[3.12] gh-105829: Fix concurrent.futures.ProcessPoolExecutor deadlock (GH-108513) (#109784)

This fixes issue GH-105829, https://github.com/python/cpython/issues/105829
(cherry picked from commit 405b06375a)

Lib/concurrent/futures/process.py

@@ -71,6 +71,11 @@ class _ThreadWakeup:
         self._reader, self._writer = mp.Pipe(duplex=False)
 
     def close(self):
+        # Please note that we do not take the shutdown lock when
+        # calling clear() (to avoid deadlocking) so this method can
+        # only be called safely from the same thread as all calls to
+        # clear() even if you hold the shutdown lock. Otherwise we
+        # might try to read from the closed pipe.
         if not self._closed:
             self._closed = True
             self._writer.close()
@@ -426,8 +431,12 @@ class _ExecutorManagerThread(threading.Thread):
         elif wakeup_reader in ready:
             is_broken = False
 
-        with self.shutdown_lock:
-            self.thread_wakeup.clear()
+        # No need to hold the _shutdown_lock here because:
+        # 1. we're the only thread to use the wakeup reader
+        # 2. we're also the only thread to call thread_wakeup.close()
+        # 3. we want to avoid a possible deadlock when both reader and writer
+        #    would block (gh-105829)
+        self.thread_wakeup.clear()
 
         return result_item, is_broken, cause
 
@@ -710,7 +719,10 @@ class ProcessPoolExecutor(_base.Executor):
         # as it could result in a deadlock if a worker process dies with the
         # _result_queue write lock still acquired.
         #
-        # _shutdown_lock must be locked to access _ThreadWakeup.
+        # _shutdown_lock must be locked to access _ThreadWakeup.close() and
+        # .wakeup(). Care must also be taken to not call clear or close from
+        # more than one thread since _ThreadWakeup.clear() is not protected by
+        # the _shutdown_lock
         self._executor_manager_thread_wakeup = _ThreadWakeup()
 
         # Create communication channels for the executor