lnrpc: wrap subservers in GrpcHandler

In order to be able to register the subservers with the root grpc server
before we have all dependencies available, we wrap them in an
GrpcHandler struct. This struct will initially hold an empty reference
to the subservers, which allows us to register with the GRPC server, and
later populate and create the subserver instance.

lnrpc/sub_server.go

@@ -32,7 +32,14 @@ type SubServer interface {
 	// Name returns a unique string representation of the sub-server. This
 	// can be used to identify the sub-server and also de-duplicate them.
 	Name() string
+}
 
+// GrpcHandler is the interface that should be registered with the root gRPC
+// server, and is the interface that implements the subserver's defined RPC.
+// Before the actual sub server has been created, this will be an empty shell
+// allowing us to start the gRPC server before we have all the dependencies
+// needed to create the subserver itself.
+type GrpcHandler interface {
 	// RegisterWithRootServer will be called by the root gRPC server to
 	// direct a sub RPC server to register itself with the main gRPC root
 	// server. Until this is called, each sub-server won't be able to have
@@ -45,6 +52,14 @@ type SubServer interface {
 	// routed towards it.
 	RegisterWithRestServer(context.Context, *runtime.ServeMux, string,
 		[]grpc.DialOption) error
+
+	// CreateSubServer populates the subserver's dependencies using the
+	// passed SubServerConfigDispatcher. This method should fully
+	// initialize the sub-server instance, making it ready for action. It
+	// returns the macaroon permissions that the sub-server wishes to pass
+	// on to the root server for all methods routed towards it.
+	CreateSubServer(subCfgs SubServerConfigDispatcher) (SubServer,
+		MacaroonPerms, error)
 }
 
 // SubServerConfigDispatcher is an interface that all sub-servers will use to
@@ -60,22 +75,18 @@ type SubServerConfigDispatcher interface {
 }
 
 // SubServerDriver is a template struct that allows the root server to create a
-// sub-server with minimal knowledge. The root server only need a fully
-// populated SubServerConfigDispatcher and with the aide of the
-// RegisterSubServers method, it's able to create and initialize all
-// sub-servers.
+// sub-server gRPC handler with minimal knowledge.
 type SubServerDriver struct {
 	// SubServerName is the full name of a sub-sever.
 	//
 	// NOTE: This MUST be unique.
 	SubServerName string
 
-	// New creates, and fully initializes a new sub-server instance with
-	// the aide of the SubServerConfigDispatcher. This closure should
-	// return the SubServer, ready for action, along with the set of
-	// macaroon permissions that the sub-server wishes to pass on to the
-	// root server for all methods routed towards it.
-	New func(subCfgs SubServerConfigDispatcher) (SubServer, MacaroonPerms, error)
+	// NewGrpcHandler creates a a new sub-server gRPC interface that can be
+	// registered with the root gRPC server. It is not expected that the
+	// SubServer is ready for operation before its CreateSubServer and
+	// Start methods have been called.
+	NewGrpcHandler func() GrpcHandler
 }
 
 var (