package flock

import (
	"errors"
	"os"

	"golang.org/x/sys/unix"
)

// 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, unix.LOCK_EX)
}

// TryLock will return a nil file if the file is already locked.
func TryLock(path string) (*os.File, error) {
	return lock(path, unix.LOCK_EX|unix.LOCK_NB)
}

func LockFile(f *os.File) error {
	_, err := lockFile(f, unix.LOCK_EX)
	return err
}

// Returns true if the lock was successfully acquired.
func TryLockFile(f *os.File) (bool, error) {
	return lockFile(f, unix.LOCK_EX|unix.LOCK_NB)
}

func lockFile(f *os.File, flags int) (bool, error) {
	if err := unix.Flock(int(f.Fd()), flags); err != nil {
		if flags&unix.LOCK_NB != 0 && errors.Is(err, unix.EAGAIN) {
			return false, nil
		}
		return false, err
	}
	return true, 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
	}
	ok, err := lockFile(f, flags)
	if err != nil || !ok {
		f.Close()
		f = nil
	}
	return f, err
}

// Unlock releases the lock acquired via the Lock function.
func Unlock(f *os.File) error {
	return f.Close()
}