Reduce chan state logic from ChannelManager when disconnecting

After lightningdevkit#3513 we have a bit more encapsulation of channel logic in
channel.rs with channelmanager.rs needing a bit less knowledge of
which specific state a channel is in.

This continues that trend slightly when a peer disconnects.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -1278,9 +1278,18 @@ impl<SP: Deref> Channel<SP> where
 		}
 	}
 
-	pub fn is_resumable(&self) -> bool {
+	/// Should be called when the peer is disconnected. Returns true if the channel can be resumed
+	/// when the peer reconnects (via [`Self::peer_connected_get_handshake`]). If not, the channel
+	/// must be immediately closed.
+	pub fn peer_disconnected_is_resumable<L: Deref>(&mut self, logger: &L) -> bool where L::Target: Logger {
 		match self {
-			Channel::Funded(_) => false,
+			Channel::Funded(chan) => chan.remove_uncommitted_htlcs_and_mark_paused(logger).is_ok(),
+			// If we get disconnected and haven't yet committed to a funding
+			// transaction, we can replay the `open_channel` on reconnection, so don't
+			// bother dropping the channel here. However, if we already committed to
+			// the funding transaction we don't yet support replaying the funding
+			// handshake (and bailing if the peer rejects it), so we force-close in
+			// that case.
 			Channel::UnfundedOutboundV1(chan) => chan.is_resumable(),
 			Channel::UnfundedInboundV1(_) => false,
 			Channel::UnfundedV2(_) => false,
@@ -6050,7 +6059,7 @@ impl<SP: Deref> FundedChannel<SP> where
 	/// No further message handling calls may be made until a channel_reestablish dance has
 	/// completed.
 	/// May return `Err(())`, which implies [`ChannelContext::force_shutdown`] should be called immediately.
-	pub fn remove_uncommitted_htlcs_and_mark_paused<L: Deref>(&mut self, logger: &L) -> Result<(), ()> where L::Target: Logger {
+	fn remove_uncommitted_htlcs_and_mark_paused<L: Deref>(&mut self, logger: &L) -> Result<(), ()> where L::Target: Logger {
 		assert!(!matches!(self.context.channel_state, ChannelState::ShutdownComplete));
 		if self.context.channel_state.is_pre_funded_state() {
 			return Err(())

lightning/src/ln/channelmanager.rs

@@ -11473,26 +11473,10 @@ where
 				let peer_state = &mut *peer_state_lock;
 				let pending_msg_events = &mut peer_state.pending_msg_events;
 				peer_state.channel_by_id.retain(|_, phase| {
-					match phase.as_funded_mut() {
-						Some(chan) => {
-							let logger = WithChannelContext::from(&self.logger, &chan.context, None);
-							if chan.remove_uncommitted_htlcs_and_mark_paused(&&logger).is_ok() {
-								// We only retain funded channels that are not shutdown.
-								return true;
-							}
-						},
-						// If we get disconnected and haven't yet committed to a funding
-						// transaction, we can replay the `open_channel` on reconnection, so don't
-						// bother dropping the channel here. However, if we already committed to
-						// the funding transaction we don't yet support replaying the funding
-						// handshake (and bailing if the peer rejects it), so we force-close in
-						// that case.
-						None => {
-							if phase.is_resumable() {
-								return true;
-							}
-						},
-					};
+					let logger = WithChannelContext::from(&self.logger, &phase.context(), None);
+					if phase.peer_disconnected_is_resumable(&&logger) {
+						return true;
+					}
 					// Clean up for removal.
 					let context = phase.context_mut();
 					let mut close_res = context.force_shutdown(false, ClosureReason::DisconnectedPeer);