Fix overcounting of pushers when they are replaced (#13296)

Signed-off-by: Sean Quah <seanq@matrix.org>

changelog.d/13296.bugfix

+Fix a bug introduced in v1.18.0 where the `synapse_pushers` metric would overcount pushers when they are replaced.

synapse/push/pusherpool.py

@@ -328,7 +328,7 @@ class PusherPool:
             return None
 
         try:
-            p = self.pusher_factory.create_pusher(pusher_config)
+            pusher = self.pusher_factory.create_pusher(pusher_config)
         except PusherConfigException as e:
             logger.warning(
                 "Pusher incorrectly configured id=%i, user=%s, appid=%s, pushkey=%s: %s",
@@ -346,23 +346,28 @@ class PusherPool:
             )
             return None
 
-        if not p:
+        if not pusher:
             return None
 
-        appid_pushkey = "%s:%s" % (pusher_config.app_id, pusher_config.pushkey)
+        appid_pushkey = "%s:%s" % (pusher.app_id, pusher.pushkey)
 
-        byuser = self.pushers.setdefault(pusher_config.user_name, {})
+        byuser = self.pushers.setdefault(pusher.user_id, {})
         if appid_pushkey in byuser:
-            byuser[appid_pushkey].on_stop()
-        byuser[appid_pushkey] = p
+            previous_pusher = byuser[appid_pushkey]
+            previous_pusher.on_stop()
 
-        synapse_pushers.labels(type(p).__name__, p.app_id).inc()
+            synapse_pushers.labels(
+                type(previous_pusher).__name__, previous_pusher.app_id
+            ).dec()
+        byuser[appid_pushkey] = pusher
+
+        synapse_pushers.labels(type(pusher).__name__, pusher.app_id).inc()
 
         # Check if there *may* be push to process. We do this as this check is a
         # lot cheaper to do than actually fetching the exact rows we need to
         # push.
-        user_id = pusher_config.user_name
-        last_stream_ordering = pusher_config.last_stream_ordering
+        user_id = pusher.user_id
+        last_stream_ordering = pusher.last_stream_ordering
         if last_stream_ordering:
             have_notifs = await self.store.get_if_maybe_push_in_range_for_user(
                 user_id, last_stream_ordering
@@ -372,9 +377,9 @@ class PusherPool:
             # risk missing push.
             have_notifs = True
 
-        p.on_started(have_notifs)
+        pusher.on_started(have_notifs)
 
-        return p
+        return pusher
 
     async def remove_pusher(self, app_id: str, pushkey: str, user_id: str) -> None:
         appid_pushkey = "%s:%s" % (app_id, pushkey)