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
15package concurrency
16
17import (
18	"context"
19	"fmt"
20	"sync"
21
22	v3 "go.etcd.io/etcd/clientv3"
23	pb "go.etcd.io/etcd/etcdserver/etcdserverpb"
24)
25
26// Mutex implements the sync Locker interface with etcd
27type Mutex struct {
28	s *Session
29
30	pfx   string
31	myKey string
32	myRev int64
33	hdr   *pb.ResponseHeader
34}
35
36func 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.
42func (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
80func (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
90func (m *Mutex) IsOwner() v3.Cmp {
91	return v3.Compare(v3.CreateRevision(m.myKey), "=", m.myRev)
92}
93
94func (m *Mutex) Key() string { return m.myKey }
95
96// Header is the response header received from etcd on acquiring the lock.
97func (m *Mutex) Header() *pb.ResponseHeader { return m.hdr }
98
99type lockerMutex struct{ *Mutex }
100
101func (lm *lockerMutex) Lock() {
102	client := lm.s.Client()
103	if err := lm.Mutex.Lock(client.Ctx()); err != nil {
104		panic(err)
105	}
106}
107func (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.
115func NewLocker(s *Session, pfx string) sync.Locker {
116	return &lockerMutex{NewMutex(s, pfx)}
117}
118