Description

Users registering on HS B can't auto-join a room on HS A that's listed in auto_join_rooms if they are either the 1st user on HS B or if there are no users on HS B that are in the room on HS A.

Steps to reproduce

• HS A has a public, federatable room with an alias
• HS B sets auto_join_rooms to include the room on HS A
• If there are no existing users on HS B:
  • 1st user registers
  • Auto-joining skipped with Cannot create room alias #HS-A:room-alias it does not match server domain
• If there are existing users on HS B and no-one on HS B is in this room
  • Registrations after setting auto_join_rooms as per the above fail with Failed to join new user to '#HS-A:room-alias': AssertionError()
  • This repeats until someone manually joins the room on HS A
  • I believe this is because if there's no local copy of the state, then an invite is assumed - https://github.com/matrix-org/synapse/blob/1c8045f67477599fabc5759205c018e44d770078/synapse/handlers/register.py (Lines 453-471)

The expectation is that auto_join_rooms works with any public room local or federated.

Version information

• Homeserver: Any 2 publicly federated servers
• Version: 1.36.0
• Install method: Docker
• Platform: Linux