text: // Copyright 2016 The etcd Authors
  //
  // 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 recipe
  
  import (
  	"context"
  
  	v3 "go.etcd.io/etcd/clientv3"
  	"go.etcd.io/etcd/mvcc/mvccpb"
  )
  
  // Barrier creates a key in etcd to block processes, then deletes the key to
  // release all blocked processes.
  type Barrier struct {
  	client *v3.Client
  	ctx    context.Context
  
  	key string
  }
  
  func NewBarrier(client *v3.Client, key string) *Barrier {
  	return &Barrier{client, context.TODO(), key}
  }
  
  // Hold creates the barrier key causing processes to block on Wait.
  func (b *Barrier) Hold() error {
  	_, err := newKey(b.client, b.key, v3.NoLease)
  	return err
  }
  
  // Release deletes the barrier key to unblock all waiting processes.
  func (b *Barrier) Release() error {
  	_, err := b.client.Delete(b.ctx, b.key)
  	return err
  }
  
  // Wait blocks on the barrier key until it is deleted. If there is no key, Wait
  // assumes Release has already been called and returns immediately.
  func (b *Barrier) Wait() error {
  	resp, err := b.client.Get(b.ctx, b.key, v3.WithFirstKey()...)
  	if err != nil {
  		return err
  	}
  	if len(resp.Kvs) == 0 {
  		// key already removed
  		return nil
  	}
  	_, err = WaitEvents(
  		b.client,
  		b.key,
  		resp.Header.Revision,
  		[]mvccpb.Event_EventType{mvccpb.PUT, mvccpb.DELETE})
  	return err
  }
  