Fix message duplication if something goes wrong after persisting the event (#8476)

Should fix #3365.

1 files changed, 12 insertions, 1 deletions

diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index ffbc62ff44..0080eeaf8d 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -171,6 +171,17 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         if requester.is_guest:
             content["kind"] = "guest"
 
+        # Check if we already have an event with a matching transaction ID. (We
+        # do this check just before we persist an event as well, but may as well
+        # do it up front for efficiency.)
+        if txn_id and requester.access_token_id:
+            existing_event_id = await self.store.get_event_id_from_transaction_id(
+                room_id, requester.user.to_string(), requester.access_token_id, txn_id,
+            )
+            if existing_event_id:
+                event_pos = await self.store.get_position_for_event(existing_event_id)
+                return existing_event_id, event_pos.stream
+
         event, context = await self.event_creation_handler.create_event(
             requester,
             {
@@ -679,7 +690,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
             if is_blocked:
                 raise SynapseError(403, "This room has been blocked on this server")
 
-        await self.event_creation_handler.handle_new_client_event(
+        event = await self.event_creation_handler.handle_new_client_event(
             requester, event, context, extra_users=[target_user], ratelimit=ratelimit
         )