Add hold_htlc param to internal Channel APIs

As part of supporting sending payments as an often-offline sender, the sender
needs to be able to set a flag in their update_add_htlc message indicating that
the HTLC should be held until receipt of a release_held_htlc onion message from
the often-offline payment recipient.

We don't yet ever set this flag, but lay the groundwork by including a
parameter for it in internal channel send_htlc APIs, including serializing the
field when the parameter is set.

    See-also BOLTs PR 989

Author: valentinewallace

lightning/src/ln/channel.rs

@@ -8067,7 +8067,7 @@ where
 						ref onion_routing_packet,
 						skimmed_fee_msat,
 						blinding_point,
-						hold_htlc: _,
+						hold_htlc,
 						..
 					} => {
 						match self.send_htlc(
@@ -8079,6 +8079,7 @@ where
 							false,
 							skimmed_fee_msat,
 							blinding_point,
+							hold_htlc,
 							fee_estimator,
 							logger,
 						) {
@@ -12053,6 +12054,8 @@ where
 			true,
 			skimmed_fee_msat,
 			blinding_point,
+			// This method is only called for forwarded HTLCs, which are never held at the next hop
+			None,
 			fee_estimator,
 			logger,
 		)
@@ -12083,7 +12086,7 @@ where
 	fn send_htlc<F: Deref, L: Deref>(
 		&mut self, amount_msat: u64, payment_hash: PaymentHash, cltv_expiry: u32,
 		source: HTLCSource, onion_routing_packet: msgs::OnionPacket, mut force_holding_cell: bool,
-		skimmed_fee_msat: Option<u64>, blinding_point: Option<PublicKey>,
+		skimmed_fee_msat: Option<u64>, blinding_point: Option<PublicKey>, hold_htlc: Option<()>,
 		fee_estimator: &LowerBoundedFeeEstimator<F>, logger: &L,
 	) -> Result<bool, (LocalHTLCFailureReason, String)>
 	where
@@ -12165,7 +12168,7 @@ where
 				onion_routing_packet,
 				skimmed_fee_msat,
 				blinding_point,
-				hold_htlc: None,
+				hold_htlc,
 			});
 			return Ok(false);
 		}
@@ -12187,7 +12190,7 @@ where
 			blinding_point,
 			skimmed_fee_msat,
 			send_timestamp,
-			hold_htlc: None,
+			hold_htlc,
 		});
 		self.context.next_holder_htlc_id += 1;
 
@@ -12422,7 +12425,7 @@ where
 	pub fn send_htlc_and_commit<F: Deref, L: Deref>(
 		&mut self, amount_msat: u64, payment_hash: PaymentHash, cltv_expiry: u32,
 		source: HTLCSource, onion_routing_packet: msgs::OnionPacket, skimmed_fee_msat: Option<u64>,
-		fee_estimator: &LowerBoundedFeeEstimator<F>, logger: &L,
+		hold_htlc: Option<()>, fee_estimator: &LowerBoundedFeeEstimator<F>, logger: &L,
 	) -> Result<Option<ChannelMonitorUpdate>, ChannelError>
 	where
 		F::Target: FeeEstimator,
@@ -12437,6 +12440,7 @@ where
 			false,
 			skimmed_fee_msat,
 			None,
+			hold_htlc,
 			fee_estimator,
 			logger,
 		);

lightning/src/ln/channelmanager.rs

@@ -5040,6 +5040,7 @@ where
 							htlc_source,
 							onion_packet,
 							None,
+							None,
 							&self.fee_estimator,
 							&&logger,
 						);