package lockfile

import (
	"errors"
	"fmt"
	"io/fs"
	"os"
	"syscall"
)

type Lock struct {
	file *os.File
}

func Acquire(path, noun, holder string) (*Lock, error) {
	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR|syscall.O_NOFOLLOW|syscall.O_CLOEXEC, 0o600)
	if err != nil {
		return nil, fmt.Errorf("open %s %s: %w", noun, path, err)
	}
	fi, err := file.Stat()
	if err != nil {
		file.Close()
		return nil, fmt.Errorf("inspect %s %s: %w", noun, path, err)
	}
	if !fi.Mode().IsRegular() {
		file.Close()
		return nil, fmt.Errorf("%s %s is a %s, not a regular file", noun, path, DescribeType(fi.Mode()))
	}
	for {
		err = syscall.Flock(int(file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
		if err == nil {
			return &Lock{file: file}, nil
		}
		if err == syscall.EINTR {
			continue
		}
		file.Close()
		if err == syscall.EWOULDBLOCK || err == syscall.EAGAIN {
			return nil, fmt.Errorf("%s %s is held by another process; %s", noun, path, holder)
		}
		return nil, fmt.Errorf("acquire %s %s: %w", noun, path, err)
	}
}

func (l *Lock) Release() error {
	unlockErr := syscall.Flock(int(l.file.Fd()), syscall.LOCK_UN)
	return errors.Join(unlockErr, l.file.Close())
}

func DescribeType(m fs.FileMode) string {
	switch {
	case m.IsRegular():
		return "regular file"
	case m.IsDir():
		return "directory"
	case m&fs.ModeSymlink != 0:
		return "symbolic link"
	case m&fs.ModeSocket != 0:
		return "socket"
	case m&fs.ModeNamedPipe != 0:
		return "named pipe"
	case m&fs.ModeDevice != 0:
		return "device"
	}
	return "file of type " + m.Type().String()
}