// flock acquires an advisory lock on a file descriptor.
func flock(f *os.File, exclusive bool, timeout time.Duration) error {
var t time.Time
for {
// If we're beyond our timeout then return an error.
// This can only occur after we've attempted a flock once.
if t.IsZero() {
t = time.Now()
} else if timeout > 0 && time.Since(t) > timeout {
return ErrTimeout
}
var lock syscall.Flock_t
lock.Start = 0
lock.Len = 0
lock.Pid = 0
lock.Whence = 0
lock.Pid = 0
if exclusive {
lock.Type = syscall.F_WRLCK
} else {
lock.Type = syscall.F_RDLCK
}
err := syscall.FcntlFlock(f.Fd(), syscall.F_SETLK, &lock)
if err == nil {
return nil
} else if err != syscall.EAGAIN {
return err
}
// Wait for a bit and try again.
time.Sleep(50 * time.Millisecond)
}
}
// TryLock acquires exclusivity on the lock without blocking
func (l *lock) TryLock() error {
var lock syscall.Flock_t
lock.Start = 0
lock.Len = 0
lock.Pid = 0
lock.Type = syscall.F_WRLCK
lock.Whence = 0
lock.Pid = 0
err := syscall.FcntlFlock(uintptr(l.fd), syscall.F_SETLK, &lock)
if err != nil && err == syscall.EAGAIN {
return ErrLocked
}
return err
}
// Lock acquires exclusivity on the lock without blocking
func (l *lock) Lock() error {
var lock syscall.Flock_t
lock.Start = 0
lock.Len = 0
lock.Type = syscall.F_WRLCK
lock.Whence = 0
lock.Pid = 0
return syscall.FcntlFlock(uintptr(l.fd), syscall.F_SETLK, &lock)
}
func TryLockFile(path string, flag int, perm os.FileMode) (*LockedFile, error) {
var lock syscall.Flock_t
lock.Start = 0
lock.Len = 0
lock.Pid = 0
lock.Type = syscall.F_WRLCK
lock.Whence = 0
lock.Pid = 0
f, err := os.OpenFile(path, flag, perm)
if err != nil {
return nil, err
}
if err := syscall.FcntlFlock(f.Fd(), syscall.F_SETLK, &lock); err != nil {
f.Close()
if err == syscall.EAGAIN {
err = ErrLocked
}
return nil, err
}
return &LockedFile{f}, nil
}
// TestFcntlFlock tests whether the file locking structure matches
// the calling convention of each kernel.
// On some Linux systems, glibc uses another set of values for the
// commands and translates them to the correct value that the kernel
// expects just before the actual fcntl syscall. As Go uses raw
// syscalls directly, it must use the real value, not the glibc value.
// Thus this test also verifies that the Flock_t structure can be
// roundtripped with F_SETLK and F_GETLK.
func TestFcntlFlock(t *testing.T) {
if runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64") {
t.Skip("skipping; no child processes allowed on iOS")
}
flock := syscall.Flock_t{
Type: syscall.F_WRLCK,
Start: 31415, Len: 271828, Whence: 1,
}
if os.Getenv("GO_WANT_HELPER_PROCESS") == "" {
// parent
name := filepath.Join(os.TempDir(), "TestFcntlFlock")
fd, err := syscall.Open(name, syscall.O_CREAT|syscall.O_RDWR|syscall.O_CLOEXEC, 0)
if err != nil {
t.Fatalf("Open failed: %v", err)
}
defer syscall.Unlink(name)
defer syscall.Close(fd)
if err := syscall.Ftruncate(fd, 1<<20); err != nil {
t.Fatalf("Ftruncate(1<<20) failed: %v", err)
}
if err := syscall.FcntlFlock(uintptr(fd), syscall.F_SETLK, &flock); err != nil {
t.Fatalf("FcntlFlock(F_SETLK) failed: %v", err)
}
cmd := exec.Command(os.Args[0], "-test.run=^TestFcntlFlock$")
cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
cmd.ExtraFiles = []*os.File{os.NewFile(uintptr(fd), name)}
out, err := cmd.CombinedOutput()
if len(out) > 0 || err != nil {
t.Fatalf("child process: %q, %v", out, err)
}
} else {
// child
got := flock
// make sure the child lock is conflicting with the parent lock
got.Start--
got.Len++
if err := syscall.FcntlFlock(3, syscall.F_GETLK, &got); err != nil {
t.Fatalf("FcntlFlock(F_GETLK) failed: %v", err)
}
flock.Pid = int32(syscall.Getppid())
// Linux kernel always set Whence to 0
flock.Whence = 0
if got.Type == flock.Type && got.Start == flock.Start && got.Len == flock.Len && got.Pid == flock.Pid && got.Whence == flock.Whence {
os.Exit(0)
}
t.Fatalf("FcntlFlock got %v, want %v", got, flock)
}
}