vitess.io/vitess@v0.16.2/go/vt/topo/consultopo/election.go

/*
Copyright 2019 The Vitess 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 consultopo

import (
	"context"
	"path"

	"github.com/hashicorp/consul/api"

	"vitess.io/vitess/go/vt/log"
	"vitess.io/vitess/go/vt/topo"
)

// NewLeaderParticipation is part of the topo.Server interface
func (s *Server) NewLeaderParticipation(name, id string) (topo.LeaderParticipation, error) {
	return &consulLeaderParticipation{
		s:    s,
		name: name,
		id:   id,
		stop: make(chan struct{}),
		done: make(chan struct{}),
	}, nil
}

// consulLeaderParticipation implements topo.LeaderParticipation.
//
// We use a key with name <global>/elections/<name> for the lock,
// that contains the id.
type consulLeaderParticipation struct {
	// s is our parent consul topo Server
	s *Server

	// name is the name of this LeaderParticipation
	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{}
}

// WaitForLeadership is part of the topo.LeaderParticipation interface.
func (mp *consulLeaderParticipation) WaitForLeadership() (context.Context, error) {

	electionPath := path.Join(mp.s.root, electionsPath, mp.name)
	l, err := mp.s.client.LockOpts(&api.LockOptions{
		Key:   electionPath,
		Value: []byte(mp.id),
	})
	if err != nil {
		return nil, err
	}

	// If Stop was already called, mp.done is closed, so we are interrupted.
	select {
	case <-mp.done:
		return nil, topo.NewError(topo.Interrupted, "Leadership")
	default:
	}

	// Try to lock until mp.stop is closed.
	lost, err := l.Lock(mp.stop)
	if err != nil {
		// We can't lock. See if it was because we got canceled.
		select {
		case <-mp.stop:
			close(mp.done)
		default:
		}
		return nil, err
	}

	// We have the lock, keep primaryship until we lose it.
	lockCtx, lockCancel := context.WithCancel(context.Background())
	go func() {
		select {
		case <-lost:
			lockCancel()
			// We could have lost the lock. Per consul API, explicitly call Unlock to make sure that session will not be renewed.
			if err := l.Unlock(); err != nil {
				log.Errorf("Leader election(%v) Unlock failed: %v", mp.name, err)
			}
		case <-mp.stop:
			// Stop was called. We stop the context first,
			// so the running process is not thinking it
			// is the primary any more, then we unlock.
			lockCancel()
			if err := l.Unlock(); err != nil {
				log.Errorf("Leader election(%v) Unlock failed: %v", mp.name, err)
			}
			close(mp.done)
		}
	}()

	return lockCtx, nil
}

// Stop is part of the topo.LeaderParticipation interface
func (mp *consulLeaderParticipation) Stop() {
	close(mp.stop)
	<-mp.done
}

// GetCurrentLeaderID is part of the topo.LeaderParticipation interface
func (mp *consulLeaderParticipation) GetCurrentLeaderID(ctx context.Context) (string, error) {
	electionPath := path.Join(mp.s.root, electionsPath, mp.name)
	pair, _, err := mp.s.kv.Get(electionPath, nil)
	if err != nil {
		return "", err
	}
	if pair == nil {
		return "", nil
	}
	return string(pair.Value), nil
}

// WaitForNewLeader is part of the topo.LeaderParticipation interface
func (mp *consulLeaderParticipation) WaitForNewLeader(context.Context) (<-chan string, error) {
	// This isn't implemented yet, but likely can be implemented using List
	// with blocking logic on election path.
	// See also how WatchRecursive could be implemented as well.
	return nil, topo.NewError(topo.NoImplementation, "wait for leader not supported in Consul topo")
}