//go:build unix

package squad

import (
	"context"
	"os"
	"path/filepath"
	"syscall"
	"time"
)

// lockRunFile 对 runs/<id>/.lock 做非阻塞独占锁，并按 ctx 轮询。
// 直到拿到锁或 ctx 结束。ctx 结束时返回 ErrRunBusy。
func lockRunFile(ctx context.Context, dir string) (func(), error) {
	if err := os.MkdirAll(dir, 0o755); err != nil {
		return nil, err
	}
	f, err := os.OpenFile(filepath.Join(dir, ".lock"), os.O_CREATE|os.O_RDWR, 0o644)
	if err != nil {
		return nil, err
	}
	for {
		err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
		if err == nil {
			return func() {
				_ = syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
				_ = f.Close()
			}, nil
		}
		if err != syscall.EWOULDBLOCK && err != syscall.EAGAIN {
			_ = f.Close()
			return nil, err
		}
		select {
		case <-ctx.Done():
			_ = f.Close()
			return nil, ErrRunBusy
		case <-time.After(20 * time.Millisecond):
		}
	}
}