f fix min htlc calc

Author: valentinewallace

lightning/src/blinded_path/payment.rs

@@ -153,6 +153,29 @@ pub(super) fn blinded_hops<T: secp256k1::Signing + secp256k1::Verification>(
 	utils::construct_blinded_hops(secp_ctx, pks, tlvs, session_priv)
 }
 
+/// `None` if underflow or overflow occurs.
+fn amt_to_forward_msat(inbound_amt_msat: u64, payment_relay: &PaymentRelay) -> Option<u64> {
+	let amt_to_fwd_numerator = inbound_amt_msat.checked_sub(payment_relay.fee_base_msat as u64)
+		.and_then(|a| a.checked_mul(1_000_000))
+		.and_then(|a| a.checked_add(1_000_000 - 1))
+		.and_then(|a| a.checked_add(payment_relay.fee_proportional_millionths as u64));
+	let amt_to_fwd_denominator = payment_relay.fee_proportional_millionths.checked_add(1_000_000);
+	let mut amt_to_forward = match (amt_to_fwd_numerator, amt_to_fwd_denominator) {
+		(Some(num), Some(denom)) => num / denom as u64,
+		_ => return None,
+	};
+	let fee_opt = amt_to_forward.checked_mul(payment_relay.fee_proportional_millionths as u64)
+		.and_then(|prop| (prop / 1000000).checked_add(payment_relay.fee_base_msat as u64));
+	let fee = if let Some(f) = fee_opt { f } else { return None };
+	if inbound_amt_msat - fee < amt_to_forward {
+		// Rounding up the forwarded amount resulted in underpaying this node, so take an extra 1 msat
+		// in fee to compensate.
+		amt_to_forward -= 1;
+	}
+	debug_assert_eq!(amt_to_forward + fee, inbound_amt_msat);
+	Some(amt_to_forward)
+}
+
 pub(super) fn compute_payinfo(
 	intermediate_nodes: &[(PublicKey, ForwardTlvs)], payee_tlvs: &ReceiveTlvs
 ) -> Result<BlindedPayInfo, ()> {
@@ -185,28 +208,22 @@ 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 = intermediate_nodes.first()
-		.map_or(0, |(_, tlvs)| tlvs.payment_constraints.htlc_minimum_msat) as u128;
-	for (_, tlvs) in intermediate_nodes.iter().skip(1) {
-		let prop_fee = tlvs.payment_relay.fee_proportional_millionths as u128;
-		let base_fee = tlvs.payment_relay.fee_base_msat as u128;
-		let hop_fee = htlc_minimum_msat
-			.checked_mul(prop_fee)
-			.and_then(|prop_fee| (prop_fee / 1_000_000).checked_add(base_fee))
-			.ok_or(())?;
-		htlc_minimum_msat = core::cmp::max(
-			htlc_minimum_msat.saturating_sub(hop_fee), tlvs.payment_constraints.htlc_minimum_msat as u128
-		);
+	let mut htlc_minimum_msat: u64 = 1;
+	for (_, tlvs) in intermediate_nodes.iter() {
+		htlc_minimum_msat = amt_to_forward_msat(
+			core::cmp::max(tlvs.payment_constraints.htlc_minimum_msat, htlc_minimum_msat),
+			&tlvs.payment_relay
+		).unwrap_or(1);
 	}
 	htlc_minimum_msat = core::cmp::max(
-		payee_tlvs.payment_constraints.htlc_minimum_msat as u128, htlc_minimum_msat
+		payee_tlvs.payment_constraints.htlc_minimum_msat, 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: u64::try_from(htlc_minimum_msat).map_err(|_| ())?,
+		htlc_minimum_msat,
 		htlc_maximum_msat: 21_000_000 * 100_000_000 * 1_000, // TODO
 		features: BlindedHopFeatures::empty(),
 	})
@@ -271,7 +288,7 @@ 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, 1000);
+		assert_eq!(blinded_payinfo.htlc_minimum_msat, 900);
 	}
 
 	#[test]
@@ -368,7 +385,8 @@ mod tests {
 				htlc_minimum_msat: 1,
 			},
 		};
+		let htlc_minimum_msat = 3798;
 		let blinded_payinfo = super::compute_payinfo(&intermediate_nodes[..], &recv_tlvs).unwrap();
-		assert_eq!(blinded_payinfo.htlc_minimum_msat, 4798);
+		assert_eq!(blinded_payinfo.htlc_minimum_msat, htlc_minimum_msat);
 	}
 }