itest: test single sat keysend payment with no asset balance

Tests an edge case that previously lead to a force close due to the
following error:
unable to sort commitment transaction: output and allocation size mismatch with error

Having a below-dust satoshi balance is only allowed when there is no
asset balance. But since such a dust output isn't materialized on-chain,
tapd needs to filter it out correctly and not create an allocation.

itest/assets_test.go

@@ -1364,7 +1364,12 @@ func sendAssetKeySendPayment(t *testing.T, src, dst *HarnessNode, amt uint64,
 }
 
 func sendKeySendPayment(t *testing.T, src, dst *HarnessNode,
-	amt btcutil.Amount) {
+	amt btcutil.Amount, opts ...payOpt) {
+
+	cfg := defaultPayConfig()
+	for _, opt := range opts {
+		opt(cfg)
+	}
 
 	ctxb := context.Background()
 	ctxt, cancel := context.WithTimeout(ctxb, defaultTimeout)
@@ -1382,12 +1387,21 @@ func sendKeySendPayment(t *testing.T, src, dst *HarnessNode,
 	customRecords := make(map[uint64][]byte)
 	customRecords[record.KeySendType] = preimage[:]
 
+	for key, value := range cfg.destCustomRecords {
+		customRecords[key] = value
+	}
+
 	req := &routerrpc.SendPaymentRequest{
 		Dest:              dst.PubKey[:],
 		Amt:               int64(amt),
 		DestCustomRecords: customRecords,
 		PaymentHash:       hash[:],
 		TimeoutSeconds:    int32(PaymentTimeout.Seconds()),
+		FeeLimitMsat:      1_000_000,
+		MaxParts:          cfg.maxShards,
+		OutgoingChanIds:   cfg.outgoingChanIDs,
+		AllowSelfPayment:  cfg.allowSelfPayment,
+		RouteHints:        cfg.routeHints,
 	}
 
 	stream, err := src.RouterClient.SendPaymentV2(ctxt, req)
@@ -1548,6 +1562,7 @@ type payConfig struct {
 	groupKey          []byte
 	outgoingChanIDs   []uint64
 	allowSelfPayment  bool
+	routeHints        []*lnrpc.RouteHint
 }
 
 func defaultPayConfig() *payConfig {
@@ -1631,6 +1646,12 @@ func withAllowSelfPayment() payOpt {
 	}
 }
 
+func withPayRouteHints(hints []*lnrpc.RouteHint) payOpt {
+	return func(c *payConfig) {
+		c.routeHints = hints
+	}
+}
+
 func payInvoiceWithAssets(t *testing.T, payer, rfqPeer *HarnessNode,
 	payReq string, assetID []byte,
 	opts ...payOpt) (uint64, rfqmath.BigIntFixedPoint) {

itest/litd_custom_channels_test.go

@@ -2389,7 +2389,7 @@ func testCustomChannelsLiquidityEdgeCasesCore(ctx context.Context,
 	)
 	charlieFundingAmount := cents.Amount - uint64(2*400_000)
 
-	_, _, _ = createTestAssetNetwork(
+	_, _, chanPointEF := createTestAssetNetwork(
 		t, net, charlieTap, daveTap, erinTap, fabiaTap, yaraTap,
 		universeTap, cents, 400_000, charlieFundingAmount,
 		daveFundingAmount, erinFundingAmount, 0,
@@ -2407,6 +2407,37 @@ func testCustomChannelsLiquidityEdgeCasesCore(ctx context.Context,
 
 	logBalance(t.t, nodes, assetID, "initial")
 
+	// Edge case: We send a single satoshi keysend payment from Dave to
+	// Fabia. Which will make it so that Fabia's balance in the channel
+	// between Erin and her is 1 satoshi, which is below the dust limit.
+	// This is only allowed while Fabia doesn't have any assets on her side
+	// yet.
+	erinFabiaChan := fetchChannel(t.t, fabia, chanPointEF)
+	hinEF := &lnrpc.HopHint{
+		NodeId:                    erin.PubKeyStr,
+		ChanId:                    erinFabiaChan.PeerScidAlias,
+		CltvExpiryDelta:           80,
+		FeeBaseMsat:               1000,
+		FeeProportionalMillionths: 1,
+	}
+	sendKeySendPayment(
+		t.t, dave, fabia, 1, withPayRouteHints([]*lnrpc.RouteHint{{
+			HopHints: []*lnrpc.HopHint{hinEF},
+		}}),
+	)
+	logBalance(t.t, nodes, assetID, "after single sat keysend")
+
+	// We make sure that a single sat keysend payment is not allowed when
+	// it carries assets.
+	sendAssetKeySendPayment(
+		t.t, erin, fabia, 123, assetID, fn.Some[int64](1),
+		withPayErrSubStr(
+			fmt.Sprintf("keysend payment satoshi amount must be "+
+				"greater than or equal to %d satoshis",
+				rfqmath.DefaultOnChainHtlcSat),
+		),
+	)
+
 	// Normal case.
 	// Send 50 assets from Charlie to Dave.
 	sendAssetKeySendPayment(