package main

import (
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"syscall"
)

// tokenLockFileName is the lock file of the token file in the data directory.
const tokenLockFileName = "auth.lock"

// errTokensInUse tells that another onechat process uses the token file.
var errTokensInUse = errors.New("another onechat process uses the token file, such as the daemon")

// errLocked tells that another process owns the chat.
var errLocked = errors.New("another process owns the chat")

// lockTokens takes the lock of the token file in dir. Each process that reads
// or refreshes the tokens holds it, because a refresh in one process rotates
// the refresh token of the other and logs it out.
func lockTokens(dir string) (*os.File, error) {
	lock, err := lockFile(filepath.Join(dir, tokenLockFileName))
	if errors.Is(err, errLocked) {
		return nil, errTokensInUse
	}

	return lock, err
}

// lockFile opens the file at path and takes an exclusive lock on it. The lock
// stays until the file closes or the process ends.
func lockFile(path string) (*os.File, error) {
	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
	if err != nil {
		return nil, fmt.Errorf("open lock file: %w", err)
	}

	err = syscall.Flock(int(file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
	if errors.Is(err, syscall.EWOULDBLOCK) {
		err = errLocked
	}

	if err != nil {
		return nil, errors.Join(fmt.Errorf("lock chat: %w", err), file.Close())
	}

	return file, nil
}