From: Matt Corallo <git@bluematt.me>
Date: Mon, 9 Jan 2023 18:42:34 +0000 (+0000)
Subject: Allow htlc_satisfies_config to override the amount to forward
X-Git-Url: http://git.bitcoin.ninja/index.cgi?a=commitdiff_plain;h=657c75aa1f6260aebd573de9a81d84de57a7ead2;p=rust-lightning

Allow htlc_satisfies_config to override the amount to forward

This allows us to calculate implicit fees before forwarding.
---

diff --git a/lightning/src/ln/channel.rs b/lightning/src/ln/channel.rs
index 29f1a30a6..583a98ec6 100644
--- a/lightning/src/ln/channel.rs
+++ b/lightning/src/ln/channel.rs
@@ -4724,7 +4724,7 @@ impl<Signer: Sign> Channel<Signer> {
 
 	fn internal_htlc_satisfies_config(
 		&self, htlc: &msgs::UpdateAddHTLC, amt_to_forward: u64, outgoing_cltv_value: u32, config: &ChannelConfig,
-	) -> Result<(), (&'static str, u16)> {
+	) -> Result<u64, (&'static str, u16)> {
 		let fee = amt_to_forward.checked_mul(config.forwarding_fee_proportional_millionths as u64)
 			.and_then(|prop_fee| (prop_fee / 1000000).checked_add(config.forwarding_fee_base_msat as u64));
 		if fee.is_none() || htlc.amount_msat < fee.unwrap() ||
@@ -4740,15 +4740,17 @@ impl<Signer: Sign> Channel<Signer> {
 				0x1000 | 13, // incorrect_cltv_expiry
 			));
 		}
-		Ok(())
+		Ok(amt_to_forward)
 	}
 
 	/// Determines whether the parameters of an incoming HTLC to be forwarded satisfy the channel's
 	/// [`ChannelConfig`]. This first looks at the channel's current [`ChannelConfig`], and if
 	/// unsuccessful, falls back to the previous one if one exists.
+	///
+	/// Returns the amount we should actually forward to our counterparty.
 	pub fn htlc_satisfies_config(
 		&self, htlc: &msgs::UpdateAddHTLC, amt_to_forward: u64, outgoing_cltv_value: u32,
-	) -> Result<(), (&'static str, u16)> {
+	) -> Result<u64, (&'static str, u16)> {
 		self.internal_htlc_satisfies_config(&htlc, amt_to_forward, outgoing_cltv_value, &self.config())
 			.or_else(|err| {
 				if let Some(prev_config) = self.prev_config() {
diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index a17332225..a39ca1e20 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -2014,7 +2014,7 @@ where
 			},
 		};
 
-		let pending_forward_info = match next_hop {
+		let mut pending_forward_info = match next_hop {
 			onion_utils::Hop::Receive(next_hop_data) => {
 				// OUR PAYMENT!
 				match self.construct_recv_pending_htlc_info(next_hop_data, shared_secret, msg.payment_hash, msg.amount_msat, msg.cltv_expiry, None) {
@@ -2058,7 +2058,7 @@ where
 			}
 		};
 
-		if let &PendingHTLCStatus::Forward(PendingHTLCInfo { ref routing, ref outgoing_amt_msat, ref outgoing_cltv_value, .. }) = &pending_forward_info {
+		if let &mut PendingHTLCStatus::Forward(PendingHTLCInfo { ref routing, ref mut outgoing_amt_msat, ref outgoing_cltv_value, .. }) = &mut pending_forward_info {
 			// If short_channel_id is 0 here, we'll reject the HTLC as there cannot be a channel
 			// with a short_channel_id of 0. This is important as various things later assume
 			// short_channel_id is non-0 in any ::Forward.
@@ -2115,8 +2115,9 @@ where
 						if *outgoing_amt_msat < chan.get_counterparty_htlc_minimum_msat() { // amount_below_minimum
 							break Some(("HTLC amount was below the htlc_minimum_msat", 0x1000 | 11, chan_update_opt));
 						}
-						if let Err((err, code)) = chan.htlc_satisfies_config(&msg, *outgoing_amt_msat, *outgoing_cltv_value) {
-							break Some((err, code, chan_update_opt));
+						match chan.htlc_satisfies_config(&msg, *outgoing_amt_msat, *outgoing_cltv_value) {
+							Err((err, code)) => break Some((err, code, chan_update_opt)),
+							Ok(amt_to_forward_msat) => *outgoing_amt_msat = amt_to_forward_msat,
 						}
 						chan_update_opt
 					} else {