tapchannel: split PaymentBandwidth logic into helpers

To make `PaymentBandwidth` more readable, we separate the main codepaths
into helper functions. One is used when we're directly comparing asset
units to each other, and the other one is the path that calculates an
msat amount based on an RFQ quote.

Author: GeorgeTsagk

tapchannel/aux_traffic_shaper.go

@@ -174,18 +174,40 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 		return 0, fmt.Errorf("error decoding HTLC blob: %w", err)
 	}
 
-	// localBalance is the total number of local asset units.
-	localBalance := cmsg.OutputSum(commitment.LocalOutputs())
-
-	// There either already is an amount set in the HTLC (which would
-	// indicate it to be a direct-channel keysend payment that just sends
-	// assets to the direct peer with no conversion), in which case we don't
-	// need an RFQ ID as we can just compare the local balance and the
-	// required HTLC amount. If there is no amount set, we need to look up
-	// the RFQ ID in the HTLC blob and use the accepted quote to determine
-	// the amount.
+	// With the help of the latest HtlcView, let's calculate a more precise
+	// local balance. This is useful in order to not forward HTLCs that may
+	// never be settled. Other HTLCs that may also call into this method are
+	// not yet registered to the commitment, so we need to account for them
+	// manually.
+	computedLocal := ComputeLocalBalance(commitment)
+
+	// If the HTLC carries asset units (keysend, forwarding), then there's
+	// no need to do any RFQ related math. We can directly compare the asset
+	// units of the HTLC with those in our local balance.
 	htlcAssetAmount := htlc.Amounts.Val.Sum()
-	if htlcAssetAmount != 0 && htlcAssetAmount <= localBalance {
+	if htlcAssetAmount != 0 {
+		return paymentBandwidthAssetUnits(
+			htlcAssetAmount, computedLocal, linkBandwidth, htlcAmt,
+		)
+	}
+
+	// Otherwise, we derive the available bandwidth from the HTLC's RFQ and
+	// the asset units in our local balance.
+	return s.paymentBandwidthRfqMath(
+		htlc, computedLocal, linkBandwidth, minHtlcAmt,
+	)
+}
+
+// paymentBandwidthAssetUnits includes the asset unit related checks between the
+// HTLC carrying the units and the asset balance of our channel. The response
+// will either be infinite or zero bandwidth, as we can't really map the amount
+// to msats without an RFQ, and it's also not needed.
+func paymentBandwidthAssetUnits(htlcAssetAmount, computedLocal uint64,
+	linkBandwidth,
+	htlcAmt lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
+
+	switch {
+	case htlcAssetAmount <= computedLocal:
 		// 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 bandwidth as we
@@ -204,10 +226,29 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 		// matter too much here, we just want to signal that this
 		// channel _does_ have available bandwidth.
 		return lnwire.NewMSatFromSatoshis(btcutil.MaxSatoshi), nil
+
+	case htlcAssetAmount > computedLocal:
+		// The asset balance of the channel is simply not enough to
+		// route the asset units, we report 0 bandwidth in order for the
+		// HTLC to fail back.
+		return 0, nil
+
+	default:
+		// We shouldn't reach this case, we add it only for the function
+		// to always return something and the compiler to be happy.
+		return 0, nil
 	}
+}
 
-	// If the HTLC doesn't have an asset amount and RFQ ID, it's incomplete,
-	// and we cannot determine what channel to use.
+// paymentBandwidthRfqMath returns the available payment bandwidth of the
+// channel based on the asset rate of the RFQ quote that is included in the
+// HTLC and the asset units of the local balance.
+func (s *AuxTrafficShaper) paymentBandwidthRfqMath(htlc *rfqmsg.Htlc,
+	localBalance uint64, linkBandwidth,
+	minHtlcAmt lnwire.MilliSatoshi) (lnwire.MilliSatoshi, error) {
+
+	// If the HTLC doesn't have an RFQ ID, it's incomplete, and we cannot
+	// determine the bandwidth.
 	if htlc.RfqID.ValOpt().IsNone() {
 		log.Tracef("No RFQ ID in HTLC, cannot determine matching " +
 			"outgoing channel")
@@ -261,6 +302,16 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 	return availableBalanceMsat, nil
 }
 
+// ComputeLocalBalance combines the given commitment state with the HtlcView to
+// produce the available local balance with accuracy.
+func ComputeLocalBalance(commitment *cmsg.Commitment) uint64 {
+	// Let's get the current local asset balance of the channel as reported
+	// by the latest commitment.
+	localBalance := cmsg.OutputSum(commitment.LocalOutputs())
+
+	return localBalance
+}
+
 // ProduceHtlcExtraData is a function that, based on the previous custom record
 // blob of an HTLC, may produce a different blob or modify the amount of bitcoin
 // this HTLC should carry.