Disable caching in /sync corner case (#16080)

Fixes #15502

changelog.d/16080.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bu in `/sync` where timeout=0 does not skip caching, resulting in slow calls in cases where there are no new changes. Contributed by @PlasmaIntec.

synapse/handlers/sync.py

@@ -387,16 +387,16 @@ async def current_sync_callback(
                 from_token=since_token,
             )
 
-            # if nothing has happened in any of the users' rooms since /sync was called,
-            # the resultant next_batch will be the same as since_token (since the result
-            # is generated when wait_for_events is first called, and not regenerated
-            # when wait_for_events times out).
-            #
-            # If that happens, we mustn't cache it, so that when the client comes back
-            # with the same cache token, we don't immediately return the same empty
-            # result, causing a tightloop. (#8518)
-            if result.next_batch == since_token:
-                cache_context.should_cache = False
+        # if nothing has happened in any of the users' rooms since /sync was called,
+        # the resultant next_batch will be the same as since_token (since the result
+        # is generated when wait_for_events is first called, and not regenerated
+        # when wait_for_events times out).
+        #
+        # If that happens, we mustn't cache it, so that when the client comes back
+        # with the same cache token, we don't immediately return the same empty
+        # result, causing a tightloop. (#8518)
+        if result.next_batch == since_token:
+            cache_context.should_cache = False
 
         if result:
             if sync_config.filter_collection.lazy_load_members():