Implemented `HttpBackend._stop_workers`.

The Tornado WSGI container won't guarantee the termination of the
spawned workers upon termination, so the code of the backend has to take
care of it and terminate all the children processes of the server
process when it terminates.

This also means that `psutil` is now a required base dependency, as we
need to expand the process subtree under the webserver launcher.

platypush/backend/http/__init__.py

@@ -2,13 +2,17 @@ import asyncio
 import os
 import pathlib
 import secrets
+import signal
 import threading
 
+from functools import partial
 from multiprocessing import Process
 from time import time
-from typing import List, Mapping, Optional
+from typing import Mapping, Optional
-from tornado.httpserver import HTTPServer
 
+import psutil
+
+from tornado.httpserver import HTTPServer
 from tornado.netutil import bind_sockets
 from tornado.process import cpu_count, fork_processes
 from tornado.wsgi import WSGIContainer
@@ -18,9 +22,9 @@ from platypush.backend import Backend
 from platypush.backend.http.app import application
 from platypush.backend.http.app.utils import get_streaming_routes, get_ws_routes
 from platypush.backend.http.app.ws.events import WSEventProxy
-
 from platypush.bus.redis import RedisBus
 from platypush.config import Config
+from platypush.utils import get_remaining_timeout
 
 
 class HttpBackend(Backend):
@@ -191,6 +195,9 @@ class HttpBackend(Backend):
     _DEFAULT_HTTP_PORT = 8008
     """The default listen port for the webserver."""
 
+    _STOP_TIMEOUT = 5
+    """How long we should wait (in seconds) before killing the worker processes."""
+
     def __init__(
         self,
         port: int = _DEFAULT_HTTP_PORT,
@@ -227,7 +234,6 @@ class HttpBackend(Backend):
 
         self.port = port
         self._server_proc: Optional[Process] = None
-        self._workers: List[Process] = []
         self._service_registry_thread = None
         self.bind_address = bind_address
 
@@ -254,39 +260,37 @@ class HttpBackend(Backend):
         """On backend stop"""
         super().on_stop()
         self.logger.info('Received STOP event on HttpBackend')
-
+        start = time()
-        start_time = time()
+        remaining_time: partial[float] = partial(  # type: ignore
-        timeout = 5
+            get_remaining_timeout, timeout=self._STOP_TIMEOUT, start=start
-        workers = self._workers.copy()
+        )
-
-        for i, worker in enumerate(workers[::-1]):
-            if worker and worker.is_alive():
-                worker.terminate()
-                worker.join(timeout=max(0, start_time + timeout - time()))
-
-            if worker and worker.is_alive():
-                worker.kill()
-                self._workers.pop(i)
 
         if self._server_proc:
-            try:
+            if self._server_proc.pid:
-                self._server_proc.terminate()
+                try:
-                self._server_proc.join(timeout=5)
+                    os.kill(self._server_proc.pid, signal.SIGINT)
-            except AttributeError:
+                except OSError:
-                pass
+                    pass
 
-            self._server_proc = None
+            if self._server_proc and self._server_proc.is_alive():
+                self._server_proc.join(timeout=remaining_time() / 2)
+                try:
+                    self._server_proc.terminate()
+                    self._server_proc.join(timeout=remaining_time() / 2)
+                except AttributeError:
+                    pass
 
         if self._server_proc and self._server_proc.is_alive():
             self._server_proc.kill()
 
         self._server_proc = None
-        self.logger.info('HTTP server terminated')
 
         if self._service_registry_thread and self._service_registry_thread.is_alive():
-            self._service_registry_thread.join(timeout=5)
+            self._service_registry_thread.join(timeout=remaining_time())
             self._service_registry_thread = None
 
+        self.logger.info('HTTP server terminated')
+
     def notify_web_clients(self, event):
         """Notify all the connected web clients (over websocket) of a new event"""
         WSEventProxy.publish(event)  # noqa: E1120
@@ -348,7 +352,10 @@ class HttpBackend(Backend):
         try:
             await asyncio.Event().wait()
         except (asyncio.CancelledError, KeyboardInterrupt):
-            return
+            pass
+        finally:
+            server.stop()
+            await server.close_all_connections()
 
     def _web_server_proc(self):
         self.logger.info(
@@ -375,7 +382,65 @@ class HttpBackend(Backend):
                 future = self._post_fork_main(sockets)
                 asyncio.run(future)
             except (asyncio.CancelledError, KeyboardInterrupt):
-                return
+                pass
+            finally:
+                self._stop_workers()
+
+    def _stop_workers(self):
+        """
+        Stop all the worker processes.
+
+        We have to run this manually on server termination because of a
+        long-standing issue with Tornado not being able to wind down the forked
+        workers when the server terminates:
+        https://github.com/tornadoweb/tornado/issues/1912.
+        """
+        parent_pid = (
+            self._server_proc.pid
+            if self._server_proc and self._server_proc.pid
+            else None
+        )
+
+        if not parent_pid:
+            return
+
+        try:
+            cur_proc = psutil.Process(parent_pid)
+        except psutil.NoSuchProcess:
+            return
+
+        # Send a SIGTERM to all the children
+        children = cur_proc.children()
+        for child in children:
+            if child.pid != parent_pid and child.is_running():
+                try:
+                    os.kill(child.pid, signal.SIGTERM)
+                except OSError as e:
+                    self.logger.warning(
+                        'Could not send SIGTERM to PID %d: %s', child.pid, e
+                    )
+
+        # Initialize the timeout
+        start = time()
+        remaining_time: partial[int] = partial(  # type: ignore
+            get_remaining_timeout, timeout=self._STOP_TIMEOUT, start=start, cls=int
+        )
+
+        # Wait for all children to terminate (with timeout)
+        for child in children:
+            if child.pid != parent_pid and child.is_running():
+                try:
+                    child.wait(timeout=remaining_time())
+                except TimeoutError:
+                    pass
+
+        # Send a SIGKILL to any child process that is still running
+        for child in children:
+            if child.pid != parent_pid and child.is_running():
+                try:
+                    child.kill()
+                except OSError:
+                    pass
 
     def _start_web_server(self):
         self._server_proc = Process(target=self._web_server_proc)