synapse Faster room joins: `device_lists.left` not emitted in `/sync` response when completing the join and discovering that a user is actually not in the room

See the commented out check in the TestPartialStateJoin/Device_list_tracking/Device_list_no_longer_tracked_for_user_incorrectly_believed_to_be_in_room complement test.

That test case is roughly:

A room starts with @charlie:remote, @derek:remote (moderator) and @fred:remote (admin)
@fred:remote leaves the room.
@alice:local partial state joins the room.
@elsie:remote joins the room.
@fred:remote "bans" @derek:remote, citing their join event as auth. The local homeserver does not know that @fred:remote is no longer in the room and accepts the event.
@derek:remote kicks @elsie:remote. The local homeserver incorrectly rejects the kick because it thinks @derek:remote has been banned.
The partial state join completes and @elsie:remote is discovered to really have been kicked.
No device_lists.left entry is emitted.

Sep 23 '22 12:09 squahtx

This bug likely also applies to /keys/changes both ways (missing changed when discovering a user is actually in a room and missing left when a user is actually not in a room). We only have a complement test for the missing left scenario.

Sep 23 '22 16:09 squahtx

This is likely already an issue if the user is state-reset out.