lnd: add aux data parser

This commit adds an optional data parser that can inspect and in-place
format custom data of certain RPC messages.
We don't add an implementation of the interface itself, as that will be
provided by external components when packaging up lnd as a bundle with
other software.

rpcserver_test.go

@@ -1,14 +1,79 @@
 package lnd
 
 import (
+	"fmt"
 	"testing"
 
+	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/fn"
+	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/proto"
 )
 
 func TestGetAllPermissions(t *testing.T) {
 	perms := GetAllPermissions()
 
-	// Currently there are there are 16 entity:action pairs in use.
+	// Currently there are 16 entity:action pairs in use.
 	assert.Equal(t, len(perms), 16)
 }
+
+// mockDataParser is a mock implementation of the AuxDataParser interface.
+type mockDataParser struct {
+}
+
+// InlineParseCustomData replaces any custom data binary blob in the given RPC
+// message with its corresponding JSON formatted data. This transforms the
+// binary (likely TLV encoded) data to a human-readable JSON representation
+// (still as byte slice).
+func (m *mockDataParser) InlineParseCustomData(msg proto.Message) error {
+	switch m := msg.(type) {
+	case *lnrpc.ChannelBalanceResponse:
+		m.CustomChannelData = []byte(`{"foo": "bar"}`)
+
+		return nil
+
+	default:
+		return fmt.Errorf("mock only supports ChannelBalanceResponse")
+	}
+}
+
+func TestAuxDataParser(t *testing.T) {
+	// We create an empty channeldb, so we can fetch some channels.
+	cdb, err := channeldb.Open(t.TempDir())
+	require.NoError(t, err)
+	t.Cleanup(func() {
+		require.NoError(t, cdb.Close())
+	})
+
+	r := &rpcServer{
+		server: &server{
+			chanStateDB: cdb.ChannelStateDB(),
+			implCfg: &ImplementationCfg{
+				AuxComponents: AuxComponents{
+					AuxDataParser: fn.Some[AuxDataParser](
+						&mockDataParser{},
+					),
+				},
+			},
+		},
+	}
+
+	// With the aux data parser in place, we should get a formatted JSON
+	// in the custom channel data field.
+	resp, err := r.ChannelBalance(nil, &lnrpc.ChannelBalanceRequest{})
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+	require.Equal(t, []byte(`{"foo": "bar"}`), resp.CustomChannelData)
+
+	// If we don't supply the aux data parser, we should get the raw binary
+	// data. Which in this case is just two VarInt fields (1 byte each) that
+	// represent the value of 0 (zero active and zero pending channels).
+	r.server.implCfg.AuxComponents.AuxDataParser = fn.None[AuxDataParser]()
+
+	resp, err = r.ChannelBalance(nil, &lnrpc.ChannelBalanceRequest{})
+	require.NoError(t, err)
+	require.NotNil(t, resp)
+	require.Equal(t, []byte{0x00, 0x00}, resp.CustomChannelData)
+}