Unban state propagation isn't explained/is nonexistent for non-participating servers

[ShadowJonathan]

Problem area

Server behaviour surrounding unbans.

Related bits;

• Transactions
• C-S /unban

Issue

Basically, if a one-user server (or a server of which only one user is involved) gets banned from a room, and then unbanned, then the unban state is not propagated to that server in any way.

This was originally raised as a synapse issue, but I believe this is a wider problem for the federation; Currently, on unban, the unbanned server is not "notified" of the ban, which may lead to wedging if it fails-fast on local state. Neither is there any documentation regarding if servers from unbanned users can fetch state regarding their own users' join state (in any way).

I believe this can be rectified by having the unbanning server do/queue a txn /send to the server in question, which'd allow it to recover and reset its internal status.

Related: matrix-org/synapse#1563

[timokoesters]

I believe this can be rectified by having the unbanning server do/queue a txn /send to the server in question, which'd allow it to recover and reset its internal status.

The problem with this is that the banned server potentially has to do a lot of state res and event handling just to stay up to date with a room it is not even in. I think the better solution would be to have the banned server do a proper /make_join /send_join to rejoin the room.

In general: Every time server wants to (re-)join a room it has no members in, it should do a /make_join, /send_join

[ShadowJonathan]

In general: Every time server wants to (re-)join a room it has no members in, it should do a /make_join, /send_join

Thanks, I agree, but if this is the intention, it should have wording in the spec, currently there's just none.

[ShadowJonathan]

MSC2314 looks related here as well.