itest: add safe HTLC failure edge case for custom channels

We enhance the liquidity edge cases itest with a new case which makes a
payment attempt over a last-hop that uses assets, and does not have
sufficient asset liquidity. The healthy expectation is for the HTLCs to
be added, as there exists partial liquidity, but they should eventually
fail. As a sanity check we then end things with a small payment that can
succeed, and assert its completion.

Author: GeorgeTsagk

itest/litd_custom_channels_test.go

@@ -30,6 +30,7 @@ import (
 	"github.com/lightningnetwork/lnd/fn"
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lnrpc/invoicesrpc"
+	"github.com/lightningnetwork/lnd/lnrpc/routerrpc"
 	"github.com/lightningnetwork/lnd/lnrpc/walletrpc"
 	"github.com/lightningnetwork/lnd/lntest"
 	"github.com/lightningnetwork/lnd/lntest/node"
@@ -2601,12 +2602,12 @@ func testCustomChannelsBreach(ctx context.Context, net *NetworkHarness,
 	t.Logf("Charlie UTXOs after breach: %v", toProtoJSON(t.t, charlieUTXOs))
 }
 
-// testCustomChannelsLiquidtyEdgeCasesCore is the core logic of the liquidity
+// testCustomChannelsLiquidityEdgeCasesCore is the core logic of the liquidity
 // edge cases. This test goes through certain scenarios that expose edge cases
 // and behaviors that proved to be buggy in the past and have been directly
 // addressed. It accepts an extra parameter which dictates whether it should use
 // group keys or asset IDs.
-func testCustomChannelsLiquidtyEdgeCasesCore(ctx context.Context,
+func testCustomChannelsLiquidityEdgeCasesCore(ctx context.Context,
 	net *NetworkHarness, t *harnessTest, groupMode bool) {
 
 	lndArgs := slices.Clone(lndArgsTemplate)
@@ -3104,6 +3105,69 @@ func testCustomChannelsLiquidtyEdgeCasesCore(ctx context.Context,
 	)
 
 	logBalance(t.t, nodes, assetID, "after small manual rfq")
+
+	// Edge case: Fabia creates an invoice which Erin cannot satisfy with
+	// his side of asset liquidity. This tests that Erin will not try to
+	// add an HTLC with more asset units than what his local balance is. To
+	// validate that the channel is still healthy, we follow up with a
+	// smaller invoice payment which is meant to succeed.
+
+	// We now create a hodl invoice on Fabia, for 125k assets.
+	hodlInv = createAssetHodlInvoice(t.t, erin, fabia, 125_000, assetID)
+
+	htlcStream, err := erin.RouterClient.SubscribeHtlcEvents(
+		ctx, &routerrpc.SubscribeHtlcEventsRequest{},
+	)
+	require.NoError(t.t, err)
+
+	// Charlie tries to pay, this is not meant to succeed, as Erin does not
+	// have enough assets to forward to Fabia.
+	payInvoiceWithAssets(
+		t.t, charlie, dave, hodlInv.payReq, assetID,
+		withFailure(lnrpc.Payment_IN_FLIGHT, failureNone),
+	)
+
+	// Let's check that at least 2 HTLCs were added on the Erin->Fabia link,
+	// which means that Erin would have an extra incoming HTLC for each
+	// outgoing one. So we expect a minimum of 4 HTLCs present on Erin.
+	assertMinNumHtlcs(t.t, erin, 4)
+
+	// We also want to make sure that at least one failure occurred that
+	// hinted at the problem (not enough assets to forward).
+	assertHtlcEvents(
+		t.t, htlcStream, withNumEvents(1),
+		withLinkFailure(routerrpc.FailureDetail_INSUFFICIENT_BALANCE),
+	)
+
+	logBalance(t.t, nodes, assetID, "with min 4 present HTLCs")
+
+	// Now Fabia cancels the invoice, this is meant to cancel back any
+	// locked in HTLCs and reset Erin's local balance back to its original
+	// value.
+	payHash = hodlInv.preimage.Hash()
+	_, err = fabia.InvoicesClient.CancelInvoice(
+		ctx, &invoicesrpc.CancelInvoiceMsg{
+			PaymentHash: payHash[:],
+		},
+	)
+	require.NoError(t.t, err)
+
+	// Let's assert that Erin cancelled all his HTLCs.
+	assertNumHtlcs(t.t, erin, 0)
+
+	logBalance(t.t, nodes, assetID, "after hodl cancel & 0 present HTLCs")
+
+	// Now let's create a smaller invoice and pay it, to validate that the
+	// channel is still healthy.
+	invoiceResp = createAssetInvoice(
+		t.t, erin, fabia, 50_000, assetID,
+	)
+
+	payInvoiceWithAssets(
+		t.t, charlie, dave, invoiceResp.PaymentRequest, assetID,
+	)
+
+	logBalance(t.t, nodes, assetID, "after safe asset htlc failure")
 }
 
 // testCustomChannelsLiquidityEdgeCases is a test that runs through some
@@ -3113,7 +3177,7 @@ func testCustomChannelsLiquidityEdgeCases(ctx context.Context,
 
 	// Run liquidity edge cases and only use single asset IDs for invoices
 	// and payments.
-	testCustomChannelsLiquidtyEdgeCasesCore(ctx, net, t, false)
+	testCustomChannelsLiquidityEdgeCasesCore(ctx, net, t, false)
 }
 
 // testCustomChannelsLiquidityEdgeCasesGroup is a test that runs through some
@@ -3123,7 +3187,7 @@ func testCustomChannelsLiquidityEdgeCasesGroup(ctx context.Context,
 
 	// Run liquidity edge cases and only use group keys for invoices and
 	// payments.
-	testCustomChannelsLiquidtyEdgeCasesCore(ctx, net, t, true)
+	testCustomChannelsLiquidityEdgeCasesCore(ctx, net, t, true)
 }
 
 // testCustomChannelsStrictForwarding is a test that tests the strict forwarding