Don't return end from /messages if there are no more events (#12903)

Signed-off-by: Jacek Kusnierz <jacek.kusnierz@tum.de>

changelog.d/12903.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug which caused the `/messages` endpoint to return an incorrect `end` attribute when there were no more events. Contributed by @Vetchu.

synapse/handlers/pagination.py

@@ -515,7 +515,16 @@ class PaginationHandler:
 
             next_token = from_token.copy_and_replace(StreamKeyType.ROOM, next_key)
 
-        if events:
+        # if no events are returned from pagination, that implies
+        # we have reached the end of the available events.
+        # In that case we do not return end, to tell the client
+        # there is no need for further queries.
+        if not events:
+            return {
+                "chunk": [],
+                "start": await from_token.to_string(self.store),
+            }
+
         if event_filter:
             events = await event_filter.filter(events)
 
@@ -523,6 +532,8 @@ class PaginationHandler:
             self.storage, user_id, events, is_peeking=(member_event_id is None)
         )
 
+        # if after the filter applied there are no more events
+        # return immediately - but there might be more in next_token batch
         if not events:
             return {
                 "chunk": [],