From 7f12e9cd81783900ecfb47bb9755236cf4a5a375 Mon Sep 17 00:00:00 2001
From: "Ziegler, Tim" <tim.ziegler@intel.com>
Date: Thu, 1 Oct 2026 13:30:51 +0200
Subject: [PATCH 1/3] Fix ProcessPoolExecutor shutdown deadlock when result
 pipe is full

---
 Lib/concurrent/futures/process.py             |  8 ++++
 .../test_concurrent_futures/test_deadlock.py  | 39 +++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
index 7f4f225c0ad4fbd..df92b7ea1d28edb 100644
--- a/Lib/concurrent/futures/process.py
+++ b/Lib/concurrent/futures/process.py
@@ -678,9 +678,17 @@ def _join_executor_internals(self, broken=False):
 
         # If .join() is not called on the created processes then
         # some ctx.Queue methods may deadlock on Mac OS X.
+        result_reader = self.result_queue._reader
         for p in self.processes.values():
             if broken:
                 p.terminate()
+            else:
+                # Exiting workers put their pid on the result queue. Keep
+                # draining it, otherwise once the pipe is full a worker
+                # blocks forever writing to it and never exits.
+                while p.sentinel not in mp.connection.wait(
+                        [result_reader, p.sentinel]):
+                    result_reader.recv_bytes()
             p.join()
 
     def get_n_children_alive(self):
diff --git a/Lib/test/test_concurrent_futures/test_deadlock.py b/Lib/test/test_concurrent_futures/test_deadlock.py
index 5cd84f7e43043c6..68371fa0c250f57 100644
--- a/Lib/test/test_concurrent_futures/test_deadlock.py
+++ b/Lib/test/test_concurrent_futures/test_deadlock.py
@@ -2,6 +2,7 @@
 import queue
 import signal
 import sys
+import threading
 import time
 import unittest
 import unittest.mock
@@ -329,6 +330,44 @@ def clear(self):
                     signal.signal(signal.SIGALRM, old_handler)
 
 
+    @warnings_helper.ignore_fork_in_thread_deprecation_warnings()
+    def test_shutdown_should_not_deadlock_if_result_pipe_full(self):
+        # Exiting workers put their pid on the result queue while shutdown()
+        # joins them. If nothing drains the queue, a full pipe blocks the
+        # workers forever. Fill the pipe once the executor manager thread
+        # stops reading results, right before the workers are told to exit.
+        self.executor.shutdown(wait=True)
+
+        data = b"a" * support.PIPE_MAX_SIZE
+        fillers = []
+        shutdown_workers = futures.process._ExecutorManagerThread.shutdown_workers
+        def mock_shutdown_workers(self):
+            # The put() blocks holding the result queue's write lock until
+            # the pipe is drained, so every exiting worker blocks behind it.
+            filler = threading.Thread(target=self.result_queue.put,
+                                      args=(data,))
+            filler.start()
+            fillers.append(filler)
+            shutdown_workers(self)
+
+        executor = self.executor_type(max_workers=2,
+                                      mp_context=self.get_context())
+        self.executor = executor  # Allow clean up in fail_on_deadlock
+        with unittest.mock.patch.object(futures.process._ExecutorManagerThread,
+                                        'shutdown_workers',
+                                        mock_shutdown_workers):
+            self.assertEqual(list(executor.map(int, range(10))),
+                             list(range(10)))
+            shutdown = threading.Thread(target=executor.shutdown)
+            shutdown.start()
+            shutdown.join(self.TIMEOUT)
+            if shutdown.is_alive():
+                self._fail_on_deadlock(executor)
+
+        self.assertEqual(len(fillers), 1)
+        fillers[0].join()
+
+
 create_executor_tests(globals(), ExecutorDeadlockTest,
                       executor_mixins=(ProcessPoolForkMixin,
                                        ProcessPoolForkserverMixin,

From 6c0483c7ac93606b87421ec58b8a86f283a79d4b Mon Sep 17 00:00:00 2001
From: "Ziegler, Tim" <tim.ziegler@intel.com>
Date: Thu, 1 Oct 2026 18:46:53 +0200
Subject: [PATCH 2/3] Add NEWS entry

---
 .../Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst     | 3 +++
 1 file changed, 3 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst

diff --git a/Misc/NEWS.d/next/Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst b/Misc/NEWS.d/next/Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst
new file mode 100644
index 000000000000000..145bafda8f4c709
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst
@@ -0,0 +1,3 @@
+Fix a deadlock in :meth:`concurrent.futures.ProcessPoolExecutor.shutdown`
+when the pipe used to return results fills up with worker exit
+notifications.

From 4ec18e81c2d6d6607e00bc261c22b7d284e81366 Mon Sep 17 00:00:00 2001
From: "Ziegler, Tim" <tim.ziegler@intel.com>
Date: Fri, 2 Oct 2026 08:53:07 +0200
Subject: [PATCH 3/3] Fix NEWS entry reference

---
 .../Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/Misc/NEWS.d/next/Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst b/Misc/NEWS.d/next/Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst
index 145bafda8f4c709..68cb1602b73c8e6 100644
--- a/Misc/NEWS.d/next/Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst
+++ b/Misc/NEWS.d/next/Library/2026-10-01-18-46-33.gh-issue-158570.rixXY7.rst
@@ -1,3 +1,4 @@
-Fix a deadlock in :meth:`concurrent.futures.ProcessPoolExecutor.shutdown`
-when the pipe used to return results fills up with worker exit
-notifications.
+Fix a deadlock in :meth:`ProcessPoolExecutor.shutdown()
+<concurrent.futures.Executor.shutdown>` when the messages that exiting
+workers send back fill up the pipe used to return results. This could
+happen with many workers or a small pipe buffer.