tapchannel: add chan reserve check on PaymentBandwidth method

Author: GeorgeTsagk

server.go

@@ -990,7 +990,7 @@ 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 lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
+	linkBandwidth, htlcAmt lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
 
 	srvrLog.Debugf("PaymentBandwidth called, htlcBlob=%v, "+
 		"commitmentBlob=%v", spew.Sdump(htlcBlob),
@@ -1001,7 +1001,7 @@ func (s *Server) PaymentBandwidth(htlcBlob, commitmentBlob lfn.Option[tlv.Blob],
 	}
 
 	return s.cfg.AuxTrafficShaper.PaymentBandwidth(
-		htlcBlob, commitmentBlob, linkBandwidth,
+		htlcBlob, commitmentBlob, linkBandwidth, htlcAmt,
 	)
 }
 

tapchannel/aux_traffic_shaper.go

@@ -121,7 +121,7 @@ func (s *AuxTrafficShaper) HandleTraffic(_ lnwire.ShortChannelID,
 // called first.
 func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 	commitmentBlob lfn.Option[tlv.Blob],
-	linkBandwidth lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
+	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
@@ -140,6 +140,35 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 		return linkBandwidth, nil
 	}
 
+	// Get the minimum HTLC amount, which is just above dust.
+	minHtlcAmt := lnwire.NewMSatFromSatoshis(DefaultOnChainHtlcAmount)
+
+	// LND calls this hook twice. Once to see if the overall budget of the
+	// node is enough, and then during pathfinding to actually see if
+	// there's enough balance in the channel to make the payment attempt.
+	//
+	// When doing the overall balance check, we don't know what the actual
+	// htlcAmt is in satoshis, so a value of 0 will be passed here. Let's at
+	// least check if we can afford the min amount above dust. If the actual
+	// htlc amount ends up being greater when calling this method during
+	// pathfinding, we will still check it below.
+
+	// If the passed htlcAmt is below dust, then assume the dust amount. At
+	// this point we know we are sending assets, so we cannot anchor them to
+	// dust amounts. Dust HTLCs are added to the fees and aren't
+	// materialized in an on-chain output, so we wouldn't have anything
+	// to anchor the asset commitment to.
+	if htlcAmt < minHtlcAmt {
+		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)