go.etcd.io/etcd@v3.3.27+incompatible/clientv3/concurrency/mutex.go (about)

     1  // Copyright 2016 The etcd Authors
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package concurrency
    16  
    17  import (
    18  	"context"
    19  	"fmt"
    20  	"sync"
    21  
    22  	v3 "github.com/coreos/etcd/clientv3"
    23  	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
    24  )
    25  
    26  // Mutex implements the sync Locker interface with etcd
    27  type Mutex struct {
    28  	s *Session
    29  
    30  	pfx   string
    31  	myKey string
    32  	myRev int64
    33  	hdr   *pb.ResponseHeader
    34  }
    35  
    36  func NewMutex(s *Session, pfx string) *Mutex {
    37  	return &Mutex{s, pfx + "/", "", -1, nil}
    38  }
    39  
    40  // Lock locks the mutex with a cancelable context. If the context is canceled
    41  // while trying to acquire the lock, the mutex tries to clean its stale lock entry.
    42  func (m *Mutex) Lock(ctx context.Context) error {
    43  	s := m.s
    44  	client := m.s.Client()
    45  
    46  	m.myKey = fmt.Sprintf("%s%x", m.pfx, s.Lease())
    47  	cmp := v3.Compare(v3.CreateRevision(m.myKey), "=", 0)
    48  	// put self in lock waiters via myKey; oldest waiter holds lock
    49  	put := v3.OpPut(m.myKey, "", v3.WithLease(s.Lease()))
    50  	// reuse key in case this session already holds the lock
    51  	get := v3.OpGet(m.myKey)
    52  	// fetch current holder to complete uncontended path with only one RPC
    53  	getOwner := v3.OpGet(m.pfx, v3.WithFirstCreate()...)
    54  	resp, err := client.Txn(ctx).If(cmp).Then(put, getOwner).Else(get, getOwner).Commit()
    55  	if err != nil {
    56  		return err
    57  	}
    58  	m.myRev = resp.Header.Revision
    59  	if !resp.Succeeded {
    60  		m.myRev = resp.Responses[0].GetResponseRange().Kvs[0].CreateRevision
    61  	}
    62  	// if no key on prefix / the minimum rev is key, already hold the lock
    63  	ownerKey := resp.Responses[1].GetResponseRange().Kvs
    64  	if len(ownerKey) == 0 || ownerKey[0].CreateRevision == m.myRev {
    65  		m.hdr = resp.Header
    66  		return nil
    67  	}
    68  
    69  	// wait for deletion revisions prior to myKey
    70  	hdr, werr := waitDeletes(ctx, client, m.pfx, m.myRev-1)
    71  	// release lock key if wait failed
    72  	if werr != nil {
    73  		m.Unlock(client.Ctx())
    74  	} else {
    75  		m.hdr = hdr
    76  	}
    77  	return werr
    78  }
    79  
    80  func (m *Mutex) Unlock(ctx context.Context) error {
    81  	client := m.s.Client()
    82  	if _, err := client.Delete(ctx, m.myKey); err != nil {
    83  		return err
    84  	}
    85  	m.myKey = "\x00"
    86  	m.myRev = -1
    87  	return nil
    88  }
    89  
    90  func (m *Mutex) IsOwner() v3.Cmp {
    91  	return v3.Compare(v3.CreateRevision(m.myKey), "=", m.myRev)
    92  }
    93  
    94  func (m *Mutex) Key() string { return m.myKey }
    95  
    96  // Header is the response header received from etcd on acquiring the lock.
    97  func (m *Mutex) Header() *pb.ResponseHeader { return m.hdr }
    98  
    99  type lockerMutex struct{ *Mutex }
   100  
   101  func (lm *lockerMutex) Lock() {
   102  	client := lm.s.Client()
   103  	if err := lm.Mutex.Lock(client.Ctx()); err != nil {
   104  		panic(err)
   105  	}
   106  }
   107  func (lm *lockerMutex) Unlock() {
   108  	client := lm.s.Client()
   109  	if err := lm.Mutex.Unlock(client.Ctx()); err != nil {
   110  		panic(err)
   111  	}
   112  }
   113  
   114  // NewLocker creates a sync.Locker backed by an etcd mutex.
   115  func NewLocker(s *Session, pfx string) sync.Locker {
   116  	return &lockerMutex{NewMutex(s, pfx)}
   117  }