routing: interpret mpp timeout failues

This commit adds success mission control
results for all hops along the route in
a mpp timeout and takes no action for
the final hop along the route. This is a
temporary measure to prevent the default
logic from penalizing the final node while
we decide how to penalize mpp timeouts.

routing/result_interpretation.go

@@ -216,6 +216,22 @@ func (i *interpretedResult) processPaymentOutcomeFinal(
 		// deliberately. What to penalize?
 		i.finalFailureReason = &reasonIncorrectDetails
 
+	case *lnwire.FailMPPTimeout:
+		// TODO(carla): decide how to penalize mpp timeout. In the
+		// meantime, attribute success to the hops along the route and
+		// do not penalize the final node.
+
+		i.finalFailureReason = &reasonError
+
+		// If this is a direct payment, take no action.
+		if n == 1 {
+			return
+		}
+
+		// Assign all pairs a success result except the final hop, as
+		// the payment reached the destination correctly.
+		i.successPairRange(route, 0, n-2)
+
 	default:
 		// All other errors are considered terminal if coming from the
 		// final hop. They indicate that something is wrong at the

routing/result_interpretation_test.go

@@ -272,6 +272,39 @@ var resultTestCases = []resultTestCase{
 			nodeFailure:        &hops[1],
 		},
 	},
+
+	// Tests a single hop mpp timeout. Test that final node is not
+	// penalized. This is a temporary measure while we decide how to
+	// penalize mpp timeouts.
+	{
+		name:          "one hop mpp timeout",
+		route:         &routeOneHop,
+		failureSrcIdx: 1,
+		failure:       &lnwire.FailMPPTimeout{},
+
+		expectedResult: &interpretedResult{
+			finalFailureReason: &reasonError,
+			nodeFailure:        nil,
+		},
+	},
+
+	// Tests a two hop mpp timeout. Test that final node is not penalized
+	// and the intermediate hop is attributed the success. This is a
+	// temporary measure while we decide how to penalize mpp timeouts.
+	{
+		name:          "two hop mpp timeout",
+		route:         &routeTwoHop,
+		failureSrcIdx: 2,
+		failure:       &lnwire.FailMPPTimeout{},
+
+		expectedResult: &interpretedResult{
+			pairResults: map[DirectedNodePair]pairResult{
+				getTestPair(0, 1): successPairResult(100),
+			},
+			finalFailureReason: &reasonError,
+			nodeFailure:        nil,
+		},
+	},
 }
 
 // TestResultInterpretation executes a list of test cases that test the result