
+ testfn = os_helper.TESTFN
+ self.addCleanup(os_helper.unlink, testfn)
+ with open(testfn, 'w', encoding='utf-8') as f:
+ f.write(textwrap.dedent('''\
+ import multiprocessing as mp
+
+ ctx = mp.get_context('spawn')
+
+ global_resource = ctx.Semaphore()
+
+ def submain(): pass
+
+ if __name__ == '__main__':
+ p = ctx.Process(target=submain)
+ p.start()
+ p.join()
+ '''))
+ rc, out, err = test.support.script_helper.assert_python_ok(testfn)
+ # on error, err = 'UserWarning: resource_tracker: There appear to
+ # be 1 leaked semaphore objects to clean up at shutdown'
+ self.assertEqual(err, b'')
+
+
 class MiscTestCase(unittest.TestCase):
  def test__all__(self):
  # Just make sure names in not_exported are excluded

Misc/NEWS.d/next/Library/2022-06-07-14-53-46.gh-issue-90549.T4FMKY.rst

@@ -0,0 +1,2 @@
+Fix a multiprocessing bug where a global named resource (such as a semaphore)
+could leak when a child process is spawned (as opposed to forked).