2 files changed, 12 insertions, 0 deletions
diff --git a/changelog.d/4769.misc b/changelog.d/4769.misc
new file mode 100644
index 0000000000..89144b5425
--- /dev/null
+++ b/changelog.d/4769.misc
@@ -0,0 +1 @@
+Add more debug for #4422.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index b4c4d89945..0e504f945d 100644
--- a/synapse/handlers/sync.py
+++ b/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)