gh-128041: Add terminate_workers and kill_workers methods to ProcessPoolExecutor (GH-128043)

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>
Commit-message-mostly-authored-by: Claude Sonnet 3.7 (because why not -greg)

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)