[3.9] bpo-45021: Fix a hang in forked children (GH-28007)

Lib/concurrent/futures/thread.py

@@ -36,6 +36,12 @@ def _python_exit():
 # See bpo-39812 for context.
 threading._register_atexit(_python_exit)
 
+# At fork, reinitialize the `_global_shutdown_lock` lock in the child process
+if hasattr(os, 'register_at_fork'):
+    os.register_at_fork(before=_global_shutdown_lock.acquire,
+                        after_in_child=_global_shutdown_lock._at_fork_reinit,
+                        after_in_parent=_global_shutdown_lock.release)
+
 
 class _WorkItem(object):
     def __init__(self, future, fn, args, kwargs):

Lib/test/test_concurrent_futures.py

@@ -896,6 +896,20 @@ def test_idle_thread_reuse(self):
         self.assertEqual(len(executor._threads), 1)
         executor.shutdown(wait=True)
 
+    @unittest.skipUnless(hasattr(os, 'register_at_fork'), 'need os.register_at_fork')
+    def test_hang_global_shutdown_lock(self):
+        # bpo-45021: _global_shutdown_lock should be reinitialized in the child
+        # process, otherwise it will never exit
+        def submit(pool):
+            pool.submit(submit, pool)
+
+        with futures.ThreadPoolExecutor(1) as pool:
+            pool.submit(submit, pool)
+
+            for _ in range(50):
+                with futures.ProcessPoolExecutor(1, mp_context=get_context('fork')) as workers:
+                    workers.submit(tuple)
+
 
 class ProcessPoolExecutorTest(ExecutorTest):
 

Misc/NEWS.d/next/Library/2021-08-28-13-00-12.bpo-45021.rReeaj.rst

@@ -0,0 +1 @@
+Fix a potential deadlock at shutdown of forked children when using :mod:`concurrent.futures` module