Add more debug for #4422 (#4769)

changelog.d/4769.misc

@@ -0,0 +1 @@
+Add more debug for #4422.

synapse/handlers/sync.py

@@ -1437,6 +1437,17 @@ class SyncHandler(object):
                     old_mem_ev = yield self.store.get_event(
                         old_mem_ev_id, allow_none=True
                     )
+
+                # debug for #4422
+                if has_join:
+                    prev_membership = None
+                    if old_mem_ev:
+                        prev_membership = old_mem_ev.membership
+                    logger.info(
+                        "Previous membership for room %s with join: %s (event %s)",
+                        room_id, prev_membership, old_mem_ev_id,
+                    )
+
                 if not old_mem_ev or old_mem_ev.membership != Membership.JOIN:
                     newly_joined_rooms.append(room_id)