Implement ServerConfig admin REST API (#3741)

Returns a valid config.json of the setup. In case of distributed
setup, it checks if quorum or more number of nodes have the same
config.json.

cmd/admin-rpc-server.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"encoding/json"
 	"errors"
 	"net/rpc"
 	"time"
@@ -54,6 +55,12 @@ type UptimeReply struct {
 	Uptime time.Duration
 }
 
+// ConfigReply - wraps the server config response over RPC.
+type ConfigReply struct {
+	AuthRPCReply
+	Config []byte // json-marshalled bytes of serverConfigV13
+}
+
 // Restart - Restart this instance of minio server.
 func (s *adminCmd) Restart(args *AuthRPCArgs, reply *AuthRPCReply) error {
 	if err := args.IsAuthenticated(); err != nil {
@@ -132,6 +139,25 @@ func (s *adminCmd) Uptime(args *AuthRPCArgs, reply *UptimeReply) error {
 	return nil
 }
 
+// GetConfig - returns the config.json of this server.
+func (s *adminCmd) GetConfig(args *AuthRPCArgs, reply *ConfigReply) error {
+	if err := args.IsAuthenticated(); err != nil {
+		return err
+	}
+
+	if serverConfig == nil {
+		return errors.New("config not present")
+	}
+
+	jsonBytes, err := json.Marshal(serverConfig)
+	if err != nil {
+		return err
+	}
+
+	reply.Config = jsonBytes
+	return nil
+}
+
 // registerAdminRPCRouter - registers RPC methods for service status,
 // stop and restart commands.
 func registerAdminRPCRouter(mux *router.Router) error {