/*
 * Copyright (c) 2020 Baidu, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package waitgroup

import (
	"fmt"
	"sync"
)

// SafeWaitGroup must not be copied after first use.
type SafeWaitGroup struct {
	wg sync.WaitGroup
	mu sync.RWMutex
	// wait indicate whether Wait is called, if true,
	// then any Add with positive delta will return error.
	wait bool
}

// Add adds delta, which may be negative, similar to sync.WaitGroup.
// If Add with a positive delta happens after Wait, it will return error,
// which prevent unsafe Add.
func (wg *SafeWaitGroup) Add(delta int) error {
	wg.mu.RLock()
	defer wg.mu.RUnlock()
	if wg.wait && delta > 0 {
		return fmt.Errorf("add with postive delta after Wait is forbidden")
	}
	wg.wg.Add(delta)
	return nil
}

// Done decrements the WaitGroup counter.
func (wg *SafeWaitGroup) Done() {
	wg.wg.Done()
}

// Wait blocks until the WaitGroup counter is zero.
func (wg *SafeWaitGroup) Wait() {
	wg.mu.Lock()
	wg.wait = true
	wg.mu.Unlock()
	wg.wg.Wait()
}
