From c8f9f0f41aead3cfc8bb7c2f0699ba6096a35b32 Mon Sep 17 00:00:00 2001
From: Odysseas YIAKOUMIS <odysseasyiakoumis@gmail.com>
Date: Sat, 3 Oct 2026 23:03:17 +0400
Subject: [PATCH] Fix Pool.imap(buffersize=...) dropping results after close()

---
 Doc/library/multiprocessing.rst   |  3 +++
 Lib/multiprocessing/pool.py       | 29 +++++++++++++++++------------
 Lib/test/_test_multiprocessing.py | 16 +++++++++++++++-
 3 files changed, 35 insertions(+), 13 deletions(-)

diff --git a/Doc/library/multiprocessing.rst b/Doc/library/multiprocessing.rst
index 43fad57139057cd..6919e200299a513 100644
--- a/Doc/library/multiprocessing.rst
+++ b/Doc/library/multiprocessing.rst
@@ -2558,6 +2558,9 @@ with the :class:`Pool` class.
       set *buffersize* at least to the number of processes in pool
       (to consume *iterable* as you go), or even higher
       (to prefetch the next ``N=buffersize-processes`` arguments).
+      Once :meth:`join` is called, *buffersize* is no longer honored:
+      the rest of the *iterable* is submitted without waiting for results
+      to be yielded.
 
       .. versionchanged:: next
          Added the *buffersize* parameter.
diff --git a/Lib/multiprocessing/pool.py b/Lib/multiprocessing/pool.py
index ef9460ac0aa6817..c0844cca9bb874e 100644
--- a/Lib/multiprocessing/pool.py
+++ b/Lib/multiprocessing/pool.py
@@ -191,10 +191,13 @@ def __init__(self, processes=None, initializer=None, initargs=(),
         self._setup_queues()
         self._taskqueue = queue.SimpleQueue()
         # The _taskqueue_buffersize_semaphores exist to allow calling .release()
-        # on every active semaphore when the pool is terminating to let task_handler
-        # wake up to stop. It's a set so that each iterator object can efficiently
-        # deregister its semaphore when iterator finishes.
+        # on every active semaphore when the pool is terminating or joined to
+        # let task_handler wake up. It's a set so that each iterator object can
+        # efficiently deregister its semaphore when iterator finishes.
         self._taskqueue_buffersize_semaphores = set()
+        # Set by join(): from then on the buffersize of imap iterators is
+        # ignored, since there may be nobody left to consume their results.
+        self._taskqueue_buffersize_ignored = False
         # The _change_notifier queue exist to wake up self._handle_workers()
         # when the cache (self._cache) is empty or when there is a change in
         # the _state variable of the thread that runs _handle_workers.
@@ -402,11 +405,11 @@ def _guarded_task_generation(self, result_job, func, iterable, sema=None):
             else:
                 enumerated_iter = iter(enumerate(iterable))
                 while True:
-                    sema.acquire()
-                    if self._state != RUN:
-                        # The pool is closing or terminating; stop submitting
-                        # the still-throttled tasks so the task handler can
-                        # finish instead of blocking here forever.
+                    if not self._taskqueue_buffersize_ignored:
+                        sema.acquire()
+                    if self._state == TERMINATE:
+                        # The pool is terminating; stop submitting tasks so
+                        # the task handler can finish.
                         break
                     try:
                         i, x = next(enumerated_iter)
@@ -666,10 +669,6 @@ def close(self):
             self._state = CLOSE
             self._worker_handler._state = CLOSE
             self._change_notifier.put(None)
-            # Wake any task generator throttled on a buffersize semaphore so
-            # it observes the CLOSE state and stops submitting.
-            for sema in list(self._taskqueue_buffersize_semaphores):
-                sema.release()
 
     def terminate(self):
         util.debug('terminating pool')
@@ -682,6 +681,12 @@ def join(self):
             raise ValueError("Pool is still running")
         elif self._state not in (CLOSE, TERMINATE):
             raise ValueError("In unknown state")
+        # Wake any task generator throttled on a buffersize semaphore and let
+        # it submit the remaining tasks unthrottled: join() has to wait for
+        # them, and there may be nobody left to consume the results.
+        self._taskqueue_buffersize_ignored = True
+        for sema in list(self._taskqueue_buffersize_semaphores):
+            sema.release()
         self._worker_handler.join()
         self._task_handler.join()
         self._result_handler.join()
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 46ed8843fcd0519..8e611e31fc7cf81 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -3239,7 +3239,7 @@ def test_imap_with_buffersize_close_after_partial_consumption(
         p = self.Pool(2)
         method = getattr(p, method_name)
         it = method(sqr, range(1000), buffersize=2)
-        next(it)
+        first = next(it)
         finished = threading.Event()
         def finalize():
             p.close()
@@ -3249,6 +3249,20 @@ def finalize():
         t.start()
         t.join(support.SHORT_TIMEOUT)
         self.assertTrue(finished.is_set(), "close()/join() deadlocked")
+        # The tasks which were not submitted yet must not be dropped.
+        self.assertEqual(sorted([first, *it]), list(map(sqr, range(1000))))
+
+    @warnings_helper.ignore_fork_in_thread_deprecation_warnings()
+    @support.subTests('method_name', ("imap", "imap_unordered"))
+    def test_imap_with_buffersize_consumption_after_close(self, method_name):
+        # close() must not drop the tasks of a buffersize iterator which
+        # were not submitted yet.
+        p = self.Pool(2)
+        method = getattr(p, method_name)
+        it = method(sqr, range(100), buffersize=2)
+        p.close()
+        self.assertEqual(sorted(it), list(map(sqr, range(100))))
+        p.join()
 
     @support.subTests('method_name', ("imap", "imap_unordered"))
     def test_imap_and_imap_unordered_with_buffersize_on_empty_iterable(