package agent

import (
	"errors"
	"os"
	"path/filepath"

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

var ErrUploadBusy = errors.New("upload is already being handled by another process")

type UploadLock struct{ file *os.File }

// LockUpload uses an OS lock released on process death. The stable lock inode
// is never removed, so independent processes cannot lock different inodes.
func (s *Spool) LockUpload(id string) (*UploadLock, error) {
	if err := validateName(id); err != nil {
		return nil, err
	}
	root := filepath.Join(s.root, ".upload-locks")
	if err := os.MkdirAll(root, 0700); err != nil {
		return nil, err
	}
	file, err := os.OpenFile(filepath.Join(root, id), os.O_CREATE|os.O_RDWR, 0600)
	if err != nil {
		return nil, err
	}
	if err := unix.Flock(int(file.Fd()), unix.LOCK_EX|unix.LOCK_NB); err != nil {
		closeErr := file.Close()
		if errors.Is(err, unix.EWOULDBLOCK) {
			return nil, errors.Join(ErrUploadBusy, closeErr)
		}
		return nil, errors.Join(err, closeErr)
	}
	return &UploadLock{file: file}, nil
}

func (l *UploadLock) Close() error {
	if l == nil || l.file == nil {
		return nil
	}
	unlockErr := unix.Flock(int(l.file.Fd()), unix.LOCK_UN)
	closeErr := l.file.Close()
	l.file = nil
	return errors.Join(unlockErr, closeErr)
}