itest: add safe HTLC failure edge case for custom channels

Author: GeorgeTsagk

itest/litd_custom_channels_test.go

@@ -2237,6 +2237,57 @@ 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)
+
+	// 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)
+
+	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