Error when trying to accept a DM

[deniseal]

Steps to reproduce

When trying to accept a DM from a user on the same server as me (matrix.org) it fails but the error message does not give me useful information on why it is failing or what can be done to fix the issue:

Outcome

I would have expected to be able to accept the DM and join the room. Alternatively if there was a problem (rescinded invite, account inviting me no longer existing, etc) I would have expected an error message saying so.

Operating system

No response

Browser information

Firefox

URL for webapp

app.element.io

Application version

No response

Homeserver

matrix.org

Will you send logs?

Yes

[Half-Shot]

So this error pops up when we see a 404 error while trying to join a room via it's bare roomId, and passed in no viaServers. Unfortunately Element doesn't distinguish the mechanism of how you tried to join the room (e.g. via invite) so it gives you unhelpful advice.

[t3chguy]

@Half-Shot it is actually M_UNKNOWN last I checked.

Will you send logs?
Yes

Not seeing any logs from you

[Half-Shot]

I had a look and we only branch to this error on status 404. PR incoming.

[anoadragon453]

On the homeserver side: the room was created on matrix.org, but the original room creator (a matrix.org user) later left the room.

Since there are no local users in the room, Synapse attempts a remote join. But because there are no via servers provided, Synapse fails with a NoKnownServersError.

As @ara4n mentioned internally, we should really rescind the invite on the homeserver in this case, because the room cannot be joined. This isn't "easy" to do though, as there's many edge cases. For instance, the last user in a room may be PL0, and wouldn't have the ability to rescind all pending invites. A "proper" solution here would probably require an MSC.

[Half-Shot]

From my PoV, #32621 is the best we can do (improving the accuracy of the error) until the spec changes. I am minded towards closing this when that lands, and track the root cause in a spec or synapse issue.