Fix MQTT re-subscription logic (#18953)

* Fix MQTT re-subscription logic

* Cleanup

* Lint

* Fix

homeassistant/components/mqtt/subscription.py

@@ -5,45 +5,90 @@ For more details about this component, please refer to the documentation at
 https://home-assistant.io/components/mqtt/
 """
 import logging
+from typing import Any, Callable, Dict, Optional
+
+import attr
 
 from homeassistant.components import mqtt
-from homeassistant.components.mqtt import DEFAULT_QOS
-from homeassistant.loader import bind_hass
+from homeassistant.components.mqtt import DEFAULT_QOS, MessageCallbackType
 from homeassistant.helpers.typing import HomeAssistantType
+from homeassistant.loader import bind_hass
 
 _LOGGER = logging.getLogger(__name__)
 
 
+@attr.s(slots=True)
+class EntitySubscription:
+    """Class to hold data about an active entity topic subscription."""
+
+    topic = attr.ib(type=str)
+    message_callback = attr.ib(type=MessageCallbackType)
+    unsubscribe_callback = attr.ib(type=Optional[Callable[[], None]])
+    qos = attr.ib(type=int, default=0)
+    encoding = attr.ib(type=str, default='utf-8')
+
+    async def resubscribe_if_necessary(self, hass, other):
+        """Re-subscribe to the new topic if necessary."""
+        if not self._should_resubscribe(other):
+            return
+
+        if other is not None and other.unsubscribe_callback is not None:
+            other.unsubscribe_callback()
+
+        if self.topic is None:
+            # We were asked to remove the subscription or not to create it
+            return
+
+        self.unsubscribe_callback = await mqtt.async_subscribe(
+            hass, self.topic, self.message_callback,
+            self.qos, self.encoding
+        )
+
+    def _should_resubscribe(self, other):
+        """Check if we should re-subscribe to the topic using the old state."""
+        if other is None:
+            return True
+
+        return (self.topic, self.qos, self.encoding) != \
+            (other.topic, other.qos, other.encoding)
+
+
 @bind_hass
-async def async_subscribe_topics(hass: HomeAssistantType, sub_state: dict,
-                                 topics: dict):
+async def async_subscribe_topics(hass: HomeAssistantType,
+                                 new_state: Optional[Dict[str,
+                                                          EntitySubscription]],
+                                 topics: Dict[str, Any]):
     """(Re)Subscribe to a set of MQTT topics.
 
-    State is kept in sub_state.
+    State is kept in sub_state and a dictionary mapping from the subscription
+    key to the subscription state.
+
+    Please note that the sub state must not be shared between multiple
+    sets of topics. Every call to async_subscribe_topics must always
+    contain _all_ the topics the subscription state should manage.
     """
-    cur_state = sub_state if sub_state is not None else {}
-    sub_state = {}
-    for key in topics:
-        topic = topics[key].get('topic', None)
-        msg_callback = topics[key].get('msg_callback', None)
-        qos = topics[key].get('qos', DEFAULT_QOS)
-        encoding = topics[key].get('encoding', 'utf-8')
-        topic = (topic, msg_callback, qos, encoding)
-        (cur_topic, unsub) = cur_state.pop(
-            key, ((None, None, None, None), None))
+    current_subscriptions = new_state if new_state is not None else {}
+    new_state = {}
+    for key, value in topics.items():
+        # Extract the new requested subscription
+        requested = EntitySubscription(
+            topic=value.get('topic', None),
+            message_callback=value.get('msg_callback', None),
+            unsubscribe_callback=None,
+            qos=value.get('qos', DEFAULT_QOS),
+            encoding=value.get('encoding', 'utf-8'),
+        )
+        # Get the current subscription state
+        current = current_subscriptions.pop(key, None)
+        await requested.resubscribe_if_necessary(hass, current)
+        new_state[key] = requested
 
-        if topic != cur_topic and topic[0] is not None:
-            if unsub is not None:
-                unsub()
-            unsub = await mqtt.async_subscribe(
-                hass, topic[0], topic[1], topic[2], topic[3])
-        sub_state[key] = (topic, unsub)
+    # Go through all remaining subscriptions and unsubscribe them
+    for remaining in current_subscriptions.values():
+        if remaining.unsubscribe_callback is not None:
+            remaining.unsubscribe_callback()
 
-    for key, (topic, unsub) in list(cur_state.items()):
-        if unsub is not None:
-            unsub()
-
-    return sub_state
+    return new_state
 
 
 @bind_hass