Allow any participant's wallet to poll the Coordinator for a Spend transaction

[edouardparis]

Would be nice if the revaultd of a stakholder retrieves spend transaction from coordinator when it watch vaults unvaulting.
The stakeholder would have then more information of spending attempts.
Same for a manager who did not received the spend transaction during the sign round and has the notification that vaults are unvaulting.

[darosior]

That would be a pretty nice feature to have, at least at first sight. My last coordinator refactoring (coming soon ™️ contains a TODO for this, and the patch for supporting get_spend_tx from a revaultd would be a trivial one liner).

However i'd like to raise two concerns with doing this:

1. Extensibility. Not all Revault deployments need Cosigning Servers and in fact i believe most use cases during the first years will be with policies which don't (eg with policies only on the unvaulted amount, or with pre-signed Spend transactions). This would make this impossible.
2. It may just not be here, or wrong. The Coordinator isn't trusted and therefore we can't assume it's going to serve it to us or that it will serve us a right one. While the watchtowers will be prepared to this, if this is relied-upon by the GUI, this could lead to nasty UX.