routing: penalize all node pairs for unknown outcomes

When an undecryptable failure comes back for a payment attempt, we
previously only penalized our own outgoing connection. However,
any node could have caused this failure. It is therefore better to
penalize all node connections along the route. Then at least we know for
sure that we will hit the responsible node.

routing/result_interpretation.go

@@ -7,6 +7,11 @@ import (
 	"github.com/lightningnetwork/lnd/routing/route"
 )
 
+// Instantiate variables to allow taking a reference from the failure reason.
+var (
+	reasonError = channeldb.FailureReasonError
+)
+
 // interpretedResult contains the result of the interpretation of a payment
 // outcome.
 type interpretedResult struct {
@@ -52,21 +57,14 @@ func (i *interpretedResult) processFail(
 	rt *route.Route, errSourceIdx *int,
 	failure lnwire.FailureMessage) (bool, channeldb.FailureReason) {
 
-	var failureSourceIdxInt int
-
 	if errSourceIdx == nil {
-		// If the failure message could not be decrypted, attribute the
-		// failure to our own outgoing channel.
-		//
-		// TODO(joostager): Penalize all channels in the route.
-		failureSourceIdxInt = 0
-		failure = lnwire.NewTemporaryChannelFailure(nil)
-	} else {
-		failureSourceIdxInt = *errSourceIdx
+		i.processPaymentOutcomeUnknown(rt)
+		return false, 0
 	}
 
 	var failureVertex route.Vertex
 
+	failureSourceIdxInt := *errSourceIdx
 	if failureSourceIdxInt > 0 {
 		failureVertex = rt.Hops[failureSourceIdxInt-1].PubKeyBytes
 	} else {
@@ -241,6 +239,74 @@ func (i *interpretedResult) processFail(
 	}
 }
 
+// processPaymentOutcomeUnknown processes a payment outcome for which no failure
+// message or source is available.
+func (i *interpretedResult) processPaymentOutcomeUnknown(route *route.Route) {
+	n := len(route.Hops)
+
+	// If this is a direct payment, the destination must be at fault.
+	if n == 1 {
+		i.failNode(route, n)
+		i.finalFailureReason = &reasonError
+		return
+	}
+
+	// Otherwise penalize all channels in the route to make sure the
+	// responsible node is at least hit too. We even penalize the connection
+	// to our own peer, because that peer could also be responsible.
+	i.failPairRange(route, 0, n-1)
+}
+
+// failNode marks the node indicated by idx in the route as failed. This
+// function intentionally panics when the self node is failed.
+func (i *interpretedResult) failNode(rt *route.Route, idx int) {
+	i.nodeFailure = &rt.Hops[idx-1].PubKeyBytes
+}
+
+// failPairRange marks the node pairs from node fromIdx to node toIdx as failed.
+func (i *interpretedResult) failPairRange(
+	rt *route.Route, fromIdx, toIdx int) {
+
+	for idx := fromIdx; idx <= toIdx; idx++ {
+		i.failPair(rt, idx)
+	}
+}
+
+// failPair marks a pair as failed in both directions.
+func (i *interpretedResult) failPair(
+	rt *route.Route, idx int) {
+
+	pair, _ := getPair(rt, idx)
+
+	// Report pair in both directions without a minimum penalization amount.
+	i.pairResults[pair] = 0
+	i.pairResults[pair.Reverse()] = 0
+}
+
+// getPair returns a node pair from the route and the amount passed between that
+// pair.
+func getPair(rt *route.Route, channelIdx int) (DirectedNodePair,
+	lnwire.MilliSatoshi) {
+
+	nodeTo := rt.Hops[channelIdx].PubKeyBytes
+	var (
+		nodeFrom route.Vertex
+		amt      lnwire.MilliSatoshi
+	)
+
+	if channelIdx == 0 {
+		nodeFrom = rt.SourcePubKey
+		amt = rt.TotalAmount
+	} else {
+		nodeFrom = rt.Hops[channelIdx-1].PubKeyBytes
+		amt = rt.Hops[channelIdx-1].AmtToForward
+	}
+
+	pair := NewDirectedNodePair(nodeFrom, nodeTo)
+
+	return pair, amt
+}
+
 // getFailedPair tries to locate the failing pair given a route and the pubkey
 // of the node that sent the failure. It will assume that the failure is
 // associated with the outgoing channel set of the failing node. As a second