routing: skip amtInRange for custom HTLCs

We might be trying to send an invoice amount that's greater than the size of the channel, but once you factor in the custom channel logic, an actual HTLC can be sent over the channel to pay that larger payment.

As a result, we'll skip over this check if a have a custom HTLC.

Author: Roasbeef

routing/unified_edges.go

@@ -247,8 +247,13 @@ func (u *edgeUnifier) getEdgeLocal(netAmtReceived lnwire.MilliSatoshi,
 		// local channel.
 		amt := netAmtReceived + lnwire.MilliSatoshi(inboundFee)
 
-		// Check valid amount range for the channel.
-		if !edge.amtInRange(amt) {
+		// Check valid amount range for the channel. We skip this test
+		// for payments with custom HTLC data, as the amount sent on
+		// the BTC layer may differ from the amount that is actually
+		// forwarded in custom channels.
+		if bandwidthHints.firstHopCustomBlob().IsNone() &&
+			!edge.amtInRange(amt) {
+
 			log.Debugf("Amount %v not in range for edge %v",
 				netAmtReceived, edge.policy.ChannelID)