routing: add outgoing channel restriction

routing/pathfind.go

@@ -11,7 +11,7 @@ import (
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/coreos/bbolt"
 
-	"github.com/lightningnetwork/lightning-onion"
+	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
@@ -453,6 +453,10 @@ type restrictParams struct {
 	// feeLimit is a maximum fee amount allowed to be used on the path from
 	// the source to the target.
 	feeLimit lnwire.MilliSatoshi
+
+	// outgoingChannelID is the channel that needs to be taken to the first
+	// hop. If nil, any channel may be used.
+	outgoingChannelID *uint64
 }
 
 // findPath attempts to find a path from the source node within the
@@ -563,6 +567,7 @@ func findPath(g *graphParams, r *restrictParams,
 		// TODO(halseth): also ignore disable flags for non-local
 		// channels if bandwidth hint is set?
 		isSourceChan := fromVertex == sourceVertex
+
 		edgeFlags := edge.ChannelFlags
 		isDisabled := edgeFlags&lnwire.ChanUpdateDisabled != 0
 
@@ -570,6 +575,14 @@ func findPath(g *graphParams, r *restrictParams,
 			return
 		}
 
+		// If we have an outgoing channel restriction and this is not
+		// the specified channel, skip it.
+		if isSourceChan && r.outgoingChannelID != nil &&
+			*r.outgoingChannelID != edge.ChannelID {
+
+			return
+		}
+
 		// If this vertex or edge has been black listed, then we'll
 		// skip exploring this edge.
 		if _, ok := r.ignoredNodes[fromVertex]; ok {