Add some logging to help track down #13444 (#13679)

changelog.d/13679.misc

@@ -0,0 +1 @@
+Add some logging to help track down #13444.

synapse/federation/sender/__init__.py

@@ -441,6 +441,19 @@ class FederationSender(AbstractFederationSender):
                             destinations = await self._external_cache.get(
                                 "get_joined_hosts", str(sg)
                             )
+                            if destinations is None:
+                                # Add logging to help track down #13444
+                                logger.info(
+                                    "Unexpectedly did not have cached destinations for %s / %s",
+                                    sg,
+                                    event.event_id,
+                                )
+                        else:
+                            # Add logging to help track down #13444
+                            logger.info(
+                                "Unexpectedly did not have cached prev group for %s",
+                                event.event_id,
+                            )
 
                     if destinations is None:
                         try: