Extract method to dedup pre-decode update_add

We have an overarching goal of (mostly) getting rid of ChannelManager
persistence and rebuilding the ChannelManager's state from existing
ChannelMonitors, due to issues when the two structs are out-of-sync on restart.
The main issue that can arise is channel force closure.

We'll use this new util when reconstructing the
ChannelManager::decode_update_add_htlcs map from Channel data in upcoming
commits. While the Channel data is not included in the monitors yet, it will be
in future work.

Author: valentinewallace

lightning/src/ln/channelmanager.rs

@@ -16819,6 +16819,33 @@ where
 	}
 }
 
+// If the HTLC corresponding to `prev_hop_data` is present in `decode_update_add_htlcs`, remove it
+// from the map as it is already being stored and processed elsewhere.
+fn dedup_decode_update_add_htlcs<L: Deref>(
+	decode_update_add_htlcs: &mut HashMap<u64, Vec<msgs::UpdateAddHTLC>>,
+	prev_hop_data: &HTLCPreviousHopData, removal_reason: &'static str, logger: &L,
+) where
+	L::Target: Logger,
+{
+	decode_update_add_htlcs.retain(|src_outb_alias, update_add_htlcs| {
+		update_add_htlcs.retain(|update_add| {
+			let matches = *src_outb_alias == prev_hop_data.prev_outbound_scid_alias
+				&& update_add.htlc_id == prev_hop_data.htlc_id;
+			if matches {
+				let logger = WithContext::from(
+					logger,
+					prev_hop_data.counterparty_node_id,
+					Some(update_add.channel_id),
+					Some(update_add.payment_hash),
+				);
+				log_info!(logger, "Removing pending to-decode HTLC: {}", removal_reason);
+			}
+			!matches
+		});
+		!update_add_htlcs.is_empty()
+	});
+}
+
 // Implement ReadableArgs for an Arc'd ChannelManager to make it a bit easier to work with the
 // SipmleArcChannelManager type:
 impl<
@@ -17686,19 +17713,11 @@ where
 								// still have an entry for this HTLC in `forward_htlcs` or
 								// `pending_intercepted_htlcs`, we were apparently not persisted after
 								// the monitor was when forwarding the payment.
-								decode_update_add_htlcs.retain(
-									|src_outb_alias, update_add_htlcs| {
-										update_add_htlcs.retain(|update_add_htlc| {
-											let matches = *src_outb_alias
-												== prev_hop_data.prev_outbound_scid_alias
-												&& update_add_htlc.htlc_id == prev_hop_data.htlc_id;
-											if matches {
-												log_info!(logger, "Removing pending to-decode HTLC as it was forwarded to the closed channel");
-											}
-											!matches
-										});
-										!update_add_htlcs.is_empty()
-									},
+								dedup_decode_update_add_htlcs(
+									&mut decode_update_add_htlcs,
+									&prev_hop_data,
+									"HTLC was forwarded to the closed channel",
+									&args.logger,
 								);
 								forward_htlcs.retain(|_, forwards| {
 									forwards.retain(|forward| {