New client ID generation logic (closes #205)

MQTT client IDs are now generated as a function of
`(client_id, host, port, topics, on_message)` to
prevent client ID clashes.

platypush/backend/mqtt/__init__.py

@@ -1,3 +1,4 @@
+import hashlib
 import json
 import os
 import threading
@@ -190,7 +191,7 @@ class MqttBackend(Backend):
 
         self.topic = '{}/{}'.format(topic, self.device_id)
         self.subscribe_default_topic = subscribe_default_topic
-        self._listeners = {}  # (host, port, msg_handler) -> MqttClient map
+        self._listeners = {}  # client_id -> MqttClient map
         self.listeners_conf = listeners or []
 
     def send_message(self, msg, topic: Optional[str] = None, **kwargs):
@@ -248,22 +249,34 @@ class MqttBackend(Backend):
             if not client.is_alive():
                 client.start()
 
+    def _get_client_id(
+            self, host: str, port: int, topics: Optional[List[str]] = None,
+            client_id: Optional[str] = None, on_message: Optional[bool] = None,
+    ) -> str:
+        return '{client_id}-{client_hash}'.format(
+            client_id=client_id or self.client_id,
+            client_hash=hashlib.sha1('|'.join([
+                host, str(port),
+                json.dumps(sorted(topics or [])),
+                str(id(on_message))
+            ]).encode()).hexdigest(),
+        )
+
     def _get_client(self, host: str, port: int, topics: Optional[List[str]] = None, username: Optional[str] = None,
                     password: Optional[str] = None, client_id: Optional[str] = None, tls_cafile: Optional[str] = None,
                     tls_certfile: Optional[str] = None, tls_keyfile: Optional[str] = None, tls_version: Optional = None,
                     tls_ciphers: Optional = None, tls_insecure: bool = False, on_message: Optional[Callable] = None) \
             -> MqttClient:
-        on_message = on_message or self.on_mqtt_message()
+        client_id = self._get_client_id(host=host, port=port, topics=topics, client_id=client_id, on_message=on_message)
-        on_message_name = repr(on_message)
+        client = self._listeners.get(client_id)
-        client = self._listeners.get((host, port, on_message_name))
 
         if not (client and client.is_alive()):
-            client = MqttClient(host=host, port=port, topics=topics, username=username, password=password,
+            client = self._listeners[client_id] = MqttClient(
+                host=host, port=port, topics=topics, username=username, password=password,
                 client_id=client_id, tls_cafile=tls_cafile, tls_certfile=tls_certfile,
                 tls_keyfile=tls_keyfile, tls_version=tls_version, tls_ciphers=tls_ciphers,
-                                tls_insecure=tls_insecure, on_message=on_message)
+                tls_insecure=tls_insecure, on_message=on_message
-
+            )
-            self._listeners[(host, port, on_message_name)] = client
 
         client.subscribe(*topics)
         return client
@@ -343,13 +356,11 @@ class MqttBackend(Backend):
     def on_stop(self):
         self.logger.info('Received STOP event on the MQTT backend')
 
-        for ((host, port, _), listener) in self._listeners.items():
+        for listener in self._listeners.values():
             try:
                 listener.stop()
             except Exception as e:
-                # noinspection PyProtectedMember
+                self.logger.warning(f'Could not stop MQTT listener: {e}')
-                self.logger.warning('Could not stop listener {host}:{port}: {error}'.
-                                    format(host=host, port=port, error=str(e)))
 
         self.logger.info('MQTT backend terminated')