routing: don't assume HTLC amount for custom channel

It seems like the amount given to the getBandwidth is 0 in some cases
(keysend) and non-zero in other cases (invoice payment).
If we use custom channels, then that amount will likely not be the
actual HTLC amount we're going to put on chain. So we have to use a zero
amount for the MayAddOutgoingHtlc check, otherwise we might seem to go
below the local channel reserve even though we'll end up sending a very
small amount only.
And since the check here is just to find out we're not already at the
maximum HTLC capacity for this channel, we can safely use the amount of
0 for the check, which is handled properly in the link logic.

Author: guggero

routing/bandwidth.go

@@ -138,6 +138,13 @@ func (b *bandwidthManager) getBandwidth(cid lnwire.ShortChannelID,
 
 		auxBandwidth           lnwire.MilliSatoshi
 		auxBandwidthDetermined bool
+
+		// htlcAmount is the amount we're going to use to check if we
+		// can add another HTLC to the channel. If the external traffic
+		// shaper is handling the channel, we'll use 0 to just sanity
+		// check the number of HTLCs on the channel, since we don't know
+		// the actual HTLC amount that will be sent.
+		htlcAmount = amount
 	)
 	err = fn.MapOptionZ(b.trafficShaper, func(ts TlvTrafficShaper) error {
 		fundingBlob := link.FundingCustomBlob()
@@ -171,6 +178,15 @@ func (b *bandwidthManager) getBandwidth(cid lnwire.ShortChannelID,
 
 		auxBandwidthDetermined = true
 
+		// We don't know the actual HTLC amount that will be sent using
+		// the custom channel. But we'll still want to make sure we can
+		// add another HTLC, using the MayAddOutgoingHtlc method below.
+		// Passing 0 into that method will use the minimum HTLC value
+		// for the channel, which is okay to just check we don't exceed
+		// the max number of HTLCs on the channel. A proper balance
+		// check is done elsewhere.
+		htlcAmount = 0
+
 		return nil
 	})
 	if err != nil {
@@ -180,11 +196,11 @@ func (b *bandwidthManager) getBandwidth(cid lnwire.ShortChannelID,
 		return 0
 	}
 
-	// If our link isn't currently in a state where it can add
-	// another outgoing htlc, treat the link as unusable.
-	if err := link.MayAddOutgoingHtlc(amount); err != nil {
+	// If our link isn't currently in a state where it can add another
+	// outgoing htlc, treat the link as unusable.
+	if err := link.MayAddOutgoingHtlc(htlcAmount); err != nil {
 		log.Warnf("ShortChannelID=%v: cannot add outgoing "+
-			"htlc: %v", cid, err)
+			"htlc with amount %v: %v", cid, htlcAmount, err)
 		return 0
 	}