Trampoline payload construction method.

Author: arik-so

lightning/src/ln/onion_utils.rs

@@ -314,6 +314,62 @@ pub(super) fn build_onion_payloads(
 	Ok((res, cur_value_msat, cur_cltv))
 }
 
+/// returns the hop data, as well as the first-hop value_msat and CLTV value we should send.
+pub(super) fn build_trampoline_payloads(
+	path: &[TrampolineHop], total_msat: u64, mut recipient_onion: RecipientOnionFields,
+	starting_htlc_offset: u32, keysend_preimage: &Option<PaymentPreimage>,
+) -> Result<(Vec<msgs::OutboundOnionPayload>, u64, u32), APIError> {
+	let mut cur_value_msat = 0u64;
+	let mut cur_cltv = starting_htlc_offset;
+	let mut res: Vec<msgs::OutboundOnionPayload> = Vec::with_capacity(path.len());
+	let mut last_node_id = None;
+
+	for (idx, hop) in path.iter().rev().enumerate() {
+		// First hop gets special values so that it can check, on receipt, that everything is
+		// exactly as it should be (and the next hop isn't trying to probe to find out if we're
+		// the intended recipient).
+		let value_msat = if cur_value_msat == 0 { hop.fee_msat } else { cur_value_msat };
+		let cltv = if cur_cltv == starting_htlc_offset {
+			hop.cltv_expiry_delta + starting_htlc_offset
+		} else {
+			cur_cltv
+		};
+		if idx == 0 {
+			res.push(msgs::OutboundOnionPayload::Receive {
+				payment_data: if let Some(secret) = recipient_onion.payment_secret.take() {
+					Some(msgs::FinalOnionHopData { payment_secret: secret, total_msat })
+				} else {
+					None
+				},
+				payment_metadata: recipient_onion.payment_metadata.take(),
+				keysend_preimage: *keysend_preimage,
+				custom_tlvs: recipient_onion.custom_tlvs.clone(),
+				sender_intended_htlc_amt_msat: value_msat,
+				cltv_expiry_height: cltv,
+			});
+		} else {
+			res.insert(
+				0,
+				msgs::OutboundOnionPayload::TrampolineForward {
+					amt_to_forward: value_msat,
+					outgoing_cltv_value: cltv,
+					outgoing_node_id: last_node_id.unwrap(),
+				},
+			);
+		}
+		cur_value_msat += hop.fee_msat;
+		if cur_value_msat >= 21000000 * 100000000 * 1000 {
+			return Err(APIError::InvalidRoute { err: "Channel fees overflowed?".to_owned() });
+		}
+		cur_cltv += hop.cltv_expiry_delta as u32;
+		if cur_cltv >= 500000000 {
+			return Err(APIError::InvalidRoute { err: "Channel CLTV overflowed?".to_owned() });
+		}
+		last_node_id = Some(hop.pubkey);
+	}
+	Ok((res, cur_value_msat, cur_cltv))
+}
+
 /// Length of the onion data packet. Before TLV-based onions this was 20 65-byte hops, though now
 /// the hops can be of variable length.
 pub(crate) const ONION_DATA_LEN: usize = 20 * 65;