project/cache_lock.go

// Copyright 2026 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package project

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

// AcquireCacheLock implements fail-fast concurrency control across worktrees.
// If another worktree is currently synchronizing the shared cache, it returns an immediate error.
func AcquireCacheLock(cacheRoot string) (unlock func(), err error) {
	if err := os.MkdirAll(cacheRoot, 0755); err != nil {
		return nil, err
	}
	lockPath := filepath.Join(cacheRoot, ".cache.lock")
	f, err := os.OpenFile(lockPath, os.O_CREATE|os.O_RDWR, 0644)
	if err != nil {
		return nil, err
	}
	// Non-blocking exclusive lock
	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
		f.Close()
		return nil, fmt.Errorf("another Jiri process is currently synchronizing the package cache (lock held at %s); please wait and retry (error: %v)", lockPath, err)
	}
	return func() {
		syscall.Flock(int(f.Fd()), syscall.LOCK_UN)
		f.Close()
	}, nil
}