// Package lockfile provides a simple file-based exclusive lock primitive.
// It is used to serialize writes to ctask metadata files across cooperating
// ctask processes running in the same workspace.
package lockfile

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

// ErrTimeout is returned when Acquire cannot obtain the lock within its timeout.
var ErrTimeout = errors.New("lockfile: acquire timeout")

// Acquire attempts to create lockPath exclusively. If the file already exists
// and is older than staleAfter, it is removed and creation is retried.
// Retries with short backoff for up to timeout. On success, returns a
// release function that removes the lock file.
func Acquire(lockPath string, timeout, staleAfter time.Duration) (func(), error) {
	if err := os.MkdirAll(filepath.Dir(lockPath), 0755); err != nil {
		return nil, fmt.Errorf("preparing lock dir: %w", err)
	}

	deadline := time.Now().Add(timeout)
	backoff := 25 * time.Millisecond

	for {
		f, err := os.OpenFile(lockPath, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0644)
		if err == nil {
			f.Close()
			return func() { os.Remove(lockPath) }, nil
		}
		if !errors.Is(err, os.ErrExist) {
			return nil, fmt.Errorf("creating lock: %w", err)
		}

		if info, statErr := os.Stat(lockPath); statErr == nil {
			if time.Since(info.ModTime()) > staleAfter {
				os.Remove(lockPath)
				continue
			}
		}

		if time.Now().After(deadline) {
			return nil, ErrTimeout
		}
		time.Sleep(backoff)
		if backoff < 200*time.Millisecond {
			backoff *= 2
		}
	}
}