itest: verify attributable failure hold times

Add an integration test that sets up Alice -> Bob -> Carol, triggers a
payment failure at Carol (unknown payment hash), and verifies that
Alice receives hold times in the RPC failure response. The test checks:
- hold_times has one entry per route hop (1:1 correspondence)
- failure source index matches Carol (index 2)
- each hold time is within a reasonable bound

Author: GeorgeTsagk

itest/list_on_test.go

@@ -723,6 +723,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "experimental accountability",
 		TestFunc: testExperimentalAccountability,
 	},
+	{
+		Name:     "attributable failure hold times",
+		TestFunc: testAttributableFailureHoldTimes,
+	},
 	{
 		Name:     "quiescence",
 		TestFunc: testQuiescence,

itest/lnd_attributable_failure_test.go

@@ -0,0 +1,103 @@
+package itest
+
+import (
+	"github.com/lightningnetwork/lnd/funding"
+	"github.com/lightningnetwork/lnd/lnrpc"
+	"github.com/lightningnetwork/lnd/lnrpc/routerrpc"
+	"github.com/lightningnetwork/lnd/lntest"
+	"github.com/stretchr/testify/require"
+)
+
+// testAttributableFailureHoldTimes verifies that when a payment fails at a
+// downstream node, the sender receives hold times in the failure response via
+// attributable errors. It sets up Alice -> Bob -> Carol, triggers a failure at
+// Carol (unknown payment hash), and checks that Alice's payment result includes
+// hold times that correctly correspond to the route hops.
+func testAttributableFailureHoldTimes(ht *lntest.HarnessTest) {
+	const chanAmt = funding.MaxBtcFundingAmount
+
+	alice := ht.NewNodeWithCoins("Alice", nil)
+	bob := ht.NewNodeWithCoins("Bob", nil)
+	carol := ht.NewNode("Carol", nil)
+
+	ht.EnsureConnected(alice, bob)
+	ht.ConnectNodes(bob, carol)
+
+	// Open channels: Alice -> Bob -> Carol.
+	chanPointAB := ht.OpenChannel(
+		alice, bob,
+		lntest.OpenChannelParams{Amt: chanAmt},
+	)
+	chanPointBC := ht.OpenChannel(
+		bob, carol,
+		lntest.OpenChannelParams{Amt: chanAmt},
+	)
+
+	// Wait for Alice to see both channels.
+	ht.AssertChannelInGraph(alice, chanPointAB)
+	ht.AssertChannelInGraph(alice, chanPointBC)
+
+	// Create an invoice from Carol to get valid route parameters.
+	carolInvoice := carol.RPC.AddInvoice(&lnrpc.Invoice{
+		Memo:  "hold-time-test",
+		Value: 10_000,
+	})
+	carolPayReq := carol.RPC.DecodePayReq(carolInvoice.PaymentRequest)
+
+	// Send a payment with a random (wrong) payment hash so Carol rejects
+	// it with INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS. This ensures the error
+	// originates at Carol (the final hop) and propagates back through Bob
+	// to Alice, with each hop adding its hold time.
+	sendReq := &routerrpc.SendPaymentRequest{
+		PaymentHash:    ht.Random32Bytes(),
+		Dest:           carol.PubKey[:],
+		Amt:            10_000,
+		FinalCltvDelta: int32(carolPayReq.CltvExpiry),
+		FeeLimitMsat:   noFeeLimitMsat,
+		MaxParts:       1,
+	}
+
+	payment := ht.SendPaymentAssertFail(
+		alice, sendReq,
+		lnrpc.PaymentFailureReason_FAILURE_REASON_INCORRECT_PAYMENT_DETAILS, //nolint:ll
+	)
+
+	// Verify we got at least one HTLC attempt with failure info.
+	require.NotEmpty(ht, payment.Htlcs, "expected at least one HTLC")
+	htlcAttempt := payment.Htlcs[len(payment.Htlcs)-1]
+	require.NotNil(ht, htlcAttempt.Failure, "expected failure info")
+
+	// The route should have 2 hops: Alice->Bob->Carol.
+	require.Len(ht, htlcAttempt.Route.Hops, 2,
+		"expected 2-hop route (Bob, Carol)")
+
+	// Verify the failure code and source. Carol is at index 2 (0=Alice,
+	// 1=Bob, 2=Carol).
+	require.Equal(ht,
+		lnrpc.Failure_INCORRECT_OR_UNKNOWN_PAYMENT_DETAILS,
+		htlcAttempt.Failure.Code,
+	)
+	require.EqualValues(ht, 2, htlcAttempt.Failure.FailureSourceIndex,
+		"failure should originate from Carol (index 2)")
+
+	// Verify hold times. With attributable errors, we expect one hold time
+	// entry per hop in the route. hold_times[0] corresponds to
+	// route.hops[0] (Bob) and hold_times[1] to route.hops[1] (Carol).
+	holdTimes := htlcAttempt.Failure.HoldTimes
+	require.Len(ht, holdTimes, len(htlcAttempt.Route.Hops),
+		"hold_times should have one entry per route hop")
+
+	// Hold times are in 100ms units. In a test environment with local
+	// nodes, processing should be nearly instant (likely 0-2 units).
+	// We verify each value is within a reasonable upper bound (< 10s)
+	// to catch any corruption or mis-encoding.
+	const maxReasonableHoldTime = uint32(100) // 10 seconds
+	for i, holdTime := range holdTimes {
+		require.LessOrEqual(ht, holdTime, maxReasonableHoldTime,
+			"hold time for hop %d (%s) unreasonably large: "+
+				"%d (= %dms)",
+			i, htlcAttempt.Route.Hops[i].PubKey,
+			holdTime, holdTime*100,
+		)
+	}
+}