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 memorytopo

import (
	"context"
	"path"

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

// NewLeaderParticipation is part of the topo.Conn interface.
func (c *Conn) NewLeaderParticipation(name, id string) (topo.LeaderParticipation, error) {
	c.factory.callstats.Add([]string{"NewLeaderParticipation"}, 1)

	if c.closed.Load() {
		return nil, ErrConnectionClosed
	}

	c.factory.mu.Lock()
	defer c.factory.mu.Unlock()

	// Make sure the global path exists.
	electionPath := path.Join(electionsPath, name)
	if n := c.factory.getOrCreatePath(c.cell, electionPath); n == nil {
		return nil, topo.NewError(topo.NoNode, electionPath)
	}

	return &cLeaderParticipation{
		c:    c,
		name: name,
		id:   id,
		stop: make(chan struct{}),
		done: make(chan struct{}),
	}, nil
}

// cLeaderParticipation implements topo.LeaderParticipation.
//
// 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 cLeaderParticipation struct {
	// c is our memorytopo connection
	c *Conn

	// 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 *cLeaderParticipation) WaitForLeadership() (context.Context, error) {
	if mp.c.closed.Load() {
		return nil, ErrConnectionClosed
	}

	// 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:
	}

	electionPath := path.Join(electionsPath, mp.name)

	// We use a cancelable context here. If stop is closed,
	// we just cancel that context.
	lockCtx, lockCancel := context.WithCancel(context.Background())

	// Try to get the primaryship, by getting a lock.
	ld, err := mp.c.Lock(lockCtx, electionPath, mp.id)
	if err != nil {
		lockCancel()
		close(mp.done)
		// It can be that we were interrupted.
		return nil, err
	}

	go func() {
		<-mp.stop
		if err := ld.Unlock(context.Background()); err != nil {
			log.Errorf("failed to unlock LockDescriptor %v: %v", electionPath, err)
		}
		lockCancel()
		close(mp.done)
	}()

	// 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.LeaderParticipation interface
func (mp *cLeaderParticipation) Stop() {
	close(mp.stop)
	<-mp.done
}

// GetCurrentLeaderID is part of the topo.LeaderParticipation interface
func (mp *cLeaderParticipation) GetCurrentLeaderID(ctx context.Context) (string, error) {
	if mp.c.closed.Load() {
		return "", ErrConnectionClosed
	}

	electionPath := path.Join(electionsPath, mp.name)

	mp.c.factory.mu.Lock()
	defer mp.c.factory.mu.Unlock()

	n := mp.c.factory.nodeByPath(mp.c.cell, electionPath)
	if n == nil {
		return "", nil
	}

	return n.lockContents, nil
}

// WaitForNewLeader is part of the topo.LeaderParticipation interface
func (mp *cLeaderParticipation) WaitForNewLeader(ctx context.Context) (<-chan string, error) {
	if mp.c.closed.Load() {
		return nil, ErrConnectionClosed
	}

	mp.c.factory.mu.Lock()
	defer mp.c.factory.mu.Unlock()

	electionPath := path.Join(electionsPath, mp.name)
	n := mp.c.factory.nodeByPath(mp.c.cell, electionPath)
	if n == nil {
		return nil, topo.NewError(topo.NoNode, electionPath)
	}

	notifications := make(chan string, 8)
	watchIndex := n.addWatch(watch{lock: notifications})

	if n.lock != nil {
		notifications <- n.lockContents
	}

	go func() {
		defer close(notifications)

		select {
		case <-mp.stop:
		case <-ctx.Done():
		}

		mp.c.factory.mu.Lock()
		defer mp.c.factory.mu.Unlock()

		n := mp.c.factory.nodeByPath(mp.c.cell, electionPath)
		if n == nil {
			return
		}
		delete(n.watches, watchIndex)
	}()

	return notifications, nil
}