Added a multi-worker approach to the Tornado WSGI container.

The WSGI container is a good option to wrap a multi-modal webapp
(Flask + websocket routes), but it's constrained to a single-process
approach and queued/pre-buffered requests. That makes performance poor
when handling requests that may take a few seconds to complete.

platypush/backend/http/__init__.py

@@ -1,10 +1,12 @@
+import multiprocessing
 import os
 import pathlib
 import secrets
 import threading
+import time
 
 from multiprocessing import Process
-from typing import Mapping, Optional
+from typing import List, Mapping, Optional
 
 from tornado.wsgi import WSGIContainer
 from tornado.web import Application, FallbackHandler
@@ -186,6 +188,7 @@ class HttpBackend(Backend):
     """
 
     _DEFAULT_HTTP_PORT = 8008
+    """The default listen port for the webserver."""
 
     def __init__(
         self,
@@ -193,6 +196,7 @@ class HttpBackend(Backend):
         bind_address: str = '0.0.0.0',
         resource_dirs: Optional[Mapping[str, str]] = None,
         secret_key_file: Optional[str] = None,
+        num_workers: Optional[int] = None,
         **kwargs,
     ):
         """
@@ -204,12 +208,13 @@ class HttpBackend(Backend):
             the value is the absolute path to expose.
         :param secret_key_file: Path to the file containing the secret key that will be used by Flask
             (default: ``~/.local/share/platypush/flask.secret.key``).
+        :param num_workers: Number of worker processes to use (default: ``(cpu_count * 2) + 1``).
         """
 
         super().__init__(**kwargs)
 
         self.port = port
-        self.server_proc = None
+        self._workers: List[Process] = []
         self._service_registry_thread = None
         self.bind_address = bind_address
         self._io_loop: Optional[IOLoop] = None
@@ -227,6 +232,7 @@ class HttpBackend(Backend):
             or os.path.join(Config.get('workdir'), 'flask.secret.key')  # type: ignore
         )
         self.local_base_url = f'http://localhost:{self.port}'
+        self.num_workers = num_workers or (multiprocessing.cpu_count() * 2) + 1
 
     def send_message(self, *_, **__):
         self.logger.warning('Use cURL or any HTTP client to query the HTTP backend')
@@ -240,18 +246,27 @@ class HttpBackend(Backend):
             self._io_loop.stop()
             self._io_loop.close()
 
-        if self.server_proc:
-            self.server_proc.terminate()
-            self.server_proc.join(timeout=10)
-            if self.server_proc.is_alive():
-                self.server_proc.kill()
-            if self.server_proc.is_alive():
+        workers = self._workers.copy()
+        for worker in workers:
+            worker.terminate()
+
+        stop_timeout = 5
+        wait_start_time = time.time()
+        wait_max_time = wait_start_time + stop_timeout
+
+        for i, worker in enumerate(workers[::-1]):
+            worker.join(timeout=max(0, wait_max_time - time.time()))
+            if worker.is_alive():
+                worker.kill()
+
+            if worker.is_alive():
                 self.logger.info(
-                    'HTTP server process may be still alive at termination'
+                    'HTTP worker %s may be still alive at termination', worker.name
                 )
             else:
-                self.logger.info('HTTP server process terminated')
+                self._workers.pop(i)
 
+        self.logger.info('HTTP workers terminated')
         if self._service_registry_thread and self._service_registry_thread.is_alive():
             self._service_registry_thread.join(timeout=5)
             self._service_registry_thread = None
@@ -282,33 +297,28 @@ class HttpBackend(Backend):
             raise e
 
     def _web_server_proc(self):
-        def proc():
-            self.logger.info('Starting local web server on port %s', self.port)
-            assert isinstance(
-                self.bus, RedisBus
-            ), 'The HTTP backend only works if backed by a Redis bus'
+        assert isinstance(
+            self.bus, RedisBus
+        ), 'The HTTP backend only works if backed by a Redis bus'
 
-            application.config['redis_queue'] = self.bus.redis_queue
-            application.secret_key = self._get_secret_key()
+        application.config['redis_queue'] = self.bus.redis_queue
+        application.secret_key = self._get_secret_key()
+        container = WSGIContainer(application)
+        server = Application(
+            [
+                *[(route.path(), route) for route in get_ws_routes()],
+                (r'.*', FallbackHandler, {'fallback': container}),
+            ]
+        )
 
-            container = WSGIContainer(application)
-            server = Application(
-                [
-                    *[(route.path(), route) for route in get_ws_routes()],
-                    (r'.*', FallbackHandler, {'fallback': container}),
-                ]
-            )
+        server.listen(address=self.bind_address, port=self.port, reuse_port=True)
+        self._io_loop = IOLoop.instance()
 
-            server.listen(address=self.bind_address, port=self.port)
-            self._io_loop = IOLoop.instance()
-
-            try:
-                self._io_loop.start()
-            except Exception as e:
-                if not self.should_stop():
-                    raise e
-
-        return proc
+        try:
+            self._io_loop.start()
+        except Exception as e:
+            if not self.should_stop():
+                raise e
 
     def _register_service(self):
         try:
@@ -325,9 +335,22 @@ class HttpBackend(Backend):
         self._service_registry_thread.start()
 
     def _run_web_server(self):
-        self.server_proc = Process(target=self._web_server_proc(), name='WebServer')
-        self.server_proc.start()
-        self.server_proc.join()
+        self.logger.info(
+            'Starting local web server on port %s with %d service workers',
+            self.port,
+            self.num_workers,
+        )
+
+        workers = [
+            Process(target=self._web_server_proc, name=f'WebWorker#{i + 1}')
+            for i in range(self.num_workers)
+        ]
+
+        for worker in workers:
+            worker.start()
+
+        for worker in workers:
+            worker.join()
 
     def run(self):
         super().run()