fix `docker cp -a` failing to access `/` in container (run `getent` with a noop stdin)
fixes https://github.com/moby/moby/issues/45719 (actually, just a workaround)

**- What I did**
configure command to run `getent` to use a noop readCloser as stdin to prevent ENOENT failure to open `/dev/null`

**- How I did it**

**- How to verify it**
test case included

**- Description for the changelog**

**- A picture of a cute animal (not mandatory but encouraged)**
![image](https://github.com/moby/moby/assets/132757/19e58e57-3606-4a67-895e-cd450aa97ed2)


pkg/idtools/idtools_unix.go
//go:build !windows

package idtools // import "github.com/docker/docker/pkg/idtools"

import (
	"bytes"
	"fmt"
	"io"
	"os"
	"os/exec"
	"path/filepath"
	"strconv"
	"sync"
	"syscall"

	"github.com/opencontainers/runc/libcontainer/user"
)

var (
	entOnce   sync.Once
	getentCmd string
)

func mkdirAs(path string, mode os.FileMode, owner Identity, mkAll, chownExisting bool) error {
	path, err := filepath.Abs(path)
	if err != nil {
		return err
	}

	stat, err := os.Stat(path)
	if err == nil {
		if !stat.IsDir() {
			return &os.PathError{Op: "mkdir", Path: path, Err: syscall.ENOTDIR}
		}
		if !chownExisting {
			return nil
		}

		// short-circuit -- we were called with an existing directory and chown was requested
		return setPermissions(path, mode, owner, stat)
	}

	// make an array containing the original path asked for, plus (for mkAll == true)
	// all path components leading up to the complete path that don't exist before we MkdirAll
	// so that we can chown all of them properly at the end.  If chownExisting is false, we won't
	// chown the full directory path if it exists
	var paths []string
	if os.IsNotExist(err) {
		paths = []string{path}
	}

	if mkAll {
		// walk back to "/" looking for directories which do not exist
		// and add them to the paths array for chown after creation
		dirPath := path
		for {
			dirPath = filepath.Dir(dirPath)
			if dirPath == "/" {
				break
			}
			if _, err = os.Stat(dirPath); err != nil && os.IsNotExist(err) {
				paths = append(paths, dirPath)
			}
		}
		if err = os.MkdirAll(path, mode); err != nil {
			return err
		}
	} else if err = os.Mkdir(path, mode); err != nil {
		return err
	}
	// even if it existed, we will chown the requested path + any subpaths that
	// didn't exist when we called MkdirAll
	for _, pathComponent := range paths {
		if err = setPermissions(pathComponent, mode, owner, nil); err != nil {
			return err
		}
	}
	return nil
}

// LookupUser uses traditional local system files lookup (from libcontainer/user) on a username,
// followed by a call to `getent` for supporting host configured non-files passwd and group dbs
func LookupUser(name string) (user.User, error) {
	// first try a local system files lookup using existing capabilities
	usr, err := user.LookupUser(name)
	if err == nil {
		return usr, nil
	}
	// local files lookup failed; attempt to call `getent` to query configured passwd dbs
	usr, err = getentUser(name)
	if err != nil {
		return user.User{}, err
	}
	return usr, nil
}

// LookupUID uses traditional local system files lookup (from libcontainer/user) on a uid,
// followed by a call to `getent` for supporting host configured non-files passwd and group dbs
func LookupUID(uid int) (user.User, error) {
	// first try a local system files lookup using existing capabilities
	usr, err := user.LookupUid(uid)
	if err == nil {
		return usr, nil
	}
	// local files lookup failed; attempt to call `getent` to query configured passwd dbs
	return getentUser(strconv.Itoa(uid))
}

func getentUser(name string) (user.User, error) {
	reader, err := callGetent("passwd", name)
	if err != nil {
		return user.User{}, err
	}
	users, err := user.ParsePasswd(reader)
	if err != nil {
		return user.User{}, err
	}
	if len(users) == 0 {
		return user.User{}, fmt.Errorf("getent failed to find passwd entry for %q", name)
	}
	return users[0], nil
}

// LookupGroup uses traditional local system files lookup (from libcontainer/user) on a group name,
// followed by a call to `getent` for supporting host configured non-files passwd and group dbs
func LookupGroup(name string) (user.Group, error) {
	// first try a local system files lookup using existing capabilities
	group, err := user.LookupGroup(name)
	if err == nil {
		return group, nil
	}
	// local files lookup failed; attempt to call `getent` to query configured group dbs
	return getentGroup(name)
}

// LookupGID uses traditional local system files lookup (from libcontainer/user) on a group ID,
// followed by a call to `getent` for supporting host configured non-files passwd and group dbs
func LookupGID(gid int) (user.Group, error) {
	// first try a local system files lookup using existing capabilities
	group, err := user.LookupGid(gid)
	if err == nil {
		return group, nil
	}
	// local files lookup failed; attempt to call `getent` to query configured group dbs
	return getentGroup(strconv.Itoa(gid))
}

func getentGroup(name string) (user.Group, error) {
	reader, err := callGetent("group", name)
	if err != nil {
		return user.Group{}, err
	}
	groups, err := user.ParseGroup(reader)
	if err != nil {
		return user.Group{}, err
	}
	if len(groups) == 0 {
		return user.Group{}, fmt.Errorf("getent failed to find groups entry for %q", name)
	}
	return groups[0], nil
}

func callGetent(database, key string) (io.Reader, error) {
	entOnce.Do(func() { getentCmd, _ = resolveBinary("getent") })
	// if no `getent` command on host, can't do anything else
	if getentCmd == "" {
		return nil, fmt.Errorf("unable to find getent command")
	}
	out, err := exec.Command(getentCmd, database, key).CombinedOutput()
	if err != nil {
		exitCode, errC := getExitCode(err)
		if errC != nil {
			return nil, err
		}
		switch exitCode {
		case 1:
			return nil, fmt.Errorf("getent reported invalid parameters/database unknown")
		case 2:
			return nil, fmt.Errorf("getent unable to find entry %q in %s database", key, database)
		case 3:
			return nil, fmt.Errorf("getent database doesn't support enumeration")
		default:
			return nil, err
		}
	}
	return bytes.NewReader(out), nil
}

// getExitCode returns the ExitStatus of the specified error if its type is
// exec.ExitError, returns 0 and an error otherwise.
func getExitCode(err error) (int, error) {
	exitCode := 0
	if exiterr, ok := err.(*exec.ExitError); ok {
		if procExit, ok := exiterr.Sys().(syscall.WaitStatus); ok {
			return procExit.ExitStatus(), nil
		}
	}
	return exitCode, fmt.Errorf("failed to get exit code")
}

// setPermissions performs a chown/chmod only if the uid/gid don't match what's requested
// Normally a Chown is a no-op if uid/gid match, but in some cases this can still cause an error, e.g. if the
// dir is on an NFS share, so don't call chown unless we absolutely must.
// Likewise for setting permissions.
func setPermissions(p string, mode os.FileMode, owner Identity, stat os.FileInfo) error {
	if stat == nil {
		var err error
		stat, err = os.Stat(p)
		if err != nil {
			return err
		}
	}
	if stat.Mode().Perm() != mode.Perm() {
		if err := os.Chmod(p, mode.Perm()); err != nil {
			return err
		}
	}
	ssi := stat.Sys().(*syscall.Stat_t)
	if ssi.Uid == uint32(owner.UID) && ssi.Gid == uint32(owner.GID) {
		return nil
	}
	return os.Chown(p, owner.UID, owner.GID)
}

// LoadIdentityMapping takes a requested username and
// using the data from /etc/sub{uid,gid} ranges, creates the
// proper uid and gid remapping ranges for that user/group pair
func LoadIdentityMapping(name string) (IdentityMapping, error) {
	usr, err := LookupUser(name)
	if err != nil {
		return IdentityMapping{}, fmt.Errorf("could not get user for username %s: %v", name, err)
	}

	subuidRanges, err := lookupSubUIDRanges(usr)
	if err != nil {
		return IdentityMapping{}, err
	}
	subgidRanges, err := lookupSubGIDRanges(usr)
	if err != nil {
		return IdentityMapping{}, err
	}

	return IdentityMapping{
		UIDMaps: subuidRanges,
		GIDMaps: subgidRanges,
	}, nil
}

func lookupSubUIDRanges(usr user.User) ([]IDMap, error) {
	rangeList, err := parseSubuid(strconv.Itoa(usr.Uid))
	if err != nil {
		return nil, err
	}
	if len(rangeList) == 0 {
		rangeList, err = parseSubuid(usr.Name)
		if err != nil {
			return nil, err
		}
	}
	if len(rangeList) == 0 {
		return nil, fmt.Errorf("no subuid ranges found for user %q", usr.Name)
	}
	return createIDMap(rangeList), nil
}

func lookupSubGIDRanges(usr user.User) ([]IDMap, error) {
	rangeList, err := parseSubgid(strconv.Itoa(usr.Uid))
	if err != nil {
		return nil, err
	}
	if len(rangeList) == 0 {
		rangeList, err = parseSubgid(usr.Name)
		if err != nil {
			return nil, err
		}
	}
	if len(rangeList) == 0 {
		return nil, fmt.Errorf("no subgid ranges found for user %q", usr.Name)
	}
	return createIDMap(rangeList), nil
}

//go:build !windows

package idtools // import "github.com/docker/docker/pkg/idtools"

import (
	"bytes"
	"fmt"
	"io"
	"os"
	"os/exec"
	"path/filepath"
	"strconv"
	"sync"
	"syscall"

	"github.com/opencontainers/runc/libcontainer/user"
)

var (
	entOnce   sync.Once
	getentCmd string
)

func mkdirAs(path string, mode os.FileMode, owner Identity, mkAll, chownExisting bool) error {
	path, err := filepath.Abs(path)
	if err != nil {
		return err
	}

	stat, err := os.Stat(path)
	if err == nil {
		if !stat.IsDir() {
			return &os.PathError{Op: "mkdir", Path: path, Err: syscall.ENOTDIR}
		}
		if !chownExisting {
			return nil
		}

		// short-circuit -- we were called with an existing directory and chown was requested
		return setPermissions(path, mode, owner, stat)
	}

	// make an array containing the original path asked for, plus (for mkAll == true)
	// all path components leading up to the complete path that don't exist before we MkdirAll
	// so that we can chown all of them properly at the end.  If chownExisting is false, we won't
	// chown the full directory path if it exists
	var paths []string
	if os.IsNotExist(err) {
		paths = []string{path}
	}

	if mkAll {
		// walk back to "/" looking for directories which do not exist
		// and add them to the paths array for chown after creation
		dirPath := path
		for {
			dirPath = filepath.Dir(dirPath)
			if dirPath == "/" {
				break
			}
			if _, err = os.Stat(dirPath); err != nil && os.IsNotExist(err) {
				paths = append(paths, dirPath)
			}
		}
		if err = os.MkdirAll(path, mode); err != nil {
			return err
		}
	} else if err = os.Mkdir(path, mode); err != nil {
		return err
	}
	// even if it existed, we will chown the requested path + any subpaths that
	// didn't exist when we called MkdirAll
	for _, pathComponent := range paths {
		if err = setPermissions(pathComponent, mode, owner, nil); err != nil {
			return err
		}
	}
	return nil
}

// LookupUser uses traditional local system files lookup (from libcontainer/user) on a username,
// followed by a call to `getent` for supporting host configured non-files passwd and group dbs
func LookupUser(name string) (user.User, error) {
	// first try a local system files lookup using existing capabilities
	usr, err := user.LookupUser(name)
	if err == nil {
		return usr, nil
	}
	// local files lookup failed; attempt to call `getent` to query configured passwd dbs
	usr, err = getentUser(name)
	if err != nil {
		return user.User{}, err
	}
	return usr, nil
}

// LookupUID uses traditional local system files lookup (from libcontainer/user) on a uid,
// followed by a call to `getent` for supporting host configured non-files passwd and group dbs
func LookupUID(uid int) (user.User, error) {
	// first try a local system files lookup using existing capabilities
	usr, err := user.LookupUid(uid)
	if err == nil {
		return usr, nil
	}
	// local files lookup failed; attempt to call `getent` to query configured passwd dbs
	return getentUser(strconv.Itoa(uid))
}

func getentUser(name string) (user.User, error) {
	reader, err := callGetent("passwd", name)
	if err != nil {
		return user.User{}, err
	}
	users, err := user.ParsePasswd(reader)
	if err != nil {
		return user.User{}, err
	}
	if len(users) == 0 {
		return user.User{}, fmt.Errorf("getent failed to find passwd entry for %q", name)
	}
	return users[0], nil
}

// LookupGroup uses traditional local system files lookup (from libcontainer/user) on a group name,
// followed by a call to `getent` for supporting host configured non-files passwd and group dbs
func LookupGroup(name string) (user.Group, error) {
	// first try a local system files lookup using existing capabilities
	group, err := user.LookupGroup(name)
	if err == nil {
		return group, nil
	}
	// local files lookup failed; attempt to call `getent` to query configured group dbs
	return getentGroup(name)
}

// LookupGID uses traditional local system files lookup (from libcontainer/user) on a group ID,
// followed by a call to `getent` for supporting host configured non-files passwd and group dbs
func LookupGID(gid int) (user.Group, error) {
	// first try a local system files lookup using existing capabilities
	group, err := user.LookupGid(gid)
	if err == nil {
		return group, nil
	}
	// local files lookup failed; attempt to call `getent` to query configured group dbs
	return getentGroup(strconv.Itoa(gid))
}

func getentGroup(name string) (user.Group, error) {
	reader, err := callGetent("group", name)
	if err != nil {
		return user.Group{}, err
	}
	groups, err := user.ParseGroup(reader)
	if err != nil {
		return user.Group{}, err
	}
	if len(groups) == 0 {
		return user.Group{}, fmt.Errorf("getent failed to find groups entry for %q", name)
	}
	return groups[0], nil
}

func callGetent(database, key string) (io.Reader, error) {
	entOnce.Do(func() { getentCmd, _ = resolveBinary("getent") })
	// if no `getent` command on host, can't do anything else
	if getentCmd == "" {
		return nil, fmt.Errorf("unable to find getent command")
	}
	command := exec.Command(getentCmd, database, key)
	// we run getent within container filesystem, but without /dev so /dev/null is not available for exec to mock stdin
	command.Stdin = io.NopCloser(bytes.NewReader(nil))
	out, err := command.CombinedOutput()
	if err != nil {
		exitCode, errC := getExitCode(err)
		if errC != nil {
			return nil, err
		}
		switch exitCode {
		case 1:
			return nil, fmt.Errorf("getent reported invalid parameters/database unknown")
		case 2:
			return nil, fmt.Errorf("getent unable to find entry %q in %s database", key, database)
		case 3:
			return nil, fmt.Errorf("getent database doesn't support enumeration")
		default:
			return nil, err
		}
	}
	return bytes.NewReader(out), nil
}

// getExitCode returns the ExitStatus of the specified error if its type is
// exec.ExitError, returns 0 and an error otherwise.
func getExitCode(err error) (int, error) {
	exitCode := 0
	if exiterr, ok := err.(*exec.ExitError); ok {
		if procExit, ok := exiterr.Sys().(syscall.WaitStatus); ok {
			return procExit.ExitStatus(), nil
		}
	}
	return exitCode, fmt.Errorf("failed to get exit code")
}

// setPermissions performs a chown/chmod only if the uid/gid don't match what's requested
// Normally a Chown is a no-op if uid/gid match, but in some cases this can still cause an error, e.g. if the
// dir is on an NFS share, so don't call chown unless we absolutely must.
// Likewise for setting permissions.
func setPermissions(p string, mode os.FileMode, owner Identity, stat os.FileInfo) error {
	if stat == nil {
		var err error
		stat, err = os.Stat(p)
		if err != nil {
			return err
		}
	}
	if stat.Mode().Perm() != mode.Perm() {
		if err := os.Chmod(p, mode.Perm()); err != nil {
			return err
		}
	}
	ssi := stat.Sys().(*syscall.Stat_t)
	if ssi.Uid == uint32(owner.UID) && ssi.Gid == uint32(owner.GID) {
		return nil
	}
	return os.Chown(p, owner.UID, owner.GID)
}

// LoadIdentityMapping takes a requested username and
// using the data from /etc/sub{uid,gid} ranges, creates the
// proper uid and gid remapping ranges for that user/group pair
func LoadIdentityMapping(name string) (IdentityMapping, error) {
	usr, err := LookupUser(name)
	if err != nil {
		return IdentityMapping{}, fmt.Errorf("could not get user for username %s: %v", name, err)
	}

	subuidRanges, err := lookupSubUIDRanges(usr)
	if err != nil {
		return IdentityMapping{}, err
	}
	subgidRanges, err := lookupSubGIDRanges(usr)
	if err != nil {
		return IdentityMapping{}, err
	}

	return IdentityMapping{
		UIDMaps: subuidRanges,
		GIDMaps: subgidRanges,
	}, nil
}

func lookupSubUIDRanges(usr user.User) ([]IDMap, error) {
	rangeList, err := parseSubuid(strconv.Itoa(usr.Uid))
	if err != nil {
		return nil, err
	}
	if len(rangeList) == 0 {
		rangeList, err = parseSubuid(usr.Name)
		if err != nil {
			return nil, err
		}
	}
	if len(rangeList) == 0 {
		return nil, fmt.Errorf("no subuid ranges found for user %q", usr.Name)
	}
	return createIDMap(rangeList), nil
}

func lookupSubGIDRanges(usr user.User) ([]IDMap, error) {
	rangeList, err := parseSubgid(strconv.Itoa(usr.Uid))
	if err != nil {
		return nil, err
	}
	if len(rangeList) == 0 {
		rangeList, err = parseSubgid(usr.Name)
		if err != nil {
			return nil, err
		}
	}
	if len(rangeList) == 0 {
		return nil, fmt.Errorf("no subgid ranges found for user %q", usr.Name)
	}
	return createIDMap(rangeList), nil
}

Maybe a comment here why this is needed.