//go:build linux

// The flock package provides a file-system mediated locking mechanism on linux
// using the `flock` system call.

package flock

import (
	"errors"
	"os"
	"syscall"
)

var ErrLocked = errors.New("locked")

// Lock gets an exclusive lock on the file at the given path. If the file
// doesn't exist, it's created.
func Lock(path string) (*os.File, error) {
	return lock(path, syscall.LOCK_EX)
}

// TryLock attempts to lock the file at path. It will return ErrLocked if the
// file is already locked.
func TryLock(path string) (*os.File, error) {
	return lock(path, syscall.LOCK_EX|syscall.LOCK_NB)
}

// LockFile is like Lock, but takes an *os.File.
func LockFile(f *os.File) error {
	return lockFile(f, syscall.LOCK_EX)
}

// TryLockFile is like TryLock, but takes an *os.File.
func TryLockFile(f *os.File) error {
	return lockFile(f, syscall.LOCK_EX|syscall.LOCK_NB)
}

func lockFile(f *os.File, flags int) error {
	if err := flock(int(f.Fd()), flags); err != nil {
		if flags&syscall.LOCK_NB != 0 && errors.Is(err, syscall.EAGAIN) {
			return ErrLocked
		}
		return err
	}
	return nil
}

func flock(fd int, how int) error {
	_, _, e1 := syscall.Syscall(syscall.SYS_FLOCK, uintptr(fd), uintptr(how), 0)
	if e1 != 0 {
		return syscall.Errno(e1)
	}
	return nil
}

func lock(path string, flags int) (*os.File, error) {
	perm := os.O_CREATE | os.O_RDWR
	f, err := os.OpenFile(path, perm, 0600)
	if err != nil {
		return nil, err
	}

	err = lockFile(f, flags)
	if err != nil {
		f.Close()
		f = nil
	}
	return f, err
}

// Unlock releases the lock acquired via the Lock function.
func Unlock(f *os.File) error {
	if err := flock(int(f.Fd()), syscall.LOCK_UN); err != nil {
		return err
	}
	return f.Close()
}