Support aggregating htlc_minimum_msat for BlindedPayInfo

lightning/src/blinded_path/payment.rs

@@ -184,11 +184,38 @@ pub(super) fn compute_payinfo(
 
 		cltv_expiry_delta = cltv_expiry_delta.checked_add(tlvs.payment_relay.cltv_expiry_delta).ok_or(())?;
 	}
+
+	let mut htlc_minimum_msat = 0;
+	for (idx, node) in intermediate_nodes.iter().map(|(_, tlvs)| tlvs).enumerate() {
+		let mut htlc_min_candidate = node.payment_constraints.htlc_minimum_msat as u128;
+		// Get an iterator over `[curr_hop_tlvs..last_intermediate_hop_tlvs]`.
+		let next_nodes = intermediate_nodes.iter()
+			.enumerate()
+			.skip_while(|(i, _)| *i != idx)
+			.map(|(_, (_, tlvs))| tlvs);
+		for node in next_nodes {
+			// The min htlc for a hop is that hop's htlc_minimum_msat minus the fees paid from that hop to
+			// the end of the path, because the sender will automatically include that following fee
+			// amount in the amount that this hop receives.
+			let prop_fee = node.payment_relay.fee_proportional_millionths as u128;
+			let base_fee = node.payment_relay.fee_base_msat as u128;
+			let hop_fee = htlc_min_candidate
+				.checked_mul(prop_fee)
+				.and_then(|prop_fee| (prop_fee / 1_000_000).checked_add(base_fee))
+				.ok_or(())?;
+			htlc_min_candidate = htlc_min_candidate.saturating_sub(hop_fee);
+			if htlc_min_candidate == 0 { break }
+		}
+		htlc_minimum_msat = core::cmp::max(htlc_min_candidate, htlc_minimum_msat);
+	}
+	htlc_minimum_msat =
+		core::cmp::max(payee_tlvs.payment_constraints.htlc_minimum_msat as u128, htlc_minimum_msat);
+
 	Ok(BlindedPayInfo {
 		fee_base_msat: u32::try_from(curr_base_fee).map_err(|_| ())?,
 		fee_proportional_millionths: u32::try_from(curr_prop_mil).map_err(|_| ())?,
 		cltv_expiry_delta,
-		htlc_minimum_msat: 1, // TODO
+		htlc_minimum_msat: u64::try_from(htlc_minimum_msat).map_err(|_| ())?,
 		htlc_maximum_msat: 21_000_000 * 100_000_000 * 1_000, // TODO
 		features: BlindedHopFeatures::empty(),
 	})
@@ -253,6 +280,7 @@ mod tests {
 		assert_eq!(blinded_payinfo.fee_base_msat, 201);
 		assert_eq!(blinded_payinfo.fee_proportional_millionths, 1001);
 		assert_eq!(blinded_payinfo.cltv_expiry_delta, 288);
+		assert_eq!(blinded_payinfo.htlc_minimum_msat, 900);
 	}
 
 	#[test]
@@ -268,5 +296,89 @@ mod tests {
 		assert_eq!(blinded_payinfo.fee_base_msat, 0);
 		assert_eq!(blinded_payinfo.fee_proportional_millionths, 0);
 		assert_eq!(blinded_payinfo.cltv_expiry_delta, 0);
+		assert_eq!(blinded_payinfo.htlc_minimum_msat, 1);
+	}
+
+	#[test]
+	fn simple_aggregated_htlc_min() {
+		// If no hops charge fees, the htlc_minimum_msat should just be the maximum htlc_minimum_msat
+		// along the path.
+		let dummy_pk = PublicKey::from_slice(&[2; 33]).unwrap();
+		let intermediate_nodes = vec![(dummy_pk, ForwardTlvs {
+			short_channel_id: 0,
+			payment_relay: PaymentRelay {
+				cltv_expiry_delta: 0,
+				fee_proportional_millionths: 0,
+				fee_base_msat: 0,
+			},
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 1,
+			},
+			features: BlindedHopFeatures::empty(),
+		}), (dummy_pk, ForwardTlvs {
+			short_channel_id: 0,
+			payment_relay: PaymentRelay {
+				cltv_expiry_delta: 0,
+				fee_proportional_millionths: 0,
+				fee_base_msat: 0,
+			},
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 2_000,
+			},
+			features: BlindedHopFeatures::empty(),
+		})];
+		let recv_tlvs = ReceiveTlvs {
+			payment_secret: PaymentSecret([0; 32]),
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 3,
+			},
+		};
+		let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs).unwrap();
+		assert_eq!(blinded_payinfo.htlc_minimum_msat, 2_000);
+	}
+
+	#[test]
+	fn aggregated_htlc_min() {
+		// Create a path with varying fees and htlc_mins, and make sure htlc_minimum_msat ends up as the
+		// max (htlc_min - following_fees) along the path.
+		let dummy_pk = PublicKey::from_slice(&[2; 33]).unwrap();
+		let intermediate_nodes = vec![(dummy_pk, ForwardTlvs {
+			short_channel_id: 0,
+			payment_relay: PaymentRelay {
+				cltv_expiry_delta: 0,
+				fee_proportional_millionths: 500,
+				fee_base_msat: 1_000,
+			},
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 5_000,
+			},
+			features: BlindedHopFeatures::empty(),
+		}), (dummy_pk, ForwardTlvs {
+			short_channel_id: 0,
+			payment_relay: PaymentRelay {
+				cltv_expiry_delta: 0,
+				fee_proportional_millionths: 500,
+				fee_base_msat: 200,
+			},
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 2_000,
+			},
+			features: BlindedHopFeatures::empty(),
+		})];
+		let recv_tlvs = ReceiveTlvs {
+			payment_secret: PaymentSecret([0; 32]),
+			payment_constraints: PaymentConstraints {
+				max_cltv_expiry: 0,
+				htlc_minimum_msat: 1,
+			},
+		};
+
+		let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs).unwrap();
+		assert_eq!(blinded_payinfo.htlc_minimum_msat, 3797);
 	}
 }