The last member of a room cannot `/forget` it after leaving

Description

If you call POST /_matrix/client/v3/rooms/{roomId}/forget the user does not forget the room. This only seems to occur when the user was the last member.

Steps to reproduce

• leave a room
• try to forget the room via /forget or webclient
• get http 200 and nothing happens
• there are no changes in db table room_memberships

If you run Synapse in SQL debug mode you can see the db runs synapse.storage.txn - 691 - DEBUG - POST-148 - [TXN START] {get_filtered_current_state_ids-3c6} but no forget_membership txn.

https://github.com/matrix-org/synapse/blob/e825f7366b589f61977f63d4fdc598abe052e6a0/synapse/handlers/room_member.py#L1912-L1927

https://github.com/matrix-org/synapse/blob/e825f7366b589f61977f63d4fdc598abe052e6a0/synapse/storage/databases/main/roommember.py#L1478-L1499

I have added two lines for debugging in room_member.py

        logger.info("add info: %s", (membership,))
        logger.info("add info: %s", (member,))

• If you are member of the room, member is a FrozenEventV3 and membership join. Synapse raises http 400. 😄
• If you have left the room, member and membership is None. Nothing happens, only http 200 😢
  • If the user was the last member of the room
• If the user was not the last member, member is a FrozenEventV3 and membership leave

Homeserver

another homeserver

Synapse Version

Synapse/1.65.0rc1

Installation Method

Debian packages from packages.matrix.org

Platform

vm: Ubuntu 20.04.4 LTS

Relevant log output

see above

Anything else that would be useful to know?

Discovered while working on https://github.com/matrix-org/synapse/pull/13503#discussion_r944192792

[H-Shay]

This is reproducible-I wrote a unit test to reproduce this, and did some debugging. I suspect that after the last member leaves the room, the background update _background_remove_left_rooms is deleting rows related to this room from the table current_state_events. Thus when the /forget request executes this line:

member = await self._storage_controllers.state.get_current_state_event(
            room_id=room_id, event_type=EventTypes.Member, state_key=user_id
        )

the return value for get_current_state_events is None, as the table current_state_events now has no rows related to the room. I did verify that after the last member leaves the room, get_current_state_events no longer retains any rows from this room.

[H-Shay]

I didn’t create a PR, it was just some hacky code to try and reproduce the situation. If it’s helpful, here it is:

    def test_leave_and_forget(self):
        self.helper.join(self.room_id, user=self.second_user_id, tok=self.second_tok)
        self.helper.leave(self.room_id, user=self.creator, tok=self.creator_tok)
        self.helper.leave(self.room_id, user=self.second_user_id, tok=self.second_tok)

        channel = self.make_request(
            "POST",
            f"/_matrix/client/v3/rooms/{self.room_id}/forget",
            access_token=self.second_tok,
        )

        self.assertEqual(channel.code, HTTPStatus.OK, channel.result)
        self.assertEqual(self.get_success(
            self.hs.get_datastores().main.did_forget(self.second_user_id, self.room_id)), True)