BOLT2: Check we don't send and accept 0-msat HTLC

Failing this requirement at sending means a strict receiver would
fail our channel while processing a HTLC routed from a third-party.

Fix by enforcing check on both sender and receiver side.

Author: Antoine Riard

lightning/src/ln/channel.rs

@@ -1656,6 +1656,9 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
 		if msg.amount_msat > self.channel_value_satoshis * 1000 {
 			return Err(ChannelError::Close("Remote side tried to send more than the total value of the channel"));
 		}
+		if msg.amount_msat == 0 {
+			return Err(ChannelError::Close("Remote side tried to send a 0-msat HTLC"));
+		}
 		if msg.amount_msat < self.our_htlc_minimum_msat {
 			return Err(ChannelError::Close("Remote side tried to send less than our minimum HTLC value"));
 		}
@@ -3493,6 +3496,11 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
 		if amount_msat > self.channel_value_satoshis * 1000 {
 			return Err(ChannelError::Ignore("Cannot send more than the total value of the channel"));
 		}
+
+		if amount_msat == 0 {
+			return Err(ChannelError::Ignore("Cannot send 0-msat HTLC"));
+		}
+
 		if amount_msat < self.their_htlc_minimum_msat {
 			return Err(ChannelError::Ignore("Cannot send less than their minimum HTLC value"));
 		}

lightning/src/ln/functional_tests.rs

@@ -5486,7 +5486,6 @@ fn bolt2_open_channel_sending_node_checks_part2() {
 
 #[test]
 fn test_update_add_htlc_bolt2_sender_value_below_minimum_msat() {
-	//BOLT2 Requirement: MUST offer amount_msat greater than 0.
 	//BOLT2 Requirement: MUST NOT offer amount_msat below the receiving node's htlc_minimum_msat (same validation check catches both of these)
 	let chanmon_cfgs = create_chanmon_cfgs(2);
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
@@ -5496,7 +5495,7 @@ fn test_update_add_htlc_bolt2_sender_value_below_minimum_msat() {
 	let mut route = nodes[0].router.get_route(&nodes[1].node.get_our_node_id(), None, &[], 100000, TEST_FINAL_CLTV).unwrap();
 	let (_, our_payment_hash) = get_payment_preimage_hash!(nodes[0]);
 
-	route.hops[0].fee_msat = 0;
+	route.hops[0].fee_msat = 100;
 
 	let err = nodes[0].node.send_payment(route, our_payment_hash);
 
@@ -5509,6 +5508,30 @@ fn test_update_add_htlc_bolt2_sender_value_below_minimum_msat() {
 	nodes[0].logger.assert_log("lightning::ln::channelmanager".to_string(), "Cannot send less than their minimum HTLC value".to_string(), 1);
 }
 
+#[test]
+fn test_update_add_htlc_bolt2_sender_zero_value_msat() {
+	//BOLT2 Requirement: MUST offer amount_msat greater than 0.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let mut nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let _chan = create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100000, 95000000, InitFeatures::supported(), InitFeatures::supported());
+	let mut route = nodes[0].router.get_route(&nodes[1].node.get_our_node_id(), None, &[], 100000, TEST_FINAL_CLTV).unwrap();
+	let (_, our_payment_hash) = get_payment_preimage_hash!(nodes[0]);
+
+	route.hops[0].fee_msat = 0;
+
+	let err = nodes[0].node.send_payment(route, our_payment_hash);
+
+	if let Err(APIError::ChannelUnavailable{err}) = err {
+		assert_eq!(err, "Cannot send 0-msat HTLC");
+	} else {
+		assert!(false);
+	}
+	assert!(nodes[0].node.get_and_clear_pending_msg_events().is_empty());
+	nodes[0].logger.assert_log("lightning::ln::channelmanager".to_string(), "Cannot send 0-msat HTLC".to_string(), 1);
+}
+
 #[test]
 fn test_update_add_htlc_bolt2_sender_cltv_expiry_too_high() {
 	//BOLT 2 Requirement: MUST set cltv_expiry less than 500000000.