tapchannel: add chan reserve check on PaymentBandwidth method

tapchannel/aux_traffic_shaper.go

@@ -140,6 +140,19 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 		return linkBandwidth, nil
 	}
 
+	htlcAmountMSat := lnwire.NewMSatFromSatoshis(DefaultOnChainHtlcAmount)
+
+	// Todo: grab actual channel reserve
+	chanReserve := lnwire.MilliSatoshi(0)
+
+	// Check if the current link bandwidth can afford sending our default
+	// 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-htlcAmountMSat <= chanReserve {
+		return 0, nil
+	}
+
 	commitment, err := cmsg.DecodeCommitment(commitmentBytes)
 	if err != nil {
 		return 0, fmt.Errorf("error decoding commitment blob: %w", err)