gh-132969 Fix exception/hang when shutdown(wait=False) and a task exi…

…ted abnormally When shutdown is called with wait=False, the executor thread keeps running even after the ProcessPoolExecutor's state is reset. The executor then tries to replenish the worker processes pool resulting in an error and a potential hang when it comes across a worker that has died. Fixed the issue by having _adjust_process_count() return without doing anything if the ProcessPoolExecutor's state has been reset. Added unit tests to validate two scenarios: max_workers < num_tasks (exception) max_workers > num_tasks (exception + hang)
python · ogbiggles · Apr 30, 2025 · Apr 30, 2025 · Apr 30, 2025 · 3edad4cb415b9508a6feab58f3c5962e494bb577
commit 3edad4cb415b9508a6feab58f3c5962e494bb577
diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
@@ -766,6 +766,10 @@ def _start_executor_manager_thread(self):
                 self._executor_manager_thread_wakeup
 
     def _adjust_process_count(self):
+        # gh-132969
+        if self._processes is None:
+            return
+
         # if there's an idle process, we don't need to spawn a new one.
         if self._idle_worker_semaphore.acquire(blocking=False):
             return

diff --git a/Lib/test/test_concurrent_futures/test_shutdown.py b/Lib/test/test_concurrent_futures/test_shutdown.py
@@ -20,6 +20,16 @@ def sleep_and_print(t, msg):
     sys.stdout.flush()
 
 
+def failing_task_132969(n: int) -> int:
+    raise ValueError("failing task")
+
+
+def good_task_132969(n: int) -> int:
+    time.sleep(0.1 * n)
+    return n
+
+
+
 class ExecutorShutdownTest:
     def test_run_after_shutdown(self):
         self.executor.shutdown()
@@ -330,6 +340,43 @@ def test_shutdown_no_wait(self):
         # shutdown.
         assert all([r == abs(v) for r, v in zip(res, range(-5, 5))])
 
+    def _run_test_issue_132969(self, max_workers: int) -> int:
+        # max_workers=2 will repro exception
+        # max_workers=4 will repro exception and then hang
+
+        import multiprocessing as mp
+
+        # Repro conditions
+        #   max_tasks_per_child=1
+        #   a task ends abnormally
+        #   shutdown(wait=False) is called
+        executor = futures.ProcessPoolExecutor(
+                max_workers=max_workers,
+                max_tasks_per_child=1,
+                mp_context=mp.get_context("forkserver"))
+        f1 = executor.submit(good_task_132969, 1)
+        f2 = executor.submit(failing_task_132969, 2)
+        f3 = executor.submit(good_task_132969, 3)
+        result:int = 0
+        try:
+            result += f1.result()
+            result += f2.result()
+            result += f3.result()
+        except ValueError:
+            # stop processing results upon first exception
+            pass
+
+        executor.shutdown(wait=False)
+        return result
+
+    def test_shutdown_len_exception_132969(self):
+        result = self._run_test_issue_132969(2)
+        self.assertEqual(result, 1)
+
+    def test_shutdown_process_hang_132969(self):
+        result = self._run_test_issue_132969(4)
+        self.assertEqual(result, 1)
+
 
 create_executor_tests(globals(), ProcessPoolShutdownTest,
                       executor_mixins=(ProcessPoolForkMixin,