Don't block for full state to cache hosts in room when sending events

synapse/handlers/message.py

@@ -1531,13 +1531,19 @@ async def cache_joined_hosts_for_events(
         external federation senders don't have to recalculate it themselves.
         """
 
-        for event, _ in events_and_context:
+        for event, event_context in events_and_context:
             if not self._external_cache.is_enabled():
                 return
 
             # If external cache is enabled we should always have this.
             assert self._external_cache_joined_hosts_updates is not None
 
+            if event_context.partial_state:
+                # We can't precalculate joined hosts for a partial-state event,
+                # (at least not without blocking until full state).
+                # So skip this calculation entirely.
+                continue
+
             # We actually store two mappings, event ID -> prev state group,
             # state group -> joined hosts, which is much more space efficient
             # than event ID -> joined hosts.