htlcswitch: implement strict forwarding for locally dispatched payments

In this commit, we address an issue that could arise when using the
SendToRoute RPC. In this RPC, we specify the exact hops that a payment
should take. However, within the switch, we would set a constraint for
the first hop to be any hop as long as the first peer was at the end of
it. This would cause discrepancies when attempting to use the RPC as the
payment would actually go through another hop with the same peer. We fix
this by explicitly specifying the channel ID of the first hop.

Fixes #1500.
Fixes #1515.

htlcswitch/switch_test.go

@@ -1330,8 +1330,7 @@ func TestSkipIneligibleLinksLocalForward(t *testing.T) {
 	// We'll attempt to send out a new HTLC that has Alice as the first
 	// outgoing link. This should fail as Alice isn't yet able to forward
 	// any active HTLC's.
-	alicePub := aliceChannelLink.Peer().PubKey()
-	_, err = s.SendHTLC(alicePub, addMsg, nil)
+	_, err = s.SendHTLC(aliceChannelLink.ShortChanID(), addMsg, nil)
 	if err == nil {
 		t.Fatalf("local forward should fail due to inactive link")
 	}
@@ -1656,7 +1655,8 @@ func TestSwitchSendPayment(t *testing.T) {
 	// Handle the request and checks that bob channel link received it.
 	errChan := make(chan error)
 	go func() {
-		_, err := s.SendHTLC(aliceChannelLink.Peer().PubKey(), update,
+		_, err := s.SendHTLC(
+			aliceChannelLink.ShortChanID(), update,
 			newMockDeobfuscator())
 		errChan <- err
 	}()
@@ -1664,8 +1664,10 @@ func TestSwitchSendPayment(t *testing.T) {
 	go func() {
 		// Send the payment with the same payment hash and same
 		// amount and check that it will be propagated successfully
-		_, err := s.SendHTLC(aliceChannelLink.Peer().PubKey(), update,
-			newMockDeobfuscator())
+		_, err := s.SendHTLC(
+			aliceChannelLink.ShortChanID(), update,
+			newMockDeobfuscator(),
+		)
 		errChan <- err
 	}()
 
@@ -1793,9 +1795,10 @@ func TestLocalPaymentNoForwardingEvents(t *testing.T) {
 	// wait for Alice to receive the preimage for the payment before
 	// proceeding.
 	receiver := n.bobServer
+	firstHop := n.firstBobChannelLink.ShortChanID()
 	_, err = n.makePayment(
-		n.aliceServer, receiver, n.bobServer.PubKey(), hops, amount,
-		htlcAmt, totalTimelock,
+		n.aliceServer, receiver, firstHop, hops, amount, htlcAmt,
+		totalTimelock,
 	).Wait(30 * time.Second)
 	if err != nil {
 		t.Fatalf("unable to make the payment: %v", err)
@@ -1852,10 +1855,11 @@ func TestMultiHopPaymentForwardingEvents(t *testing.T) {
 		finalAmt, testStartingHeight, n.firstBobChannelLink,
 		n.carolChannelLink,
 	)
+	firstHop := n.firstBobChannelLink.ShortChanID()
 	for i := 0; i < numPayments; i++ {
 		_, err := n.makePayment(
-			n.aliceServer, n.carolServer, n.bobServer.PubKey(),
-			hops, finalAmt, htlcAmt, totalTimelock,
+			n.aliceServer, n.carolServer, firstHop, hops, finalAmt,
+			htlcAmt, totalTimelock,
 		).Wait(30 * time.Second)
 		if err != nil {
 			t.Fatalf("unable to send payment: %v", err)