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.go

@@ -87,6 +87,7 @@ import (
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
+	"google.golang.org/protobuf/proto"
 	"gopkg.in/macaroon-bakery.v2/bakery"
 )
 
@@ -579,6 +580,17 @@ func MainRPCServerPermissions() map[string][]bakery.Op {
 	}
 }
 
+// AuxDataParser is an interface that is used to parse auxiliary custom data
+// within RPC messages. This is used to transform binary blobs to human-readable
+// JSON representations.
+type AuxDataParser interface {
+	// 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).
+	InlineParseCustomData(msg proto.Message) error
+}
+
 // rpcServer is a gRPC, RPC front end to the lnd daemon.
 // TODO(roasbeef): pagination support for the list-style calls
 type rpcServer struct {
@@ -731,6 +743,20 @@ func (r *rpcServer) addDeps(s *server, macService *macaroons.Service,
 		},
 		SetChannelAuto:     s.chanStatusMgr.RequestAuto,
 		UseStatusInitiated: subServerCgs.RouterRPC.UseStatusInitiated,
+		ParseCustomChannelData: func(msg proto.Message) error {
+			err = fn.MapOptionZ(
+				r.server.implCfg.AuxDataParser,
+				func(parser AuxDataParser) error {
+					return parser.InlineParseCustomData(msg)
+				},
+			)
+			if err != nil {
+				return fmt.Errorf("error parsing custom data: "+
+					"%w", err)
+			}
+
+			return nil
+		},
 	}
 
 	genInvoiceFeatures := func() *lnwire.FeatureVector {
@@ -3596,7 +3622,7 @@ func (r *rpcServer) ChannelBalance(ctx context.Context,
 		unsettledRemoteBalance, pendingOpenLocalBalance,
 		pendingOpenRemoteBalance)
 
-	return &lnrpc.ChannelBalanceResponse{
+	resp := &lnrpc.ChannelBalanceResponse{
 		LocalBalance: &lnrpc.Amount{
 			Sat:  uint64(localBalance.ToSatoshis()),
 			Msat: uint64(localBalance),
@@ -3626,7 +3652,19 @@ func (r *rpcServer) ChannelBalance(ctx context.Context,
 		// Deprecated fields.
 		Balance:            int64(localBalance.ToSatoshis()),
 		PendingOpenBalance: int64(pendingOpenLocalBalance.ToSatoshis()),
-	}, nil
+	}
+
+	err = fn.MapOptionZ(
+		r.server.implCfg.AuxDataParser,
+		func(parser AuxDataParser) error {
+			return parser.InlineParseCustomData(resp)
+		},
+	)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing custom data: %w", err)
+	}
+
+	return resp, nil
 }
 
 type (
@@ -4068,6 +4106,16 @@ func (r *rpcServer) PendingChannels(ctx context.Context,
 	resp.WaitingCloseChannels = waitingCloseChannels
 	resp.TotalLimboBalance += limbo
 
+	err = fn.MapOptionZ(
+		r.server.implCfg.AuxDataParser,
+		func(parser AuxDataParser) error {
+			return parser.InlineParseCustomData(resp)
+		},
+	)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing custom data: %w", err)
+	}
+
 	return resp, nil
 }
 
@@ -4382,6 +4430,16 @@ func (r *rpcServer) ListChannels(ctx context.Context,
 		resp.Channels = append(resp.Channels, channel)
 	}
 
+	err = fn.MapOptionZ(
+		r.server.implCfg.AuxDataParser,
+		func(parser AuxDataParser) error {
+			return parser.InlineParseCustomData(resp)
+		},
+	)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing custom data: %w", err)
+	}
+
 	return resp, nil
 }