Detect and fail-back monitor-blocked un-forwarded HTLCs at close

[TheBlueMatt]

If we have pending HTLCs which we intended to forward, but which were waiting on a ChannelMonitorUpdate to be forwarded when we closed, they will neither be in the ChannelMonitor nor in the Channel in a state which indicates they need to be failed (i.e. in the holding cell). As a result, we previously did not fail such HTLCs back. Note that the catch-all
fail-back-before-channel-closure logic is run by the ChannelMonitor so is also unaware of these HTLCs.

Here we fix this by detecting the specific case - HTLCs which are in LocalSent (i.e. the counterparty has not provided an RAA yet) and we have a blocked ChannelMonitorUpdate containing a remote commitment transaction update (which will always contain the HTLC).

In such a case, we can be confident the counterparty does not have a commitment transaction containing the HTLC, and can fail it back immediately.

[ldk-reviews-bot]

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[codecov]

Codecov Report

❌ Patch coverage is 88.42105% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.98%. Comparing base (e01663a) to head (7c39480).
⚠️ Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	74.07%	6 Missing and 1 partial ⚠️
lightning/src/ln/shutdown_tests.rs	94.11%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3989      +/-   ##
==========================================
+ Coverage   88.97%   88.98%   +0.01%     
==========================================
  Files         174      174              
  Lines      124161   124394     +233     
  Branches   124161   124394     +233     
==========================================
+ Hits       110470   110693     +223     
- Misses      11216    11219       +3     
- Partials     2475     2482       +7     

Flag	Coverage Δ
fuzzing	22.22% <7.40%> (-0.43%)	⬇️
tests	88.81% <88.42%> (+0.01%)	⬆️

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[tankyleo]

Quick first pass

[ldk-reviews-bot]

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[TheBlueMatt]

Fixed handling the missing monitor update type:

$ git diff-tree -U2 34abcf6603^1 7c394804b0
diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index 5a95295f8f..115b917649 100644
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -5351,25 +5351,33 @@ where
 				for update in self.blocked_monitor_updates.iter() {
 					for update in update.update.updates.iter() {
-						match update {
-							ChannelMonitorUpdateStep::LatestCounterpartyCommitmentTXInfo {
-								htlc_outputs,
+						let have_htlc = match update {
+							ChannelMonitorUpdateStep::LatestCounterpartyCommitment {
+								htlc_data,
 								..
 							} => {
-								let have_htlc = htlc_outputs.iter().any(|(_, source)| {
-									source.as_ref().map(|s| &**s) == Some(&htlc.source)
-								});
-								debug_assert!(have_htlc);
-								if have_htlc {
-									dropped_outbound_htlcs.push((
-										htlc.source.clone(),
-										htlc.payment_hash,
-										counterparty_node_id,
-										self.channel_id,
-									));
-								}
-								continue 'htlc_iter;
+								let dust =
+									htlc_data.dust_htlcs.iter().map(|(_, source)| source.as_ref());
+								let nondust =
+									htlc_data.nondust_htlc_sources.iter().map(|s| Some(s));
+								dust.chain(nondust).any(|source| source == Some(&htlc.source))
 							},
-							_ => {},
+							ChannelMonitorUpdateStep::LatestCounterpartyCommitmentTXInfo {
+								htlc_outputs,
+								..
+							} => htlc_outputs.iter().any(|(_, source)| {
+								source.as_ref().map(|s| &**s) == Some(&htlc.source)
+							}),
+							_ => continue,
+						};
+						debug_assert!(have_htlc);
+						if have_htlc {
+							dropped_outbound_htlcs.push((
+								htlc.source.clone(),
+								htlc.payment_hash,
+								counterparty_node_id,
+								self.channel_id,
+							));
 						}
+						continue 'htlc_iter;
 					}
 				}