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William Kurkianea869482019-04-09 15:16:11 -04001// 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
15package concurrency
16
17import (
18 "context"
Devmalya Pauleb2e9552019-08-27 19:42:00 -040019 "errors"
William Kurkianea869482019-04-09 15:16:11 -040020 "fmt"
21 "sync"
22
23 v3 "go.etcd.io/etcd/clientv3"
24 pb "go.etcd.io/etcd/etcdserver/etcdserverpb"
25)
26
Devmalya Pauleb2e9552019-08-27 19:42:00 -040027// ErrLocked is returned by TryLock when Mutex is already locked by another session.
28var ErrLocked = errors.New("mutex: Locked by another session")
29
William Kurkianea869482019-04-09 15:16:11 -040030// Mutex implements the sync Locker interface with etcd
31type Mutex struct {
32 s *Session
33
34 pfx string
35 myKey string
36 myRev int64
37 hdr *pb.ResponseHeader
38}
39
40func NewMutex(s *Session, pfx string) *Mutex {
41 return &Mutex{s, pfx + "/", "", -1, nil}
42}
43
Devmalya Pauleb2e9552019-08-27 19:42:00 -040044// TryLock locks the mutex if not already locked by another session.
45// If lock is held by another session, return immediately after attempting necessary cleanup
46// The ctx argument is used for the sending/receiving Txn RPC.
47func (m *Mutex) TryLock(ctx context.Context) error {
48 resp, err := m.tryAcquire(ctx)
49 if err != nil {
50 return err
51 }
52 // if no key on prefix / the minimum rev is key, already hold the lock
53 ownerKey := resp.Responses[1].GetResponseRange().Kvs
54 if len(ownerKey) == 0 || ownerKey[0].CreateRevision == m.myRev {
55 m.hdr = resp.Header
56 return nil
57 }
58 client := m.s.Client()
59 // Cannot lock, so delete the key
60 if _, err := client.Delete(ctx, m.myKey); err != nil {
61 return err
62 }
63 m.myKey = "\x00"
64 m.myRev = -1
65 return ErrLocked
66}
67
William Kurkianea869482019-04-09 15:16:11 -040068// Lock locks the mutex with a cancelable context. If the context is canceled
69// while trying to acquire the lock, the mutex tries to clean its stale lock entry.
70func (m *Mutex) Lock(ctx context.Context) error {
Devmalya Pauleb2e9552019-08-27 19:42:00 -040071 resp, err := m.tryAcquire(ctx)
72 if err != nil {
73 return err
74 }
75 // if no key on prefix / the minimum rev is key, already hold the lock
76 ownerKey := resp.Responses[1].GetResponseRange().Kvs
77 if len(ownerKey) == 0 || ownerKey[0].CreateRevision == m.myRev {
78 m.hdr = resp.Header
79 return nil
80 }
81 client := m.s.Client()
82 // wait for deletion revisions prior to myKey
83 hdr, werr := waitDeletes(ctx, client, m.pfx, m.myRev-1)
84 // release lock key if wait failed
85 if werr != nil {
86 m.Unlock(client.Ctx())
87 } else {
88 m.hdr = hdr
89 }
90 return werr
91}
92
93func (m *Mutex) tryAcquire(ctx context.Context) (*v3.TxnResponse, error) {
William Kurkianea869482019-04-09 15:16:11 -040094 s := m.s
95 client := m.s.Client()
96
97 m.myKey = fmt.Sprintf("%s%x", m.pfx, s.Lease())
98 cmp := v3.Compare(v3.CreateRevision(m.myKey), "=", 0)
99 // put self in lock waiters via myKey; oldest waiter holds lock
100 put := v3.OpPut(m.myKey, "", v3.WithLease(s.Lease()))
101 // reuse key in case this session already holds the lock
102 get := v3.OpGet(m.myKey)
103 // fetch current holder to complete uncontended path with only one RPC
104 getOwner := v3.OpGet(m.pfx, v3.WithFirstCreate()...)
105 resp, err := client.Txn(ctx).If(cmp).Then(put, getOwner).Else(get, getOwner).Commit()
106 if err != nil {
Devmalya Pauleb2e9552019-08-27 19:42:00 -0400107 return nil, err
William Kurkianea869482019-04-09 15:16:11 -0400108 }
109 m.myRev = resp.Header.Revision
110 if !resp.Succeeded {
111 m.myRev = resp.Responses[0].GetResponseRange().Kvs[0].CreateRevision
112 }
Devmalya Pauleb2e9552019-08-27 19:42:00 -0400113 return resp, nil
William Kurkianea869482019-04-09 15:16:11 -0400114}
115
116func (m *Mutex) Unlock(ctx context.Context) error {
117 client := m.s.Client()
118 if _, err := client.Delete(ctx, m.myKey); err != nil {
119 return err
120 }
121 m.myKey = "\x00"
122 m.myRev = -1
123 return nil
124}
125
126func (m *Mutex) IsOwner() v3.Cmp {
127 return v3.Compare(v3.CreateRevision(m.myKey), "=", m.myRev)
128}
129
130func (m *Mutex) Key() string { return m.myKey }
131
132// Header is the response header received from etcd on acquiring the lock.
133func (m *Mutex) Header() *pb.ResponseHeader { return m.hdr }
134
135type lockerMutex struct{ *Mutex }
136
137func (lm *lockerMutex) Lock() {
138 client := lm.s.Client()
139 if err := lm.Mutex.Lock(client.Ctx()); err != nil {
140 panic(err)
141 }
142}
143func (lm *lockerMutex) Unlock() {
144 client := lm.s.Client()
145 if err := lm.Mutex.Unlock(client.Ctx()); err != nil {
146 panic(err)
147 }
148}
149
150// NewLocker creates a sync.Locker backed by an etcd mutex.
151func NewLocker(s *Session, pfx string) sync.Locker {
152 return &lockerMutex{NewMutex(s, pfx)}
153}