gh-128041: Add terminate_workers and kill_workers methods to ProcessPoolExecutor (GH-130849)

This adds two new methods to `multiprocessing`'s `ProcessPoolExecutor`:
- **`terminate_workers()`**: forcefully terminates worker processes using `Process.terminate()`
- **`kill_workers()`**: forcefully kills worker processes using `Process.kill()`

These methods provide users with a direct way to stop worker processes without `shutdown()` or relying on implementation details, addressing situations where immediate termination is needed.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Sam Gross @colesbury
Commit-message-mostly-authored-by: Claude Sonnet 3.7 (because why not -greg)

Doc/library/concurrent.futures.rst

@@ -415,6 +415,30 @@ to a :class:`ProcessPoolExecutor` will result in deadlock.
       require the *fork* start method for :class:`ProcessPoolExecutor` you must
       explicitly pass ``mp_context=multiprocessing.get_context("fork")``.
 
+   .. method:: terminate_workers()
+
+      Attempt to terminate all living worker processes immediately by calling
+      :meth:`Process.terminate <multiprocessing.Process.terminate>` on each of them.
+      Internally, it will also call :meth:`Executor.shutdown` to ensure that all
+      other resources associated with the executor are freed.
+
+      After calling this method the caller should no longer submit tasks to the
+      executor.
+
+      .. versionadded:: next
+
+   .. method:: kill_workers()
+
+      Attempt to kill all living worker processes immediately by calling
+      :meth:`Process.kill <multiprocessing.Process.kill>` on each of them.
+      Internally, it will also call :meth:`Executor.shutdown` to ensure that all
+      other resources associated with the executor are freed.
+
+      After calling this method the caller should no longer submit tasks to the
+      executor.
+
+      .. versionadded:: next
+
 .. _processpoolexecutor-example:
 
 ProcessPoolExecutor Example

Doc/whatsnew/3.14.rst

@@ -444,6 +444,11 @@ contextvars
 * Support context manager protocol by :class:`contextvars.Token`.
   (Contributed by Andrew Svetlov in :gh:`129889`.)
 
+* Add :meth:`concurrent.futures.ProcessPoolExecutor.terminate_workers` and
+  :meth:`concurrent.futures.ProcessPoolExecutor.kill_workers` as
+  ways to terminate or kill all living worker processes in the given pool.
+  (Contributed by Charles Machalow in :gh:`130849`.)
+
 
 ctypes
 ------

Lib/concurrent/futures/process.py

@@ -626,6 +626,14 @@ class BrokenProcessPool(_base.BrokenExecutor):
     while a future was in the running state.
     """
 
+_TERMINATE = "terminate"
+_KILL = "kill"
+
+_SHUTDOWN_CALLBACK_OPERATION = {
+    _TERMINATE,
+    _KILL
+}
+
 
 class ProcessPoolExecutor(_base.Executor):
     def __init__(self, max_workers=None, mp_context=None,
@@ -855,3 +863,66 @@ class ProcessPoolExecutor(_base.Executor):
         self._executor_manager_thread_wakeup = None
 
     shutdown.__doc__ = _base.Executor.shutdown.__doc__
+
+    def _force_shutdown(self, operation):
+        """Attempts to terminate or kill the executor's workers based off the
+        given operation. Iterates through all of the current processes and
+        performs the relevant task if the process is still alive.
+
+        After terminating workers, the pool will be in a broken state
+        and no longer usable (for instance, new tasks should not be
+        submitted).
+        """
+        if operation not in _SHUTDOWN_CALLBACK_OPERATION:
+            raise ValueError(f"Unsupported operation: {operation!r}")
+
+        processes = {}
+        if self._processes:
+            processes = self._processes.copy()
+
+        # shutdown will invalidate ._processes, so we copy it right before
+        # calling. If we waited here, we would deadlock if a process decides not
+        # to exit.
+        self.shutdown(wait=False, cancel_futures=True)
+
+        if not processes:
+            return
+
+        for proc in processes.values():
+            try:
+                if not proc.is_alive():
+                    continue
+            except ValueError:
+                # The process is already exited/closed out.
+                continue
+
+            try:
+                if operation == _TERMINATE:
+                    proc.terminate()
+                elif operation == _KILL:
+                    proc.kill()
+            except ProcessLookupError:
+                # The process just ended before our signal
+                continue
+
+    def terminate_workers(self):
+        """Attempts to terminate the executor's workers.
+        Iterates through all of the current worker processes and terminates
+        each one that is still alive.
+
+        After terminating workers, the pool will be in a broken state
+        and no longer usable (for instance, new tasks should not be
+        submitted).
+        """
+        return self._force_shutdown(operation=_TERMINATE)
+
+    def kill_workers(self):
+        """Attempts to kill the executor's workers.
+        Iterates through all of the current worker processes and kills
+        each one that is still alive.
+
+        After killing workers, the pool will be in a broken state
+        and no longer usable (for instance, new tasks should not be
+        submitted).
+        """
+        return self._force_shutdown(operation=_KILL)

Lib/test/test_concurrent_futures/test_process_pool.py

@@ -1,13 +1,17 @@
 import os
+import queue
+import signal
 import sys
 import threading
 import time
 import unittest
+import unittest.mock
 from concurrent import futures
 from concurrent.futures.process import BrokenProcessPool
 
 from test import support
 from test.support import hashlib_helper
+from test.test_importlib.metadata.fixtures import parameterize
 
 from .executor import ExecutorTest, mul
 from .util import (
@@ -22,6 +26,21 @@ class EventfulGCObj():
     def __del__(self):
         self.event.set()
 
+TERMINATE_WORKERS = futures.ProcessPoolExecutor.terminate_workers.__name__
+KILL_WORKERS = futures.ProcessPoolExecutor.kill_workers.__name__
+FORCE_SHUTDOWN_PARAMS = [
+    dict(function_name=TERMINATE_WORKERS),
+    dict(function_name=KILL_WORKERS),
+]
+
+def _put_wait_put(queue, event):
+    """ Used as part of test_terminate_workers """
+    queue.put('started')
+    event.wait()
+
+    # We should never get here since the event will not get set
+    queue.put('finished')
+
 
 class ProcessPoolExecutorTest(ExecutorTest):
 
@@ -218,6 +237,107 @@ class ProcessPoolExecutorTest(ExecutorTest):
                     list(executor.map(mul, [(2, 3)] * 10))
             executor.shutdown()
 
+    def test_terminate_workers(self):
+        mock_fn = unittest.mock.Mock()
+        with self.executor_type(max_workers=1) as executor:
+            executor._force_shutdown = mock_fn
+            executor.terminate_workers()
+
+        mock_fn.assert_called_once_with(operation=futures.process._TERMINATE)
+
+    def test_kill_workers(self):
+        mock_fn = unittest.mock.Mock()
+        with self.executor_type(max_workers=1) as executor:
+            executor._force_shutdown = mock_fn
+            executor.kill_workers()
+
+        mock_fn.assert_called_once_with(operation=futures.process._KILL)
+
+    def test_force_shutdown_workers_invalid_op(self):
+        with self.executor_type(max_workers=1) as executor:
+            self.assertRaises(ValueError,
+                              executor._force_shutdown,
+                              operation='invalid operation'),
+
+    @parameterize(*FORCE_SHUTDOWN_PARAMS)
+    def test_force_shutdown_workers(self, function_name):
+        manager = self.get_context().Manager()
+        q = manager.Queue()
+        e = manager.Event()
+
+        with self.executor_type(max_workers=1) as executor:
+            executor.submit(_put_wait_put, q, e)
+
+            # We should get started, but not finished since we'll terminate the
+            # workers just after and never set the event.
+            self.assertEqual(q.get(timeout=support.SHORT_TIMEOUT), 'started')
+
+            worker_process = list(executor._processes.values())[0]
+
+            Mock = unittest.mock.Mock
+            worker_process.terminate = Mock(wraps=worker_process.terminate)
+            worker_process.kill = Mock(wraps=worker_process.kill)
+
+            getattr(executor, function_name)()
+            worker_process.join()
+
+            if function_name == TERMINATE_WORKERS:
+                worker_process.terminate.assert_called()
+            elif function_name == KILL_WORKERS:
+                worker_process.kill.assert_called()
+            else:
+                self.fail(f"Unknown operation: {function_name}")
+
+            self.assertRaises(queue.Empty, q.get, timeout=0.01)
+
+    @parameterize(*FORCE_SHUTDOWN_PARAMS)
+    def test_force_shutdown_workers_dead_workers(self, function_name):
+        with self.executor_type(max_workers=1) as executor:
+            future = executor.submit(os._exit, 1)
+            self.assertRaises(BrokenProcessPool, future.result)
+
+            # even though the pool is broken, this shouldn't raise
+            getattr(executor, function_name)()
+
+    @parameterize(*FORCE_SHUTDOWN_PARAMS)
+    def test_force_shutdown_workers_not_started_yet(self, function_name):
+        ctx = self.get_context()
+        with unittest.mock.patch.object(ctx, 'Process') as mock_process:
+            with self.executor_type(max_workers=1, mp_context=ctx) as executor:
+                # The worker has not been started yet, terminate/kill_workers
+                # should basically no-op
+                getattr(executor, function_name)()
+
+            mock_process.return_value.kill.assert_not_called()
+            mock_process.return_value.terminate.assert_not_called()
+
+    @parameterize(*FORCE_SHUTDOWN_PARAMS)
+    def test_force_shutdown_workers_stops_pool(self, function_name):
+        with self.executor_type(max_workers=1) as executor:
+            task = executor.submit(time.sleep, 0)
+            self.assertIsNone(task.result())
+
+            worker_process = list(executor._processes.values())[0]
+            getattr(executor, function_name)()
+
+            self.assertRaises(RuntimeError, executor.submit, time.sleep, 0)
+
+            # A signal sent, is not a signal reacted to.
+            # So wait a moment here for the process to die.
+            # If we don't, every once in a while we may get an ENV CHANGE
+            # error since the process would be alive immediately after the
+            # test run.. and die a moment later.
+            worker_process.join(support.SHORT_TIMEOUT)
+
+            # Oddly enough, even though join completes, sometimes it takes a
+            # moment for the process to actually be marked as dead.
+            # ...  that seems a bit buggy.
+            # We need it dead before ending the test to ensure it doesn't
+            # get marked as an ENV CHANGE due to living child process.
+            for _ in support.sleeping_retry(support.SHORT_TIMEOUT):
+                if not worker_process.is_alive():
+                    break
+
 
 create_executor_tests(globals(), ProcessPoolExecutorTest,
                       executor_mixins=(ProcessPoolForkMixin,

Misc/NEWS.d/next/Library/2024-12-17-18-53-21.gh-issue-128041.W96kAr.rst

@@ -0,0 +1,4 @@
+Add :meth:`concurrent.futures.ProcessPoolExecutor.terminate_workers` and
+:meth:`concurrent.futures.ProcessPoolExecutor.kill_workers` as
+ways to terminate or kill all living worker processes in the given pool.
+(Contributed by Charles Machalow in :gh:`130849`.)