future.go

package raft

import (
	"sync"
	"time"
)

// Future is used to represent an action that may occur in the future.
type Future interface {
	// Error blocks until the future arrives and then
	// returns the error status of the future.
	// This may be called any number of times - all
	// calls will return the same value.
	// Note that it is not OK to call this method
	// twice concurrently on the same Future instance.
	Error() error
}

// ApplyFuture is used for Apply() and may return the FSM response.
type ApplyFuture interface {
	Future

	// Response returns the FSM response as returned
	// by the FSM.Apply method. This must not be called
	// until after the Error method has returned.
	Response() interface{}

	// Index holds the index of the newly applied log entry.
	// This must not be called
	// until after the Error method has returned.
	Index() uint64
}

// errorFuture is used to return a static error.
type errorFuture struct {
	err error
}

func (e errorFuture) Error() error {
	return e.err
}

func (e errorFuture) Response() interface{} {
	return nil
}

func (e errorFuture) Index() uint64 {
	return 0
}

// deferError can be embedded to allow a future
// to provide an error in the future.
type deferError struct {
	err       error
	errCh     chan error
	responded bool
}

func (d *deferError) init() {
	d.errCh = make(chan error, 1)
}

func (d *deferError) Error() error {
	if d.err != nil {
		// Note that when we've received a nil error, this
		// won't trigger, but the channel is closed after
		// send so we'll still return nil below.
		return d.err
	}
	if d.errCh == nil {
		panic("waiting for response on nil channel")
	}
	d.err = <-d.errCh
	return d.err
}

func (d *deferError) respond(err error) {
	if d.errCh == nil {
		return
	}
	if d.responded {
		return
	}
	d.errCh <- err
	close(d.errCh)
	d.responded = true
}

// logFuture is used to apply a log entry and waits until
// the log is considered committed.
type logFuture struct {
	deferError
	log      Log
	policy   quorumPolicy
	response interface{}
	dispatch time.Time
}

func (l *logFuture) Response() interface{} {
	return l.response
}

func (l *logFuture) Index() uint64 {
	return l.log.Index
}

type peerFuture struct {
	deferError
	peers []string
}

type shutdownFuture struct {
	raft *Raft
}

func (s *shutdownFuture) Error() error {
	if s.raft == nil {
		return nil
	}
	s.raft.waitShutdown()
	if closeable, ok := s.raft.trans.(WithClose); ok {
		closeable.Close()
	}
	return nil
}

// snapshotFuture is used for waiting on a snapshot to complete.
type snapshotFuture struct {
	deferError
}

// reqSnapshotFuture is used for requesting a snapshot start.
// It is only used internally.
type reqSnapshotFuture struct {
	deferError

	// snapshot details provided by the FSM runner before responding
	index    uint64
	term     uint64
	peers    []string
	snapshot FSMSnapshot
}

// restoreFuture is used for requesting an FSM to perform a
// snapshot restore. Used internally only.
type restoreFuture struct {
	deferError
	ID string
}

// verifyFuture is used to verify the current node is still
// the leader. This is to prevent a stale read.
type verifyFuture struct {
	deferError
	notifyCh   chan *verifyFuture
	quorumSize int
	votes      int
	voteLock   sync.Mutex
}

// vote is used to respond to a verifyFuture.
// This may block when responding on the notifyCh.
func (v *verifyFuture) vote(leader bool) {
	v.voteLock.Lock()
	defer v.voteLock.Unlock()

	// Guard against having notified already
	if v.notifyCh == nil {
		return
	}

	if leader {
		v.votes++
		if v.votes >= v.quorumSize {
			v.notifyCh <- v
			v.notifyCh = nil
		}
	} else {
		v.notifyCh <- v
		v.notifyCh = nil
	}
}

// appendFuture is used for waiting on a pipelined append
// entries RPC.
type appendFuture struct {
	deferError
	start time.Time
	args  *AppendEntriesRequest
	resp  *AppendEntriesResponse
}

func (a *appendFuture) Start() time.Time {
	return a.start
}

func (a *appendFuture) Request() *AppendEntriesRequest {
	return a.args
}

func (a *appendFuture) Response() *AppendEntriesResponse {
	return a.resp
}