Disinviting users over federation doesn't work

[turt2live]

Riot issue: element-hq/element-web#7318

As mentioned in element-hq/element-web#7318 (comment), this is a different case from #2181 due to Synapse not sending the disinvite over federation to the target homeserver.

As a simple test case:

1. Create a room on Server A
2. Invite a user from Server B to the room
3. Before they accept, disinvite them
4. That user now has a stuck invite

Server A never notifies Server B of the disinvite (leave event), and therefore Server B can't really do anything about it. If the user tries to respond to the event, Server B could realize that 403 on join (or declination of the invite) means it should be locally rejected. At that point, you're in #2181 territory.

When Server A is modified to make calls to Server B by sending the disinvite in a transaction, Server B accepts the transaction but ignores the event because it cannot determine the room version (and therefore the event format version). A lightly tested and highly unstable branch for modifying Server A to send the event is available here: 16c8b4e...travis/fix-stuck-invites

Patching Server B's behaviour is non-trivial to me, otherwise I'd just fix it. Would be good if this fix was partnered with spec to clarify that disinvite (leave) events should be sent to target hosts, and that target hosts should expect them. I don't think this is introducing anything new, and shouldn't need a dedicated endpoint either.

[turt2live]

worth noting that fixing this would also fix #1563 (probably)

[richvdh]

This is no longer a stuck-invite issue thanks to the fix for #2181; however it's still a bug because the invite should disappear when the user is disinvited.