invoicesrpc: add InvoiceAcceptor RPC endpoint and acceptor RPC server

This commit introduces a singleton invoice acceptor RPC server and
an endpoint to activate it. The server interfaces with the internal
invoice settlement interpreter, handling the marshalling between RPC
types and internal formats.

Named "acceptor," it allows clients to accept invoice settlements, but
not to reject them.

Author: ffranr

lnrpc/invoicesrpc/invoice_acceptor.go

@@ -0,0 +1,111 @@
+package invoicesrpc
+
+import (
+	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/lightningnetwork/lnd/invoices"
+	"github.com/lightningnetwork/lnd/lntypes"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+// invoiceAcceptorConfig contains the configuration for an RPC invoice acceptor
+// server.
+type invoiceAcceptorConfig struct {
+	// chainParams is required to properly marshall an invoice for RPC.
+	chainParams *chaincfg.Params
+
+	// rpcServer is a bidirectional RPC server to send invoices to a client
+	// and receive accept responses from the client.
+	rpcServer Invoices_InvoiceAcceptorServer
+
+	// interceptor is the invoice interceptor that will be used to intercept
+	// and resolve invoices.
+	interceptor invoices.SettlementInterceptorInterface
+}
+
+// invoiceAcceptor is a helper struct that handles the lifecycle of an RPC
+// invoice acceptor server instance.
+//
+// This struct handles passing send and receive RPC messages between the client
+// and the invoice service.
+type invoiceAcceptor struct {
+	// cfg contains the configuration for the invoice acceptor.
+	cfg invoiceAcceptorConfig
+}
+
+// newInvoiceAcceptor creates a new RPC invoice acceptor handler.
+func newInvoiceAcceptor(cfg invoiceAcceptorConfig) *invoiceAcceptor {
+	return &invoiceAcceptor{
+		cfg: cfg,
+	}
+}
+
+// run sends the intercepted invoices to the client and receives the
+// corresponding responses.
+func (r *invoiceAcceptor) run() error {
+	// Register our invoice interceptor.
+	r.cfg.interceptor.SetClientCallback(r.onIntercept)
+	defer r.cfg.interceptor.SetClientCallback(nil)
+
+	// Listen for a response from the client in a loop.
+	for {
+		resp, err := r.cfg.rpcServer.Recv()
+		if err != nil {
+			return err
+		}
+
+		if err := r.resolveFromClient(resp); err != nil {
+			return err
+		}
+	}
+}
+
+// onIntercept is called when an invoice is intercepted by the invoice
+// interceptor. This method sends the invoice to the client.
+func (r *invoiceAcceptor) onIntercept(
+	req invoices.InterceptClientRequest) error {
+
+	// Convert the circuit key to an RPC circuit key.
+	rpcCircuitKey := &CircuitKey{
+		ChanId: req.ExitHtlcCircuitKey.ChanID.ToUint64(),
+		HtlcId: req.ExitHtlcCircuitKey.HtlcID,
+	}
+
+	// Convert the invoice to an RPC invoice.
+	rpcInvoice, err := CreateRPCInvoice(&req.Invoice, r.cfg.chainParams)
+	if err != nil {
+		return err
+	}
+
+	return r.cfg.rpcServer.Send(&InvoiceAcceptorRequest{
+		Invoice:            rpcInvoice,
+		ExitHtlcCircuitKey: rpcCircuitKey,
+		ExitHtlcAmt:        uint64(req.ExitHtlcAmt),
+		ExitHtlcExpiry:     req.ExitHtlcExpiry,
+		CurrentHeight:      req.CurrentHeight,
+	})
+}
+
+// resolveFromClient handles an invoice resolution received from the client.
+func (r *invoiceAcceptor) resolveFromClient(
+	in *InvoiceAcceptorResponse) error {
+
+	log.Tracef("Resolving invoice acceptor response %v", in)
+
+	// Parse the invoice preimage from the response.
+	if len(in.Preimage) != lntypes.HashSize {
+		return status.Errorf(codes.InvalidArgument,
+			"Preimage has invalid length: %d", len(in.Preimage))
+	}
+	preimage, err := lntypes.MakePreimage(in.Preimage)
+	if err != nil {
+		return status.Errorf(codes.InvalidArgument,
+			"Preimage is invalid: %v", err)
+	}
+
+	// Derive the payment hash from the preimage.
+	paymentHash := preimage.Hash()
+
+	// Pass the resolution to the interceptor.
+	return r.cfg.interceptor.Resolve(paymentHash, in.SkipAmountCheck)
+}