Fix sync tightloop bug.

changelog.d/5507.bugfix

@@ -0,0 +1 @@
+Fix bug where clients could tight loop calling `/sync` for a period.

synapse/handlers/presence.py

@@ -1017,11 +1017,28 @@ def get_new_events(
             if from_key is not None:
                 from_key = int(from_key)
 
+            max_token = self.store.get_current_presence_token()
+            if from_key == max_token:
+                # This is necessary as due to the way stream ID generators work
+                # we may get updates that have a stream ID greater than the max
+                # token (e.g. max_token is N but stream generator may return
+                # results for N+2, due to N+1 not having finished being
+                # persisted yet).
+                #
+                # This is usually fine, as it just means that we may send down
+                # some presence updates multiple times. However, we need to be
+                # careful that the sync stream either actually does make some
+                # progress or doesn't return, otherwise clients will end up
+                # tight looping calling /sync due to it immediately returning
+                # the same token repeatedly.
+                #
+                # Hence this guard where we just return nothing so that the sync
+                # doesn't return. C.f. #5503.
+                defer.returnValue(([], max_token))
+
             presence = self.get_presence_handler()
             stream_change_cache = self.store.presence_stream_cache
 
-            max_token = self.store.get_current_presence_token()
-
             users_interested_in = yield self._get_interested_in(user, explicit_room_id)
 
             user_ids_changed = set()