lnrpc+rpcserver: encode custom records as custom channel data

With this commit we encode the custom records as a TLV stream into the
custom channel data field of the invoice HTLC.
This allows the custom data parser to parse those records and replace it
with human-readable JSON on the RPC interface.

lnrpc/invoicesrpc/config_active.go

@@ -10,6 +10,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/macaroons"
 	"github.com/lightningnetwork/lnd/netann"
+	"google.golang.org/protobuf/proto"
 )
 
 // Config is the primary configuration struct for the invoices RPC server. It
@@ -69,4 +70,8 @@ type Config struct {
 	// GetAlias returns the peer's alias SCID if it exists given the
 	// 32-byte ChannelID.
 	GetAlias func(lnwire.ChannelID) (lnwire.ShortChannelID, error)
+
+	// ParseAuxData is a function that can be used to parse the auxiliary
+	// data from the invoice.
+	ParseAuxData func(message proto.Message) error
 }

lnrpc/invoicesrpc/invoices.swagger.json

@@ -784,6 +784,11 @@
         "amp": {
           "$ref": "#/definitions/lnrpcAMP",
           "description": "Details relevant to AMP HTLCs, only populated if this is an AMP HTLC."
+        },
+        "custom_channel_data": {
+          "type": "string",
+          "format": "byte",
+          "description": "Custom channel data that might be populated in custom channels."
         }
       },
       "title": "Details of an HTLC that paid to an invoice"

lnrpc/invoicesrpc/invoices_server.go

@@ -222,8 +222,9 @@ func (r *ServerShell) RegisterWithRestServer(ctx context.Context,
 // methods routed towards it.
 //
 // NOTE: This is part of the lnrpc.GrpcHandler interface.
-func (r *ServerShell) CreateSubServer(configRegistry lnrpc.SubServerConfigDispatcher) (
-	lnrpc.SubServer, lnrpc.MacaroonPerms, error) {
+func (r *ServerShell) CreateSubServer(
+	configRegistry lnrpc.SubServerConfigDispatcher) (lnrpc.SubServer,
+	lnrpc.MacaroonPerms, error) {
 
 	subServer, macPermissions, err := createNewSubServer(configRegistry)
 	if err != nil {
@@ -264,6 +265,14 @@ func (s *Server) SubscribeSingleInvoice(req *SubscribeSingleInvoiceRequest,
 				return err
 			}
 
+			// Give the aux data parser a chance to format the
+			// custom data in the invoice HTLCs.
+			err = s.cfg.ParseAuxData(rpcInvoice)
+			if err != nil {
+				return fmt.Errorf("error parsing custom data: "+
+					"%w", err)
+			}
+
 			if err := updateStream.Send(rpcInvoice); err != nil {
 				return err
 			}
@@ -451,7 +460,19 @@ func (s *Server) LookupInvoiceV2(ctx context.Context,
 		return nil, err
 	}
 
-	return CreateRPCInvoice(&invoice, s.cfg.ChainParams)
+	rpcInvoice, err := CreateRPCInvoice(&invoice, s.cfg.ChainParams)
+	if err != nil {
+		return nil, err
+	}
+
+	// Give the aux data parser a chance to format the custom data in the
+	// invoice HTLCs.
+	err = s.cfg.ParseAuxData(rpcInvoice)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing custom data: %w", err)
+	}
+
+	return rpcInvoice, nil
 }
 
 // HtlcModifier is a bidirectional streaming RPC that allows a client to

lnrpc/invoicesrpc/utils.go

@@ -123,6 +123,16 @@ func CreateRPCInvoice(invoice *invoices.Invoice,
 			MppTotalAmtMsat: uint64(htlc.MppTotalAmt),
 		}
 
+		// The custom channel data is currently just the raw bytes of
+		// the encoded custom records.
+		customData, err := lnwire.CustomRecords(
+			htlc.CustomRecords,
+		).Serialize()
+		if err != nil {
+			return nil, err
+		}
+		rpcHtlc.CustomChannelData = customData
+
 		// Populate any fields relevant to AMP payments.
 		if htlc.AMP != nil {
 			rootShare := htlc.AMP.Record.RootShare()