Users whose servers were unreachable when you logged in will send you undecryptable messages

[richvdh]

• Alice logs in on a new device
• Alice's server tries to tell everyone about her new device
• Bob's server is unreachable at that moment
• Later, Bob sends a message. He doesn't know about Alice's new device. Alice sees a UISI.

[richvdh]

It's hard to see what to do about this - Alice doesn't know that Bob's server is now reachable (if, indeed, it is), and Bob has no way to know about Alice's new device.

One possibility might be for Alice's server to tell Bob's server about the new device in the response to the to-device message.

[richvdh]

One possibility might be for Alice's server to tell Bob's server about the new device in the response to the to-device message.

What to-device message? Bob may not send Alice a to-device message if he doesn't know about any of her devices (though we could potentially solve that by making sure that Bob's server sends a to-device message to every other server in the room).

In any case, if Bob and Alice are having connectivity issues, then any to-device message may not get through.

So this might solve the problem in the situation that Bob was just restarting his server or having some other blip. But a more general solution ends up looking like Alice needing to ask for history in the room - ie https://github.com/vector-im/riot-web/issues/2286.

[richvdh]

This still exists, even though we now don't rely on to-device messages for this.