// Package lock provides per-app exclusive locking, so that concurrent skiff
// invocations against the same app - a deploy racing a config change, either
// racing a manual rollback - can't interleave their snapshot/apply/restart
// sequences against the same dataset and jail.
package lock

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

// Handle is a held lock; call Release to give it up.
type Handle struct {
	f *os.File
}

// Acquire blocks until it holds an exclusive lock on path, creating the file
// (and its parent directory) if needed. The lock is held via flock(2) on the
// open file descriptor, so it's released automatically if the process dies -
// a crash never leaves a stale lock behind.
func Acquire(path string) (*Handle, error) {
	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
		return nil, fmt.Errorf("create lock dir: %w", err)
	}
	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDONLY, 0644)
	if err != nil {
		return nil, fmt.Errorf("open lock file: %w", err)
	}
	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
		f.Close()
		return nil, fmt.Errorf("flock: %w", err)
	}
	return &Handle{f: f}, nil
}

// Release gives up the lock. Safe to call via defer immediately after a
// successful Acquire.
func (h *Handle) Release() error {
	defer h.f.Close()
	return syscall.Flock(int(h.f.Fd()), syscall.LOCK_UN)
}