Don't lock up when joining large rooms (#16903)

Co-authored-by: Andrew Morgan <andrew@amorgan.xyz>

changelog.d/16903.bugfix

@@ -0,0 +1 @@
+Fix performance issue when joining very large rooms that can cause the server to lock up. Introduced in v1.100.0.

synapse/handlers/federation_event.py

@@ -1757,13 +1757,21 @@ class FederationEventHandler:
 
             events_and_contexts_to_persist.append((event, context))
 
-        for event in sorted_auth_events:
+        for i, event in enumerate(sorted_auth_events):
             await prep(event)
 
+            # The above function is typically not async, and so won't yield to
+            # the reactor. For large rooms let's yield to the reactor
+            # occasionally to ensure we don't block other work.
+            if (i + 1) % 1000 == 0:
+                await self._clock.sleep(0)
+
+        # Also persist the new event in batches for similar reasons as above.
+        for batch in batch_iter(events_and_contexts_to_persist, 1000):
             await self.persist_events_and_notify(
                 room_id,
-            events_and_contexts_to_persist,
+                batch,
-            # Mark these events backfilled as they're historic events that will
+                # Mark these events as backfilled as they're historic events that will
                 # eventually be backfilled. For example, missing events we fetch
                 # during backfill should be marked as backfilled as well.
                 backfilled=True,