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// 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 concurrency
import (
"context"
"errors"
"fmt"
"sync"
v3 "go.etcd.io/etcd/clientv3"
pb "go.etcd.io/etcd/etcdserver/etcdserverpb"
)
// ErrLocked is returned by TryLock when Mutex is already locked by another session.
var ErrLocked = errors.New("mutex: Locked by another session")
// Mutex implements the sync Locker interface with etcd
type Mutex struct {
s *Session
pfx string
myKey string
myRev int64
hdr *pb.ResponseHeader
}
func NewMutex(s *Session, pfx string) *Mutex {
return &Mutex{s, pfx + "/", "", -1, nil}
}
// TryLock locks the mutex if not already locked by another session.
// If lock is held by another session, return immediately after attempting necessary cleanup
// The ctx argument is used for the sending/receiving Txn RPC.
func (m *Mutex) TryLock(ctx context.Context) error {
resp, err := m.tryAcquire(ctx)
if err != nil {
return err
}
// if no key on prefix / the minimum rev is key, already hold the lock
ownerKey := resp.Responses[1].GetResponseRange().Kvs
if len(ownerKey) == 0 || ownerKey[0].CreateRevision == m.myRev {
m.hdr = resp.Header
return nil
}
client := m.s.Client()
// Cannot lock, so delete the key
if _, err := client.Delete(ctx, m.myKey); err != nil {
return err
}
m.myKey = "\x00"
m.myRev = -1
return ErrLocked
}
// Lock locks the mutex with a cancelable context. If the context is canceled
// while trying to acquire the lock, the mutex tries to clean its stale lock entry.
func (m *Mutex) Lock(ctx context.Context) error {
resp, err := m.tryAcquire(ctx)
if err != nil {
return err
}
// if no key on prefix / the minimum rev is key, already hold the lock
ownerKey := resp.Responses[1].GetResponseRange().Kvs
if len(ownerKey) == 0 || ownerKey[0].CreateRevision == m.myRev {
m.hdr = resp.Header
return nil
}
client := m.s.Client()
// wait for deletion revisions prior to myKey
hdr, werr := waitDeletes(ctx, client, m.pfx, m.myRev-1)
// release lock key if wait failed
if werr != nil {
m.Unlock(client.Ctx())
} else {
m.hdr = hdr
}
return werr
}
func (m *Mutex) tryAcquire(ctx context.Context) (*v3.TxnResponse, error) {
s := m.s
client := m.s.Client()
m.myKey = fmt.Sprintf("%s%x", m.pfx, s.Lease())
cmp := v3.Compare(v3.CreateRevision(m.myKey), "=", 0)
// put self in lock waiters via myKey; oldest waiter holds lock
put := v3.OpPut(m.myKey, "", v3.WithLease(s.Lease()))
// reuse key in case this session already holds the lock
get := v3.OpGet(m.myKey)
// fetch current holder to complete uncontended path with only one RPC
getOwner := v3.OpGet(m.pfx, v3.WithFirstCreate()...)
resp, err := client.Txn(ctx).If(cmp).Then(put, getOwner).Else(get, getOwner).Commit()
if err != nil {
return nil, err
}
m.myRev = resp.Header.Revision
if !resp.Succeeded {
m.myRev = resp.Responses[0].GetResponseRange().Kvs[0].CreateRevision
}
return resp, nil
}
func (m *Mutex) Unlock(ctx context.Context) error {
client := m.s.Client()
if _, err := client.Delete(ctx, m.myKey); err != nil {
return err
}
m.myKey = "\x00"
m.myRev = -1
return nil
}
func (m *Mutex) IsOwner() v3.Cmp {
return v3.Compare(v3.CreateRevision(m.myKey), "=", m.myRev)
}
func (m *Mutex) Key() string { return m.myKey }
// Header is the response header received from etcd on acquiring the lock.
func (m *Mutex) Header() *pb.ResponseHeader { return m.hdr }
type lockerMutex struct{ *Mutex }
func (lm *lockerMutex) Lock() {
client := lm.s.Client()
if err := lm.Mutex.Lock(client.Ctx()); err != nil {
panic(err)
}
}
func (lm *lockerMutex) Unlock() {
client := lm.s.Client()
if err := lm.Mutex.Unlock(client.Ctx()); err != nil {
panic(err)
}
}
// NewLocker creates a sync.Locker backed by an etcd mutex.
func NewLocker(s *Session, pfx string) sync.Locker {
return &lockerMutex{NewMutex(s, pfx)}
}