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election.go
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election.go
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/*
Copyright 2017 Google Inc.
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 etcd2topo
import (
"path"
"github.com/coreos/etcd/clientv3"
"golang.org/x/net/context"
"github.com/xsec-lab/go/vt/log"
"github.com/xsec-lab/go/vt/topo"
)
// NewMasterParticipation is part of the topo.Server interface
func (s *Server) NewMasterParticipation(name, id string) (topo.MasterParticipation, error) {
return &etcdMasterParticipation{
s: s,
name: name,
id: id,
stop: make(chan struct{}),
done: make(chan struct{}),
}, nil
}
// etcdMasterParticipation implements topo.MasterParticipation.
//
// We use a directory (in global election path, with the name) with
// ephemeral files in it, that contains the id. The oldest revision
// wins the election.
type etcdMasterParticipation struct {
// s is our parent etcd topo Server
s *Server
// name is the name of this MasterParticipation
name string
// id is the process's current id.
id string
// stop is a channel closed when Stop is called.
stop chan struct{}
// done is a channel closed when we're done processing the Stop
done chan struct{}
}
// WaitForMastership is part of the topo.MasterParticipation interface.
func (mp *etcdMasterParticipation) WaitForMastership() (context.Context, error) {
// If Stop was already called, mp.done is closed, so we are interrupted.
select {
case <-mp.done:
return nil, topo.NewError(topo.Interrupted, "mastership")
default:
}
electionPath := path.Join(electionsPath, mp.name)
var ld topo.LockDescriptor
// We use a cancelable context here. If stop is closed,
// we just cancel that context.
lockCtx, lockCancel := context.WithCancel(context.Background())
go func() {
select {
case <-mp.stop:
if ld != nil {
if err := ld.Unlock(context.Background()); err != nil {
log.Errorf("failed to unlock electionPath %v: %v", electionPath, err)
}
}
lockCancel()
close(mp.done)
}
}()
// Try to get the mastership, by getting a lock.
var err error
ld, err = mp.s.lock(lockCtx, electionPath, mp.id)
if err != nil {
// It can be that we were interrupted.
return nil, err
}
// We got the lock. Return the lockContext. If Stop() is called,
// it will cancel the lockCtx, and cancel the returned context.
return lockCtx, nil
}
// Stop is part of the topo.MasterParticipation interface
func (mp *etcdMasterParticipation) Stop() {
close(mp.stop)
<-mp.done
}
// GetCurrentMasterID is part of the topo.MasterParticipation interface
func (mp *etcdMasterParticipation) GetCurrentMasterID(ctx context.Context) (string, error) {
electionPath := path.Join(mp.s.root, electionsPath, mp.name)
// Get the keys in the directory, older first.
resp, err := mp.s.cli.Get(ctx, electionPath+"/",
clientv3.WithPrefix(),
clientv3.WithSort(clientv3.SortByModRevision, clientv3.SortAscend),
clientv3.WithLimit(1))
if err != nil {
return "", convertError(err, electionPath)
}
if len(resp.Kvs) == 0 {
// No key starts with this prefix, means nobody is the master.
return "", nil
}
return string(resp.Kvs[0].Value), nil
}