(fixup)tapchannel: conditionally check linkbandwidth against htlcAmt in PaymentBandwidth

Previously we would always perform a check between htlcAmt and link
bandwidth in the PaymentBandwidth hook. This would not work as for RFQ
payments the htlcAmt passed to this function would be the overall
translated asset value, which could easily exceed the remaining link
bandwidth expressed in sats. We now perform two distinct htlc budget
checks, once in the keysend path and once in the RFQ path.

Author: GeorgeTsagk

server.go

@@ -990,7 +990,8 @@ func (s *Server) HandleTraffic(cid lnwire.ShortChannelID,
 //
 // NOTE: This method is part of the routing.TlvTrafficShaper interface.
 func (s *Server) PaymentBandwidth(htlcBlob, commitmentBlob lfn.Option[tlv.Blob],
-	linkBandwidth, htlcAmt lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
+	linkBandwidth,
+	htlcAmt lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
 
 	srvrLog.Debugf("PaymentBandwidth called, htlcBlob=%v, "+
 		"commitmentBlob=%v", spew.Sdump(htlcBlob),

tapchannel/aux_traffic_shaper.go

@@ -120,8 +120,8 @@ func (s *AuxTrafficShaper) HandleTraffic(_ lnwire.ShortChannelID,
 // should be handled by the traffic shaper, the HandleTraffic method should be
 // called first.
 func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
-	commitmentBlob lfn.Option[tlv.Blob],
-	linkBandwidth, htlcAmt lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
+	commitmentBlob lfn.Option[tlv.Blob], linkBandwidth,
+	htlcAmt lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
 
 	// If the commitment or HTLC blob is not set, we don't have any
 	// information about the channel and cannot determine the available
@@ -162,13 +162,6 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 		htlcAmt = minHtlcAmt
 	}
 
-	// Check if the current link bandwidth can afford sending out the htlc
-	// amount without dipping into the channel reserve. If it goes below the
-	// reserve, we report zero bandwdith as we cannot push the htlc amount.
-	if linkBandwidth < htlcAmt {
-		return 0, nil
-	}
-
 	commitment, err := cmsg.DecodeCommitment(commitmentBytes)
 	if err != nil {
 		return 0, fmt.Errorf("error decoding commitment blob: %w", err)
@@ -190,6 +183,14 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 	// the amount.
 	htlcAssetAmount := htlc.Amounts.Val.Sum()
 	if htlcAssetAmount != 0 && htlcAssetAmount <= localBalance {
+		// Check if the current link bandwidth can afford sending out
+		// the htlc amount without dipping into the channel reserve. If
+		// it goes below the reserve, we report zero bandwdith as we
+		// cannot push the htlc amount.
+		if linkBandwidth < htlcAmt {
+			return 0, nil
+		}
+
 		// We signal "infinite" bandwidth by returning a very high
 		// value (number of Satoshis ever in existence), since we might
 		// not have a quote available to know what the asset amount
@@ -219,6 +220,14 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 
 	mSatPerAssetUnit := quote.BidPrice
 
+	// At this point we have acquired what we need to express the asset
+	// bandwidth expressed in satoshis. Before we return the result, we need
+	// to check if the link bandwidth can afford sending a non-dust htlc to
+	// the other side.
+	if linkBandwidth < minHtlcAmt {
+		return 0, nil
+	}
+
 	// The available balance is the local asset unit expressed in
 	// milli-satoshis.
 	return lnwire.MilliSatoshi(localBalance) * mSatPerAssetUnit, nil