itest: Add test for sendpayment with route hints

Adds an integration test to verify SendPayment successfully uses
invoice route hints for payments involving private channels.

Author: appilon

itest/list_on_test.go

@@ -177,6 +177,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "send direct payment simple taproot",
 		TestFunc: testSendDirectPaymentSimpleTaproot,
 	},
+	{
+		Name:     "sendpayment with route hints",
+		TestFunc: testSendPaymentWithRouteHints,
+	},
 	{
 		Name:     "immediate payment after channel opened",
 		TestFunc: testPaymentFollowingChannelOpen,

itest/lnd_payment_test.go

@@ -1396,3 +1396,48 @@ func testSendPaymentKeysendMPPFail(ht *lntest.HarnessTest) {
 	_, err = ht.ReceivePaymentUpdate(client)
 	require.Error(ht, err)
 }
+
+// testSendPaymentWithRouteHints tests sending a payment using route hints
+// derived from a private channel.
+func testSendPaymentWithRouteHints(ht *lntest.HarnessTest) {
+	// Setup a three-node network: Alice -> Bob -> Carol.
+	alice := ht.NewNode("Alice", nil)
+	defer ht.Shutdown(alice)
+	bob := ht.NewNode("Bob", nil)
+	defer ht.Shutdown(bob)
+	carol := ht.NewNode("Carol", nil)
+	defer ht.Shutdown(carol)
+
+	ht.ConnectNodes(alice, bob)
+	ht.ConnectNodes(bob, carol)
+
+	// Fund Alice and Bob.
+	ht.FundCoins(btcutil.SatoshiPerBitcoin, alice)
+	ht.FundCoins(btcutil.SatoshiPerBitcoin, bob)
+
+	// Open channels: Alice -> Bob (public), Bob -> Carol (private).
+	const chanAmt = btcutil.Amount(1_000_000)
+	aliceBobChanPoint := ht.OpenChannel(
+		alice, bob, lntest.OpenChannelParams{Amt: chanAmt},
+	)
+	defer ht.CloseChannel(alice, aliceBobChanPoint)
+	bobCarolChanPoint := ht.OpenChannel(
+		bob, carol, lntest.OpenChannelParams{Amt: chanAmt, Private: true},
+	)
+	defer ht.CloseChannel(bob, bobCarolChanPoint)
+
+	sendReq := &routerrpc.SendPaymentRequest{
+		PaymentRequest: carol.RPC.AddInvoice(&lnrpc.Invoice{
+			Value:   10_000,
+			Private: true, // include route hints
+		}).PaymentRequest,
+		FeeLimitSat:    int64(chanAmt),
+		TimeoutSeconds: 60,
+	}
+
+	// Send payment and assert success.
+	ht.AssertPaymentStatusFromStream(
+		alice.RPC.SendPayment(sendReq),
+		lnrpc.Payment_SUCCEEDED,
+	)
+}