itest: add safe HTLC failure edge case for custom channels

Author: GeorgeTsagk

itest/litd_custom_channels_test.go

@@ -2163,6 +2163,41 @@ func testCustomChannelsLiquidityEdgeCases(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.
+	payInvoiceWithAssets(
+		t.t, charlie, dave, hodlInv.payReq, assetID,
+		withFailure(lnrpc.Payment_IN_FLIGHT, failureNone),
+	)
+
+	payHash = hodlInv.preimage.Hash()
+	_, err = fabia.InvoicesClient.CancelInvoice(
+		ctx, &invoicesrpc.CancelInvoiceMsg{
+			PaymentHash: payHash[:],
+		},
+	)
+	require.NoError(t.t, err)
+
+	// Now let's create a smaller invoice and pay it, to validate that the
+	// channel is still healthy.
+	invoiceResp = createAssetInvoice(
+		t.t, erin, fabia, 100, assetID,
+	)
+
+	payInvoiceWithAssets(
+		t.t, charlie, dave, invoiceResp.PaymentRequest, assetID,
+	)
+
+	logBalance(t.t, nodes, assetID, "after safe htlc failure")
 }
 
 // testCustomChannelsStrictForwarding is a test that tests the strict forwarding