Don't double cancel verification request

[bwindels]

This fixes the verifier not handling incoming m.key.verification.cancel events, and just treating them as unexpected and sending a m.key.verification.cancel event of its own with an error. These double m.key.verification.cancel events make it harder to detect which user actually pressed cancel.

Initial PR for verification over DM was #1050

[uhoreg]

I think that if you just do a this.cancel(e), then it should do the right thing, because the cancel function says that it handles cancellations from the other user (if it gets passed a MatrixEvent). Though we probably also want to call this._rejectEvent if it's defined, so it may be better to just stick this in with the else clause, and assign to exception based on whether e.getType() === "m.key.verification.cancel".

Though it looks like the else clause isn't doing the right thing right now, because it really should be causing the client to send an m.key.verification.cancel with code set to m.unexpected_message. So, I think if you do something like

const exception = e.getType() === "m.key.verification.cancel" ? e : newUnexpectedMessageError();

in the else clause (you'll need to import newUnexpectedMessageError from Error.js), then it should fix that and the double-cancel issue.

[bwindels]

Hmm, I tried calling _rejectEvent, but that made it send an m.key.verification.cancel event like before. Will have another look