htlcswitch+router: define PaymentResult, GetPaymentResult

This lets us distinguish an critical error from a actual payment result
(success or failure). This is important since we know that we can only
attempt another payment when a final result from the previous payment
attempt is received.

routing/mock_test.go

@@ -1,28 +1,61 @@
 package routing
 
 import (
-	"crypto/sha256"
-
 	"github.com/lightningnetwork/lnd/htlcswitch"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
 type mockPaymentAttemptDispatcher struct {
 	onPayment func(firstHop lnwire.ShortChannelID) ([32]byte, error)
+	results   map[uint64]*htlcswitch.PaymentResult
 }
 
 var _ PaymentAttemptDispatcher = (*mockPaymentAttemptDispatcher)(nil)
 
 func (m *mockPaymentAttemptDispatcher) SendHTLC(firstHop lnwire.ShortChannelID,
-	_ uint64,
+	pid uint64,
 	_ *lnwire.UpdateAddHTLC,
-	_ htlcswitch.ErrorDecrypter) ([sha256.Size]byte, error) {
+	_ htlcswitch.ErrorDecrypter) error {
 
-	if m.onPayment != nil {
-		return m.onPayment(firstHop)
+	if m.onPayment == nil {
+		return nil
 	}
 
-	return [sha256.Size]byte{}, nil
+	if m.results == nil {
+		m.results = make(map[uint64]*htlcswitch.PaymentResult)
+	}
+
+	var result *htlcswitch.PaymentResult
+	preimage, err := m.onPayment(firstHop)
+	if err != nil {
+		fwdErr, ok := err.(*htlcswitch.ForwardingError)
+		if !ok {
+			return err
+		}
+		result = &htlcswitch.PaymentResult{
+			Error: fwdErr,
+		}
+	} else {
+		result = &htlcswitch.PaymentResult{Preimage: preimage}
+	}
+
+	m.results[pid] = result
+
+	return nil
+}
+
+func (m *mockPaymentAttemptDispatcher) GetPaymentResult(paymentID uint64) (
+	<-chan *htlcswitch.PaymentResult, error) {
+
+	c := make(chan *htlcswitch.PaymentResult, 1)
+	res, ok := m.results[paymentID]
+	if !ok {
+		return nil, htlcswitch.ErrPaymentIDNotFound
+	}
+	c <- res
+
+	return c, nil
+
 }
 
 func (m *mockPaymentAttemptDispatcher) setPaymentResult(