rpcserver+itest: enforce keysend minimum carrier HTLC amount

Fixes an issue where attempting to pay a below-dust carrier HTLC satoshi
amount would knock a channel offline.

Author: guggero

itest/channels_test.go

@@ -3,6 +3,7 @@ package itest
 import (
 	"context"
 
+	"github.com/lightninglabs/taproot-assets/internal/test"
 	"github.com/lightninglabs/taproot-assets/rfqmsg"
 	tchrpc "github.com/lightninglabs/taproot-assets/taprpc/tapchannelrpc"
 	"github.com/lightningnetwork/lnd/lnrpc"
@@ -66,6 +67,25 @@ func testChannelRPCs(t *harnessTest) {
 			"payment",
 	)
 
+	// Make sure that the minimum satoshi amount for a keysend payment is
+	// enforced.
+	stream, err = t.tapd.SendPayment(ctx, &tchrpc.SendPaymentRequest{
+		AssetAmount: 123,
+		AssetId:     dummyByteArr[:],
+		PaymentRequest: &routerrpc.SendPaymentRequest{
+			Dest: test.RandPubKey(t.t).SerializeCompressed(),
+			DestCustomRecords: map[uint64][]byte{
+				record.KeySendType: dummyByteArr[:],
+			},
+		},
+	})
+	require.NoError(t.t, err)
+
+	_, err = stream.Recv()
+	require.ErrorContains(
+		t.t, err, "keysend payment satoshi amount must be greater",
+	)
+
 	// Now let's also try the invoice path, which should fail because we
 	// don't have any asset channels with peers that we could ask for a
 	// quote.

rpcserver.go

@@ -7798,6 +7798,16 @@ func (r *rpcServer) SendPayment(req *tchrpc.SendPaymentRequest,
 				"pubkey: %w", err)
 		}
 
+		// We must enforce a minimum satoshi amount to make sure the
+		// carrier HTLC is above dust. This is handled by the traffic
+		// shaper for invoice-based payments, but for keysend payments
+		// we need to do it here.
+		if pReq.Amt < int64(rfqmath.DefaultOnChainHtlcSat) {
+			return fmt.Errorf("keysend payment satoshi amount "+
+				"must be greater than or equal to %d satoshis",
+				rfqmath.DefaultOnChainHtlcSat)
+		}
+
 		// We check that we have the asset amount available in the
 		// channel.
 		_, err = r.cfg.RfqManager.FetchChannel(