Don't generate a `ChannelMonitorUpdate` for closed chans on shutdown

The `Channel::get_shutdown` docs are very clear - if the channel
jumps to `Shutdown` as a result of not being funded when we go to
initiate shutdown we should not generate a `ChannelMonitorUpdate`
as there's no need to bother with the shutdown script - we're
force-closing anyway.

However, this wasn't actually implemented, potentially causing a
spurious monitor update for no reason.

diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index 515e10264fac63070d0588d9b0a3c57471d27eeb..e3641a6204a96d6685098a86ba6fc9d67309e15d 100644 (file)
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -5965,9 +5965,16 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
                        return Err(APIError::ChannelUnavailable{err: "Cannot begin shutdown while peer is disconnected or we're waiting on a monitor update, maybe force-close instead?".to_owned()});
                }
 
+               // If we haven't funded the channel yet, we don't need to bother ensuring the shutdown
+               // script is set, we just force-close and call it a day.
+               let mut chan_closed = false;
+               if self.channel_state < ChannelState::FundingSent as u32 {
+                       chan_closed = true;
+               }
+
                let update_shutdown_script = match self.shutdown_scriptpubkey {
                        Some(_) => false,
-                       None => {
+                       None if !chan_closed => {
                                let shutdown_scriptpubkey = signer_provider.get_shutdown_scriptpubkey();
                                if !shutdown_scriptpubkey.is_compatible(their_features) {
                                        return Err(APIError::IncompatibleShutdownScript { script: shutdown_scriptpubkey.clone() });
@@ -5975,6 +5982,7 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
                                self.shutdown_scriptpubkey = Some(shutdown_scriptpubkey);
                                true
                        },
+                       None => false,
                };
 
                // From here on out, we may not fail!