Implement option_simple_close
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This feature adds two new messages: - `closing_complete` sent after exchanging `shutdown` - `closing_sig` sent in response to `closing_complete`
The spec allows the closer to use an OP_RETURN output if their amount is too low when using `option_simple_close`.
We introduce a new `NEGOTIATING_SIMPLE` state where we exchange the `closing_complete` and `closing_sig` messages, and allow RBF-ing previous transactions and updating our closing script. We stay in that state until one of the transactions confirms, or a force close is detected. This is important to ensure we're able to correctly reconnect and negotiate RBF candidates. We keep this separate from the previous NEGOTIATING state to make it easier to remove support for the older mutual close protocols once we're confident the network has been upgraded.
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Whenever one side sends `shutdown`, we restart a signing round from scratch. To be compatible with future taproot channels, we require the receiver to also send `shutdown` before moving on to exchanging `closing_complete` and `closing_sig`. This will give nodes a message to exchange fresh musig2 nonces before producing signatures. On reconnection, we also restart a signing session from scratch and discard pending partial signatures.
Whenever we exchange `shutdown`, we now require that new closing txs are signed before allowing another `shutdown` message to be sent to start a new signing round. This creates more risk of deadlock when one side fails to send their sigs, where we'll need to disconnect to start a new signing round. But that shouldn't happen if nodes are honest and not buggy, so it probably doesn't matter. If nodes are buggy or malicious, we will need to force-close anyway.
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Implement
option_simple_closeas defined in lightning/bolts#1096Hopefully this is the last time we change the mutual close protocol! And at some point that will let us entirely remove all the code supporting the two previous mutual close protocols (this is why I kept the code as separate as possible instead of trying to fit into the existing
NEGOTIATINGstate).Note that this is a prerequisite for taproot channels: this protocol allows nodes to safely exchange nonces in
shutdownandclosing_completeto spend a musig2 channel output.