// TODO(mknyszek): Check other flags.
	check := func(name string, got, want int) {
		if got != want {
			t.Errorf("flag %s does not line up: got %d, want %d", name, got, want)
		}
	}
	check("O_WRONLY", runtime.O_WRONLY, syscall.O_WRONLY)
	check("O_CREAT", runtime.O_CREAT, syscall.O_CREAT)
	check("O_TRUNC", runtime.O_TRUNC, syscall.O_TRUNC)

	// consistently.

	f, err := os.OpenFile(name, flag&^os.O_TRUNC, perm)
	if err != nil {
		return nil, err
	}

	switch flag & (os.O_RDONLY | os.O_WRONLY | os.O_RDWR) {
	case os.O_WRONLY, os.O_RDWR:
		err = filelock.Lock(f)
	default:
		err = filelock.RLock(f)
	}
	if err != nil {
		f.Close()
		return nil, err
	}

	if flag&os.O_TRUNC == os.O_TRUNC {

#include <errno.h>
#include <signal.h>
*/
import "C"

const (
	EINTR     = C.EINTR
	EFAULT    = C.EFAULT
	EBUSY     = C.EBUSY
	EAGAIN    = C.EAGAIN
	ETIMEDOUT = C.ETIMEDOUT

	O_WRONLY   = C.O_WRONLY
	O_NONBLOCK = C.O_NONBLOCK
	O_CREAT    = C.O_CREAT
	O_TRUNC    = C.O_TRUNC
	O_CLOEXEC  = C.O_CLOEXEC

	PROT_NONE  = C.PROT_NONE
	PROT_READ  = C.PROT_READ
	PROT_WRITE = C.PROT_WRITE

func lockedOpenFile(path string, flag int, perm os.FileMode, lockType int) (*LockedFile, error) {
	switch flag {
	case syscall.O_RDONLY:
		lockType |= syscall.LOCK_SH
	case syscall.O_WRONLY:
		fallthrough
	case syscall.O_RDWR:
		fallthrough
	case syscall.O_WRONLY | syscall.O_CREAT:
		fallthrough
	case syscall.O_RDWR | syscall.O_CREAT:
		lockType |= syscall.LOCK_EX
	default:
		return nil, &os.PathError{
			Op:   "open",

#include <sys/time.h>
#include <sys/ucontext.h>
#include <sys/unistd.h>
#include <errno.h>
#include <signal.h>
*/
import "C"

const (
	EINTR  = C.EINTR
	EFAULT = C.EFAULT
	EAGAIN = C.EAGAIN

	O_WRONLY   = C.O_WRONLY
	O_NONBLOCK = C.O_NONBLOCK
	O_CREAT    = C.O_CREAT
	O_TRUNC    = C.O_TRUNC
	O_CLOEXEC  = C.O_CLOEXEC

	PROT_NONE  = C.PROT_NONE
	PROT_READ  = C.PROT_READ
	PROT_WRITE = C.PROT_WRITE

	var lockType int16
	switch flag {
	case syscall.O_RDONLY:
		lockType = syscall.F_RDLCK
	case syscall.O_WRONLY:
		fallthrough
	case syscall.O_RDWR:
		fallthrough
	case syscall.O_WRONLY | syscall.O_CREAT:
		fallthrough
	case syscall.O_RDWR | syscall.O_CREAT:
		lockType = syscall.F_WRLCK
	default:
		return nil, &os.PathError{
			Op:   "open",

package plan9

// Plan 9 Constants

// Open modes
const (
	O_RDONLY  = 0
	O_WRONLY  = 1
	O_RDWR    = 2
	O_TRUNC   = 16
	O_CLOEXEC = 32
	O_EXCL    = 0x1000
)

// Rfork flags
const (
	RFNAMEG  = 1 << 0
	RFENVG   = 1 << 1
	RFFDG    = 1 << 2
	RFNOTEG  = 1 << 3
	RFPROC   = 1 << 4
	RFMEM    = 1 << 5
	RFNOWAIT = 1 << 6
	RFCNAMEG = 1 << 10
	RFCENVG  = 1 << 11
	RFCFDG   = 1 << 12
	RFREND   = 1 << 13

	"io/fs"
	"os"
)

// hasWritePerm reports whether the current user has permission to write to the
// file with the given info.
func hasWritePerm(path string, _ fs.FileInfo) bool {
	if f, err := os.OpenFile(path, os.O_WRONLY, 0); err == nil {
		f.Close()
		return true
	}
	return false

  // When creating file, use the same permissions as original
  mode_t open_mode = mode & (S_IRWXU | S_IRWXG | S_IRWXO);

  // O_WRONLY | O_CREAT | O_TRUNC:
  //   Open file for write and if file does not exist, create the file.
  //   If file exists, truncate its size to 0.
  int dst_fd = open(dst, O_WRONLY | O_CREAT | O_TRUNC, open_mode);
  if (dst_fd < 0) {
    close(src_fd);
    return -1;
  }

			t.Fatal(err)
		}
	}()

	// lock the file
	l, err := LockedOpenFile(f.Name(), os.O_WRONLY, 0o600)
	if err != nil {
		t.Fatal(err)
	}

	// unlock the file
	if err = l.Close(); err != nil {
		t.Fatal(err)
	}

	// try lock the unlocked file
	dupl, err := LockedOpenFile(f.Name(), os.O_WRONLY|os.O_CREATE, 0o600)
	if err != nil {
		t.Errorf("err = %v, want %v", err, nil)
	}