f - move logic to be after on-chain claim checks

Author: alecchendev

lightning/src/chain/channelmonitor.rs

@@ -3662,6 +3662,63 @@ impl<Signer: WriteableEcdsaChannelSigner> ChannelMonitorImpl<Signer> {
 			}
 		}
 
+		// Fail back HTLCs on backwards channels if they expire within `LATENCY_GRACE_PERIOD_BLOCKS`
+		// blocks. If we haven't seen the preimage for an HTLC by the time the previous hop's
+		// timeout expires, we've lost that HTLC, so we might as well fail it back instead of having our
+		// counterparty force-close the channel.
+		let current_holder_htlcs = self.current_holder_commitment_tx.htlc_outputs.iter()
+			.map(|&(ref a, _, ref b)| (a, b.as_ref()));
+
+		let current_counterparty_htlcs = if let Some(txid) = self.current_counterparty_commitment_txid {
+			if let Some(htlc_outputs) = self.counterparty_claimable_outpoints.get(&txid) {
+				Some(htlc_outputs.iter().map(|&(ref a, ref b)| (a, b.as_ref().map(|boxed| &**boxed))))
+			} else { None }
+		} else { None }.into_iter().flatten();
+
+		let prev_counterparty_htlcs = if let Some(txid) = self.prev_counterparty_commitment_txid {
+			if let Some(htlc_outputs) = self.counterparty_claimable_outpoints.get(&txid) {
+				Some(htlc_outputs.iter().map(|&(ref a, ref b)| (a, b.as_ref().map(|boxed| &**boxed))))
+			} else { None }
+		} else { None }.into_iter().flatten();
+
+		let htlcs = current_holder_htlcs
+			.chain(current_counterparty_htlcs)
+			.chain(prev_counterparty_htlcs);
+
+		let height = self.best_block.height();
+		for (htlc, source_opt) in htlcs {
+			// Only check forwarded HTLCs' previous hops
+			let source = match source_opt {
+				Some(source) => source,
+				None => continue,
+			};
+			let (cltv_expiry, htlc_id) = match source {
+				HTLCSource::PreviousHopData(HTLCPreviousHopData {
+					htlc_id, cltv_expiry: Some(cltv_expiry), ..
+				}) if !self.failed_back_htlc_ids.contains(htlc_id) => (*cltv_expiry, *htlc_id),
+				_ => continue,
+			};
+			if cltv_expiry <= height + LATENCY_GRACE_PERIOD_BLOCKS {
+				let duplicate_event = self.pending_monitor_events.iter().any(
+					|update| if let &MonitorEvent::HTLCEvent(ref upd) = update {
+						upd.source == *source
+					} else { false });
+				if !duplicate_event {
+					log_debug!(logger, "Failing back HTLC {} upstream to preserve the \
+						channel as the forward HTLC hasn't resolved and our backward HTLC \
+						expires soon at {}", log_bytes!(htlc.payment_hash.0), cltv_expiry);
+					self.pending_monitor_events.push(MonitorEvent::HTLCEvent(HTLCUpdate {
+						source: source.clone(),
+						payment_preimage: None,
+						payment_hash: htlc.payment_hash,
+						htlc_value_satoshis: Some(htlc.amount_msat / 1000),
+						awaiting_downstream_confirmation: true,
+					}));
+					self.failed_back_htlc_ids.push(htlc_id);
+				}
+			}
+		}
+
 		self.onchain_tx_handler.update_claims_view_from_requests(claimable_outpoints, conf_height, self.best_block.height(), broadcaster, fee_estimator, logger);
 		self.onchain_tx_handler.update_claims_view_from_matched_txn(&txn_matched, conf_height, conf_hash, self.best_block.height(), broadcaster, fee_estimator, logger);