rpc: SendOutputs considers anchor reserve

lnrpc/walletrpc/errors.go

@@ -0,0 +1,17 @@
+package walletrpc
+
+import "errors"
+
+var (
+
+	// ErrZeroLabel is returned when an attempt is made to label a
+	// transaction with an empty label.
+	ErrZeroLabel = errors.New("cannot label transaction with empty " +
+		"label")
+
+	// ErrInsufficientReserve is returned when SendOutputs wouldn't leave
+	// enough funds in the wallet to cover for the anchor reserve.
+	ErrInsufficientReserve = errors.New("the outputs to be sent " +
+		"would leave insufficient reserves for anchor channels in " +
+		"the wallet")
+)

lnrpc/walletrpc/walletkit_server.go

@@ -222,10 +222,6 @@ var (
 	}
 )
 
-// ErrZeroLabel is returned when an attempt is made to label a transaction with
-// an empty label.
-var ErrZeroLabel = errors.New("cannot label transaction with empty label")
-
 // ServerShell is a shell struct holding a reference to the actual sub-server.
 // It is used to register the gRPC sub-server with the root server before we
 // have the necessary dependencies to populate the actual sub-server.
@@ -666,12 +662,14 @@ func (w *WalletKit) SendOutputs(ctx context.Context,
 	// Before we can request this transaction to be created, we'll need to
 	// amp the protos back into the format that the internal wallet will
 	// recognize.
+	var totalOutputValue int64
 	outputsToCreate := make([]*wire.TxOut, 0, len(req.Outputs))
 	for _, output := range req.Outputs {
 		outputsToCreate = append(outputsToCreate, &wire.TxOut{
 			Value:    output.Value,
 			PkScript: output.PkScript,
 		})
+		totalOutputValue += output.Value
 	}
 
 	// Then, we'll extract the minimum number of confirmations that each
@@ -681,13 +679,36 @@ func (w *WalletKit) SendOutputs(ctx context.Context,
 		return nil, err
 	}
 
+	// Before sending out funds we need to ensure that the remainder of our
+	// wallet funds would cover for the anchor reserve requirement. We'll
+	// also take unconfirmed funds into account.
+	walletBalance, err := w.cfg.Wallet.ConfirmedBalance(
+		0, lnwallet.DefaultAccountName,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	// We'll get the currently required reserve amount.
+	reserve, err := w.RequiredReserve(ctx, &RequiredReserveRequest{})
+	if err != nil {
+		return nil, err
+	}
+
+	// Then we check if our current wallet balance undershoots the required
+	// reserve if we'd send out the outputs specified in the request.
+	if int64(walletBalance)-totalOutputValue < reserve.RequiredReserve {
+		return nil, ErrInsufficientReserve
+	}
+
 	label, err := labels.ValidateAPI(req.Label)
 	if err != nil {
 		return nil, err
 	}
 
-	// Now that we have the outputs mapped, we can request that the wallet
-	// attempt to create this transaction.
+	// Now that we have the outputs mapped and checked for the reserve
+	// requirement, we can request that the wallet attempts to create this
+	// transaction.
 	tx, err := w.cfg.Wallet.SendOutputs(
 		outputsToCreate, chainfee.SatPerKWeight(req.SatPerKw), minConfs, label,
 	)

lntest/rpc/wallet_kit.go

@@ -290,3 +290,18 @@ func (h *HarnessRPC) ImportTapscript(
 
 	return resp
 }
+
+// RequiredReserve makes a RPC call to the node's WalletKitClient and asserts.
+//
+//nolint:lll
+func (h *HarnessRPC) RequiredReserve(
+	req *walletrpc.RequiredReserveRequest) *walletrpc.RequiredReserveResponse {
+
+	ctxt, cancel := context.WithTimeout(h.runCtx, DefaultTimeout)
+	defer cancel()
+
+	resp, err := h.WalletKit.RequiredReserve(ctxt, req)
+	h.NoError(err, "RequiredReserve")
+
+	return resp
+}