You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: user_linux.go
    path of file: ./repos/docker-ce/components/engine/vendor/github.com/moby/buildkit/solver/llbsolver/file
    
    the code of the file until where you have to start completion: package file

import (
	"os"

	"github.com/containerd/continuity/fs"
	"github.com/moby/buildkit/snapshot"
	"github.com/moby/buildkit/solver/llbsolver/ops/fileoptypes"
	"github.com/moby/buildkit/solver/pb"
	"github.com/opencontainers/runc/libcontain
    
func readUser(chopt *pb.ChownOpt, mu, mg fileoptypes.Mount) (*copy.User, error) {
	if chopt == nil {
		return nil, nil
	}
	var us copy.User
	if chopt.User != nil {
		switch u := chopt.User.User.(type) {
		case *pb.UserOpt_ByName:
			if mu == nil {
				return nil, errors.Errorf("invalid missing user mount")
			}
			mmu, ok := mu.(*Mount)
			if !ok {
				return nil, errors.Errorf("invalid mount type %T", mu)
			}
			lm := snapshot.LocalMounter(mmu.m)
			dir, err := lm.Mount()
			if err != nil {
				return nil, err
			}
			defer lm.Unmount()

			passwdPath, err := user.GetPasswdPath()
			if err != nil {
				return nil, err
			}

			passwdPath, err = fs.RootPath(dir, passwdPath)
			if err != nil {
				return nil, err
			}

			ufile, err := os.Open(passwdPath)
			if err != nil {
				return nil, err
			}
			defer ufile.Close()

			users, err := user.ParsePasswdFilter(ufile, func(uu user.User) bool {
				return uu.Name == u.ByName.Name
			})
			if err != nil {
				return nil, err
			}

			if len(users) > 0 {
				us.UID = users[0].Uid
				us.GID = users[0].Gid
			}
		case *pb.UserOpt_ByID:
			us.UID = int(u.ByID)
			us.GID = int(u.ByID)
		}
	}

	if chopt.Group != nil {
		switch u := chopt.Group.User.(type) {
		case *pb.UserOpt_ByName:
			if mg == nil {
				return nil, errors.Errorf("invalid missing group mount")
			}
			mmg, ok := mg.(*Mount)
			if !ok {
				return nil, errors.Errorf("invalid mount type %T", mg)
			}
			lm := snapshot.LocalMounter(mmg.m)
			dir, err := lm.Mount()
			if err != nil {
				return nil, err
			}
			defer lm.Unmount()

			groupPath, err := user.GetGroupPath()
			if err != nil {
				return nil, err
			}

			groupPath, err = fs.RootPath(dir, groupPath)
			if err != nil {
				return nil, err
			}

			gfile, err := os.Open(groupPath)
			if err != nil {
				return nil, err
			}
			defer gfile.Close()

			groups, err := user.ParseGroupFilter(gfile, func(g user.Group) bool {
				return g.Name == u.ByName.Name
			})
			if err != nil {
				return nil, err
			}

			if len(groups) > 0 {
				us.GID = groups[0].Gid
			}
		case *pb.UserOpt_ByID:
			us.GID = int(u.ByID)
		}
	}

	return &us, nil
}