routing: fix gap in result interpretation

Fixes an off by one error that skipped assigning a success pair. Added a
test that previously failed because of this bug.

routing/result_interpretation.go

@@ -191,10 +191,8 @@ func (i *interpretedResult) processPaymentOutcomeFinal(
 		i.failPair(route, n-1)
 
 		// The other hops relayed corectly, so assign those pairs a
-		// success result.
+		// success result. At this point, n >= 2.
-		if n > 2 {
+		i.successPairRange(route, 0, n-2)
-			i.successPairRange(route, 0, n-2)
-		}
 
 	// We are using wrong payment hash or amount, fail the payment.
 	case *lnwire.FailIncorrectPaymentAmount,
@@ -223,7 +221,7 @@ func (i *interpretedResult) processPaymentOutcomeFinal(
 		i.failNode(route, n)
 
 		// Other channels in the route forwarded correctly.
-		if n > 2 {
+		if n >= 2 {
 			i.successPairRange(route, 0, n-2)
 		}
 

routing/result_interpretation_test.go

@@ -32,6 +32,16 @@ var (
 		},
 	}
 
+	routeThreeHop = route.Route{
+		SourcePubKey: hops[0],
+		TotalAmount:  100,
+		Hops: []*route.Hop{
+			{PubKeyBytes: hops[1], AmtToForward: 99},
+			{PubKeyBytes: hops[2], AmtToForward: 97},
+			{PubKeyBytes: hops[3], AmtToForward: 94},
+		},
+	}
+
 	routeFourHop = route.Route{
 		SourcePubKey: hops[0],
 		TotalAmount:  100,
@@ -203,7 +213,7 @@ var resultTestCases = []resultTestCase{
 	// be failed while the proceeding hops are reproed as successes. The
 	// failure is terminal since the receiver can't process our onion.
 	{
-		name:          "fail invalid onion payload final hop",
+		name:          "fail invalid onion payload final hop four",
 		route:         &routeFourHop,
 		failureSrcIdx: 4,
 		failure:       lnwire.NewInvalidOnionPayload(0, 0),
@@ -229,6 +239,30 @@ var resultTestCases = []resultTestCase{
 		},
 	},
 
+	// Tests an invalid onion payload from a final hop on a three hop route.
+	{
+		name:          "fail invalid onion payload final hop three",
+		route:         &routeThreeHop,
+		failureSrcIdx: 3,
+		failure:       lnwire.NewInvalidOnionPayload(0, 0),
+
+		expectedResult: &interpretedResult{
+			pairResults: map[DirectedNodePair]pairResult{
+				getTestPair(0, 1): {
+					success: true,
+					amt:     100,
+				},
+				getTestPair(1, 2): {
+					success: true,
+					amt:     99,
+				},
+				getTestPair(3, 2): {},
+			},
+			finalFailureReason: &reasonError,
+			nodeFailure:        &hops[3],
+		},
+	},
+
 	// Tests an invalid onion payload from an intermediate hop. Only the
 	// reporting node should be failed. The failure is non-terminal since we
 	// can still try other paths.