vendor/go.etcd.io/etcd/clientv3/lease.go - voltha-go - Gitiles

 // 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 clientv3

 import (
 	"context"
 	"sync"
 	"time"

 	"github.com/coreos/etcd/etcdserver/api/v3rpc/rpctypes"
 	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"

 	"google.golang.org/grpc"
 	"google.golang.org/grpc/metadata"
 )

 type (
 	LeaseRevokeResponse pb.LeaseRevokeResponse
 	LeaseID             int64
 )

 // LeaseGrantResponse wraps the protobuf message LeaseGrantResponse.
 type LeaseGrantResponse struct {
 	*pb.ResponseHeader
 	ID    LeaseID
 	TTL   int64
 	Error string
 }

 // LeaseKeepAliveResponse wraps the protobuf message LeaseKeepAliveResponse.
 type LeaseKeepAliveResponse struct {
 	*pb.ResponseHeader
 	ID  LeaseID
 	TTL int64
 }

 // LeaseTimeToLiveResponse wraps the protobuf message LeaseTimeToLiveResponse.
 type LeaseTimeToLiveResponse struct {
 	*pb.ResponseHeader
 	ID LeaseID `json:"id"`

 	// TTL is the remaining TTL in seconds for the lease; the lease will expire in under TTL+1 seconds. Expired lease will return -1.
 	TTL int64 `json:"ttl"`

 	// GrantedTTL is the initial granted time in seconds upon lease creation/renewal.
 	GrantedTTL int64 `json:"granted-ttl"`

 	// Keys is the list of keys attached to this lease.
 	Keys [][]byte `json:"keys"`
 }

 // LeaseStatus represents a lease status.
 type LeaseStatus struct {
 	ID LeaseID `json:"id"`
 	// TODO: TTL int64
 }

 // LeaseLeasesResponse wraps the protobuf message LeaseLeasesResponse.
 type LeaseLeasesResponse struct {
 	*pb.ResponseHeader
 	Leases []LeaseStatus `json:"leases"`
 }

 const (
 	// defaultTTL is the assumed lease TTL used for the first keepalive
 	// deadline before the actual TTL is known to the client.
 	defaultTTL = 5 * time.Second
 	// NoLease is a lease ID for the absence of a lease.
 	NoLease LeaseID = 0

 	// retryConnWait is how long to wait before retrying request due to an error
 	retryConnWait = 500 * time.Millisecond
 )

 // LeaseResponseChSize is the size of buffer to store unsent lease responses.
 // WARNING: DO NOT UPDATE.
 // Only for testing purposes.
 var LeaseResponseChSize = 16

 // ErrKeepAliveHalted is returned if client keep alive loop halts with an unexpected error.
 //
 // This usually means that automatic lease renewal via KeepAlive is broken, but KeepAliveOnce will still work as expected.
 type ErrKeepAliveHalted struct {
 	Reason error
 }

 func (e ErrKeepAliveHalted) Error() string {
 	s := "etcdclient: leases keep alive halted"
 	if e.Reason != nil {
 		s += ": " + e.Reason.Error()
 	}
 	return s
 }

 type Lease interface {
 	// Grant creates a new lease.
 	Grant(ctx context.Context, ttl int64) (*LeaseGrantResponse, error)

 	// Revoke revokes the given lease.
 	Revoke(ctx context.Context, id LeaseID) (*LeaseRevokeResponse, error)

 	// TimeToLive retrieves the lease information of the given lease ID.
 	TimeToLive(ctx context.Context, id LeaseID, opts ...LeaseOption) (*LeaseTimeToLiveResponse, error)

 	// Leases retrieves all leases.
 	Leases(ctx context.Context) (*LeaseLeasesResponse, error)

 	// KeepAlive keeps the given lease alive forever. If the keepalive response
 	// posted to the channel is not consumed immediately, the lease client will
 	// continue sending keep alive requests to the etcd server at least every
 	// second until latest response is consumed.
 	//
 	// The returned "LeaseKeepAliveResponse" channel closes if underlying keep
 	// alive stream is interrupted in some way the client cannot handle itself;
 	// given context "ctx" is canceled or timed out. "LeaseKeepAliveResponse"
 	// from this closed channel is nil.
 	//
 	// If client keep alive loop halts with an unexpected error (e.g. "etcdserver:
 	// no leader") or canceled by the caller (e.g. context.Canceled), the error
 	// is returned. Otherwise, it retries.
 	//
 	// TODO(v4.0): post errors to last keep alive message before closing
 	// (see https://github.com/coreos/etcd/pull/7866)
 	KeepAlive(ctx context.Context, id LeaseID) (<-chan *LeaseKeepAliveResponse, error)

 	// KeepAliveOnce renews the lease once. The response corresponds to the
 	// first message from calling KeepAlive. If the response has a recoverable
 	// error, KeepAliveOnce will retry the RPC with a new keep alive message.
 	//
 	// In most of the cases, Keepalive should be used instead of KeepAliveOnce.
 	KeepAliveOnce(ctx context.Context, id LeaseID) (*LeaseKeepAliveResponse, error)

 	// Close releases all resources Lease keeps for efficient communication
 	// with the etcd server.
 	Close() error
 }

 type lessor struct {
 	mu sync.Mutex // guards all fields

 	// donec is closed and loopErr is set when recvKeepAliveLoop stops
 	donec   chan struct{}
 	loopErr error

 	remote pb.LeaseClient

 	stream       pb.Lease_LeaseKeepAliveClient
 	streamCancel context.CancelFunc

 	stopCtx    context.Context
 	stopCancel context.CancelFunc

 	keepAlives map[LeaseID]*keepAlive

 	// firstKeepAliveTimeout is the timeout for the first keepalive request
 	// before the actual TTL is known to the lease client
 	firstKeepAliveTimeout time.Duration

 	// firstKeepAliveOnce ensures stream starts after first KeepAlive call.
 	firstKeepAliveOnce sync.Once

 	callOpts []grpc.CallOption
 }

 // keepAlive multiplexes a keepalive for a lease over multiple channels
 type keepAlive struct {
 	chs  []chan<- *LeaseKeepAliveResponse
 	ctxs []context.Context
 	// deadline is the time the keep alive channels close if no response
 	deadline time.Time
 	// nextKeepAlive is when to send the next keep alive message
 	nextKeepAlive time.Time
 	// donec is closed on lease revoke, expiration, or cancel.
 	donec chan struct{}
 }

 func NewLease(c *Client) Lease {
 	return NewLeaseFromLeaseClient(RetryLeaseClient(c), c, c.cfg.DialTimeout+time.Second)
 }

 func NewLeaseFromLeaseClient(remote pb.LeaseClient, c *Client, keepAliveTimeout time.Duration) Lease {
 	l := &lessor{
 		donec:                 make(chan struct{}),
 		keepAlives:            make(map[LeaseID]*keepAlive),
 		remote:                remote,
 		firstKeepAliveTimeout: keepAliveTimeout,
 	}
 	if l.firstKeepAliveTimeout == time.Second {
 		l.firstKeepAliveTimeout = defaultTTL
 	}
 	if c != nil {
 		l.callOpts = c.callOpts
 	}
 	reqLeaderCtx := WithRequireLeader(context.Background())
 	l.stopCtx, l.stopCancel = context.WithCancel(reqLeaderCtx)
 	return l
 }

 func (l *lessor) Grant(ctx context.Context, ttl int64) (*LeaseGrantResponse, error) {
 	r := &pb.LeaseGrantRequest{TTL: ttl}
 	resp, err := l.remote.LeaseGrant(ctx, r, l.callOpts...)
 	if err == nil {
 		gresp := &LeaseGrantResponse{
 			ResponseHeader: resp.GetHeader(),
 			ID:             LeaseID(resp.ID),
 			TTL:            resp.TTL,
 			Error:          resp.Error,
 		}
 		return gresp, nil
 	}
 	return nil, toErr(ctx, err)
 }

 func (l *lessor) Revoke(ctx context.Context, id LeaseID) (*LeaseRevokeResponse, error) {
 	r := &pb.LeaseRevokeRequest{ID: int64(id)}
 	resp, err := l.remote.LeaseRevoke(ctx, r, l.callOpts...)
 	if err == nil {
 		return (*LeaseRevokeResponse)(resp), nil
 	}
 	return nil, toErr(ctx, err)
 }

 func (l *lessor) TimeToLive(ctx context.Context, id LeaseID, opts ...LeaseOption) (*LeaseTimeToLiveResponse, error) {
 	r := toLeaseTimeToLiveRequest(id, opts...)
 	resp, err := l.remote.LeaseTimeToLive(ctx, r, l.callOpts...)
 	if err == nil {
 		gresp := &LeaseTimeToLiveResponse{
 			ResponseHeader: resp.GetHeader(),
 			ID:             LeaseID(resp.ID),
 			TTL:            resp.TTL,
 			GrantedTTL:     resp.GrantedTTL,
 			Keys:           resp.Keys,
 		}
 		return gresp, nil
 	}
 	return nil, toErr(ctx, err)
 }

 func (l *lessor) Leases(ctx context.Context) (*LeaseLeasesResponse, error) {
 	resp, err := l.remote.LeaseLeases(ctx, &pb.LeaseLeasesRequest{}, l.callOpts...)
 	if err == nil {
 		leases := make([]LeaseStatus, len(resp.Leases))
 		for i := range resp.Leases {
 			leases[i] = LeaseStatus{ID: LeaseID(resp.Leases[i].ID)}
 		}
 		return &LeaseLeasesResponse{ResponseHeader: resp.GetHeader(), Leases: leases}, nil
 	}
 	return nil, toErr(ctx, err)
 }

 func (l *lessor) KeepAlive(ctx context.Context, id LeaseID) (<-chan *LeaseKeepAliveResponse, error) {
 	ch := make(chan *LeaseKeepAliveResponse, LeaseResponseChSize)

 	l.mu.Lock()
 	// ensure that recvKeepAliveLoop is still running
 	select {
 	case <-l.donec:
 		err := l.loopErr
 		l.mu.Unlock()
 		close(ch)
 		return ch, ErrKeepAliveHalted{Reason: err}
 	default:
 	}
 	ka, ok := l.keepAlives[id]
 	if !ok {
 		// create fresh keep alive
 		ka = &keepAlive{
 			chs:           []chan<- *LeaseKeepAliveResponse{ch},
 			ctxs:          []context.Context{ctx},
 			deadline:      time.Now().Add(l.firstKeepAliveTimeout),
 			nextKeepAlive: time.Now(),
 			donec:         make(chan struct{}),
 		}
 		l.keepAlives[id] = ka
 	} else {
 		// add channel and context to existing keep alive
 		ka.ctxs = append(ka.ctxs, ctx)
 		ka.chs = append(ka.chs, ch)
 	}
 	l.mu.Unlock()

 	go l.keepAliveCtxCloser(id, ctx, ka.donec)
 	l.firstKeepAliveOnce.Do(func() {
 		go l.recvKeepAliveLoop()
 		go l.deadlineLoop()
 	})

 	return ch, nil
 }

 func (l *lessor) KeepAliveOnce(ctx context.Context, id LeaseID) (*LeaseKeepAliveResponse, error) {
 	for {
 		resp, err := l.keepAliveOnce(ctx, id)
 		if err == nil {
 			if resp.TTL <= 0 {
 				err = rpctypes.ErrLeaseNotFound
 			}
 			return resp, err
 		}
 		if isHaltErr(ctx, err) {
 			return nil, toErr(ctx, err)
 		}
 	}
 }

 func (l *lessor) Close() error {
 	l.stopCancel()
 	// close for synchronous teardown if stream goroutines never launched
 	l.firstKeepAliveOnce.Do(func() { close(l.donec) })
 	<-l.donec
 	return nil
 }

 func (l *lessor) keepAliveCtxCloser(id LeaseID, ctx context.Context, donec <-chan struct{}) {
 	select {
 	case <-donec:
 		return
 	case <-l.donec:
 		return
 	case <-ctx.Done():
 	}

 	l.mu.Lock()
 	defer l.mu.Unlock()

 	ka, ok := l.keepAlives[id]
 	if !ok {
 		return
 	}

 	// close channel and remove context if still associated with keep alive
 	for i, c := range ka.ctxs {
 		if c == ctx {
 			close(ka.chs[i])
 			ka.ctxs = append(ka.ctxs[:i], ka.ctxs[i+1:]...)
 			ka.chs = append(ka.chs[:i], ka.chs[i+1:]...)
 			break
 		}
 	}
 	// remove if no one more listeners
 	if len(ka.chs) == 0 {
 		delete(l.keepAlives, id)
 	}
 }

 // closeRequireLeader scans keepAlives for ctxs that have require leader
 // and closes the associated channels.
 func (l *lessor) closeRequireLeader() {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 	for _, ka := range l.keepAlives {
 		reqIdxs := 0
 		// find all required leader channels, close, mark as nil
 		for i, ctx := range ka.ctxs {
 			md, ok := metadata.FromOutgoingContext(ctx)
 			if !ok {
 				continue
 			}
 			ks := md[rpctypes.MetadataRequireLeaderKey]
 			if len(ks) < 1 || ks[0] != rpctypes.MetadataHasLeader {
 				continue
 			}
 			close(ka.chs[i])
 			ka.chs[i] = nil
 			reqIdxs++
 		}
 		if reqIdxs == 0 {
 			continue
 		}
 		// remove all channels that required a leader from keepalive
 		newChs := make([]chan<- *LeaseKeepAliveResponse, len(ka.chs)-reqIdxs)
 		newCtxs := make([]context.Context, len(newChs))
 		newIdx := 0
 		for i := range ka.chs {
 			if ka.chs[i] == nil {
 				continue
 			}
 			newChs[newIdx], newCtxs[newIdx] = ka.chs[i], ka.ctxs[newIdx]
 			newIdx++
 		}
 		ka.chs, ka.ctxs = newChs, newCtxs
 	}
 }

 func (l *lessor) keepAliveOnce(ctx context.Context, id LeaseID) (*LeaseKeepAliveResponse, error) {
 	cctx, cancel := context.WithCancel(ctx)
 	defer cancel()

 	stream, err := l.remote.LeaseKeepAlive(cctx, l.callOpts...)
 	if err != nil {
 		return nil, toErr(ctx, err)
 	}

 	err = stream.Send(&pb.LeaseKeepAliveRequest{ID: int64(id)})
 	if err != nil {
 		return nil, toErr(ctx, err)
 	}

 	resp, rerr := stream.Recv()
 	if rerr != nil {
 		return nil, toErr(ctx, rerr)
 	}

 	karesp := &LeaseKeepAliveResponse{
 		ResponseHeader: resp.GetHeader(),
 		ID:             LeaseID(resp.ID),
 		TTL:            resp.TTL,
 	}
 	return karesp, nil
 }

 func (l *lessor) recvKeepAliveLoop() (gerr error) {
 	defer func() {
 		l.mu.Lock()
 		close(l.donec)
 		l.loopErr = gerr
 		for _, ka := range l.keepAlives {
 			ka.close()
 		}
 		l.keepAlives = make(map[LeaseID]*keepAlive)
 		l.mu.Unlock()
 	}()

 	for {
 		stream, err := l.resetRecv()
 		if err != nil {
 			if canceledByCaller(l.stopCtx, err) {
 				return err
 			}
 		} else {
 			for {
 				resp, err := stream.Recv()
 				if err != nil {
 					if canceledByCaller(l.stopCtx, err) {
 						return err
 					}

 					if toErr(l.stopCtx, err) == rpctypes.ErrNoLeader {
 						l.closeRequireLeader()
 					}
 					break
 				}

 				l.recvKeepAlive(resp)
 			}
 		}

 		select {
 		case <-time.After(retryConnWait):
 			continue
 		case <-l.stopCtx.Done():
 			return l.stopCtx.Err()
 		}
 	}
 }

 // resetRecv opens a new lease stream and starts sending keep alive requests.
 func (l *lessor) resetRecv() (pb.Lease_LeaseKeepAliveClient, error) {
 	sctx, cancel := context.WithCancel(l.stopCtx)
 	stream, err := l.remote.LeaseKeepAlive(sctx, l.callOpts...)
 	if err != nil {
 		cancel()
 		return nil, err
 	}

 	l.mu.Lock()
 	defer l.mu.Unlock()
 	if l.stream != nil && l.streamCancel != nil {
 		l.streamCancel()
 	}

 	l.streamCancel = cancel
 	l.stream = stream

 	go l.sendKeepAliveLoop(stream)
 	return stream, nil
 }

 // recvKeepAlive updates a lease based on its LeaseKeepAliveResponse
 func (l *lessor) recvKeepAlive(resp *pb.LeaseKeepAliveResponse) {
 	karesp := &LeaseKeepAliveResponse{
 		ResponseHeader: resp.GetHeader(),
 		ID:             LeaseID(resp.ID),
 		TTL:            resp.TTL,
 	}

 	l.mu.Lock()
 	defer l.mu.Unlock()

 	ka, ok := l.keepAlives[karesp.ID]
 	if !ok {
 		return
 	}

 	if karesp.TTL <= 0 {
 		// lease expired; close all keep alive channels
 		delete(l.keepAlives, karesp.ID)
 		ka.close()
 		return
 	}

 	// send update to all channels
 	nextKeepAlive := time.Now().Add((time.Duration(karesp.TTL) * time.Second) / 3.0)
 	ka.deadline = time.Now().Add(time.Duration(karesp.TTL) * time.Second)
 	for _, ch := range ka.chs {
 		select {
 		case ch <- karesp:
 		default:
 		}
 		// still advance in order to rate-limit keep-alive sends
 		ka.nextKeepAlive = nextKeepAlive
 	}
 }

 // deadlineLoop reaps any keep alive channels that have not received a response
 // within the lease TTL
 func (l *lessor) deadlineLoop() {
 	for {
 		select {
 		case <-time.After(time.Second):
 		case <-l.donec:
 			return
 		}
 		now := time.Now()
 		l.mu.Lock()
 		for id, ka := range l.keepAlives {
 			if ka.deadline.Before(now) {
 				// waited too long for response; lease may be expired
 				ka.close()
 				delete(l.keepAlives, id)
 			}
 		}
 		l.mu.Unlock()
 	}
 }

 // sendKeepAliveLoop sends keep alive requests for the lifetime of the given stream.
 func (l *lessor) sendKeepAliveLoop(stream pb.Lease_LeaseKeepAliveClient) {
 	for {
 		var tosend []LeaseID

 		now := time.Now()
 		l.mu.Lock()
 		for id, ka := range l.keepAlives {
 			if ka.nextKeepAlive.Before(now) {
 				tosend = append(tosend, id)
 			}
 		}
 		l.mu.Unlock()

 		for _, id := range tosend {
 			r := &pb.LeaseKeepAliveRequest{ID: int64(id)}
 			if err := stream.Send(r); err != nil {
 				// TODO do something with this error?
 				return
 			}
 		}

 		select {
 		case <-time.After(500 * time.Millisecond):
 		case <-stream.Context().Done():
 			return
 		case <-l.donec:
 			return
 		case <-l.stopCtx.Done():
 			return
 		}
 	}
 }

 func (ka *keepAlive) close() {
 	close(ka.donec)
 	for _, ch := range ka.chs {
 		close(ch)
 	}
 }
	// 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 clientv3

	import (
	"context"
	"sync"
	"time"

	"github.com/coreos/etcd/etcdserver/api/v3rpc/rpctypes"
	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"

	"google.golang.org/grpc"
	"google.golang.org/grpc/metadata"
	)

	type (
	LeaseRevokeResponse pb.LeaseRevokeResponse
	LeaseID int64
	)

	// LeaseGrantResponse wraps the protobuf message LeaseGrantResponse.
	type LeaseGrantResponse struct {
	*pb.ResponseHeader
	ID LeaseID
	TTL int64
	Error string
	}

	// LeaseKeepAliveResponse wraps the protobuf message LeaseKeepAliveResponse.
	type LeaseKeepAliveResponse struct {
	*pb.ResponseHeader
	ID LeaseID
	TTL int64
	}

	// LeaseTimeToLiveResponse wraps the protobuf message LeaseTimeToLiveResponse.
	type LeaseTimeToLiveResponse struct {
	*pb.ResponseHeader
	ID LeaseID `json:"id"`

	// TTL is the remaining TTL in seconds for the lease; the lease will expire in under TTL+1 seconds. Expired lease will return -1.
	TTL int64 `json:"ttl"`

	// GrantedTTL is the initial granted time in seconds upon lease creation/renewal.
	GrantedTTL int64 `json:"granted-ttl"`

	// Keys is the list of keys attached to this lease.
	Keys [][]byte `json:"keys"`
	}

	// LeaseStatus represents a lease status.
	type LeaseStatus struct {
	ID LeaseID `json:"id"`
	// TODO: TTL int64
	}

	// LeaseLeasesResponse wraps the protobuf message LeaseLeasesResponse.
	type LeaseLeasesResponse struct {
	*pb.ResponseHeader
	Leases []LeaseStatus `json:"leases"`
	}

	const (
	// defaultTTL is the assumed lease TTL used for the first keepalive
	// deadline before the actual TTL is known to the client.
	defaultTTL = 5 * time.Second
	// NoLease is a lease ID for the absence of a lease.
	NoLease LeaseID = 0

	// retryConnWait is how long to wait before retrying request due to an error
	retryConnWait = 500 * time.Millisecond
	)

	// LeaseResponseChSize is the size of buffer to store unsent lease responses.
	// WARNING: DO NOT UPDATE.
	// Only for testing purposes.
	var LeaseResponseChSize = 16

	// ErrKeepAliveHalted is returned if client keep alive loop halts with an unexpected error.
	//
	// This usually means that automatic lease renewal via KeepAlive is broken, but KeepAliveOnce will still work as expected.
	type ErrKeepAliveHalted struct {
	Reason error
	}

	func (e ErrKeepAliveHalted) Error() string {
	s := "etcdclient: leases keep alive halted"
	if e.Reason != nil {
	s += ": " + e.Reason.Error()
	}
	return s
	}

	type Lease interface {
	// Grant creates a new lease.
	Grant(ctx context.Context, ttl int64) (*LeaseGrantResponse, error)

	// Revoke revokes the given lease.
	Revoke(ctx context.Context, id LeaseID) (*LeaseRevokeResponse, error)

	// TimeToLive retrieves the lease information of the given lease ID.
	TimeToLive(ctx context.Context, id LeaseID, opts ...LeaseOption) (*LeaseTimeToLiveResponse, error)

	// Leases retrieves all leases.
	Leases(ctx context.Context) (*LeaseLeasesResponse, error)

	// KeepAlive keeps the given lease alive forever. If the keepalive response
	// posted to the channel is not consumed immediately, the lease client will
	// continue sending keep alive requests to the etcd server at least every
	// second until latest response is consumed.
	//
	// The returned "LeaseKeepAliveResponse" channel closes if underlying keep
	// alive stream is interrupted in some way the client cannot handle itself;
	// given context "ctx" is canceled or timed out. "LeaseKeepAliveResponse"
	// from this closed channel is nil.
	//
	// If client keep alive loop halts with an unexpected error (e.g. "etcdserver:
	// no leader") or canceled by the caller (e.g. context.Canceled), the error
	// is returned. Otherwise, it retries.
	//
	// TODO(v4.0): post errors to last keep alive message before closing
	// (see https://github.com/coreos/etcd/pull/7866)
	KeepAlive(ctx context.Context, id LeaseID) (<-chan *LeaseKeepAliveResponse, error)

	// KeepAliveOnce renews the lease once. The response corresponds to the
	// first message from calling KeepAlive. If the response has a recoverable
	// error, KeepAliveOnce will retry the RPC with a new keep alive message.
	//
	// In most of the cases, Keepalive should be used instead of KeepAliveOnce.
	KeepAliveOnce(ctx context.Context, id LeaseID) (*LeaseKeepAliveResponse, error)

	// Close releases all resources Lease keeps for efficient communication
	// with the etcd server.
	Close() error
	}

	type lessor struct {
	mu sync.Mutex // guards all fields

	// donec is closed and loopErr is set when recvKeepAliveLoop stops
	donec chan struct{}
	loopErr error

	remote pb.LeaseClient

	stream pb.Lease_LeaseKeepAliveClient
	streamCancel context.CancelFunc

	stopCtx context.Context
	stopCancel context.CancelFunc

	keepAlives map[LeaseID]*keepAlive

	// firstKeepAliveTimeout is the timeout for the first keepalive request
	// before the actual TTL is known to the lease client
	firstKeepAliveTimeout time.Duration

	// firstKeepAliveOnce ensures stream starts after first KeepAlive call.
	firstKeepAliveOnce sync.Once

	callOpts []grpc.CallOption
	}

	// keepAlive multiplexes a keepalive for a lease over multiple channels
	type keepAlive struct {
	chs []chan<- *LeaseKeepAliveResponse
	ctxs []context.Context
	// deadline is the time the keep alive channels close if no response
	deadline time.Time
	// nextKeepAlive is when to send the next keep alive message
	nextKeepAlive time.Time
	// donec is closed on lease revoke, expiration, or cancel.
	donec chan struct{}
	}

	func NewLease(c *Client) Lease {
	return NewLeaseFromLeaseClient(RetryLeaseClient(c), c, c.cfg.DialTimeout+time.Second)
	}

	func NewLeaseFromLeaseClient(remote pb.LeaseClient, c *Client, keepAliveTimeout time.Duration) Lease {
	l := &lessor{
	donec: make(chan struct{}),
	keepAlives: make(map[LeaseID]*keepAlive),
	remote: remote,
	firstKeepAliveTimeout: keepAliveTimeout,
	}
	if l.firstKeepAliveTimeout == time.Second {
	l.firstKeepAliveTimeout = defaultTTL
	}
	if c != nil {
	l.callOpts = c.callOpts
	}
	reqLeaderCtx := WithRequireLeader(context.Background())
	l.stopCtx, l.stopCancel = context.WithCancel(reqLeaderCtx)
	return l
	}

	func (l lessor) Grant(ctx context.Context, ttl int64) (LeaseGrantResponse, error) {
	r := &pb.LeaseGrantRequest{TTL: ttl}
	resp, err := l.remote.LeaseGrant(ctx, r, l.callOpts...)
	if err == nil {
	gresp := &LeaseGrantResponse{
	ResponseHeader: resp.GetHeader(),
	ID: LeaseID(resp.ID),
	TTL: resp.TTL,
	Error: resp.Error,
	}
	return gresp, nil
	}
	return nil, toErr(ctx, err)
	}

	func (l lessor) Revoke(ctx context.Context, id LeaseID) (LeaseRevokeResponse, error) {
	r := &pb.LeaseRevokeRequest{ID: int64(id)}
	resp, err := l.remote.LeaseRevoke(ctx, r, l.callOpts...)
	if err == nil {
	return (*LeaseRevokeResponse)(resp), nil
	}
	return nil, toErr(ctx, err)
	}

	func (l lessor) TimeToLive(ctx context.Context, id LeaseID, opts ...LeaseOption) (LeaseTimeToLiveResponse, error) {
	r := toLeaseTimeToLiveRequest(id, opts...)
	resp, err := l.remote.LeaseTimeToLive(ctx, r, l.callOpts...)
	if err == nil {
	gresp := &LeaseTimeToLiveResponse{
	ResponseHeader: resp.GetHeader(),
	ID: LeaseID(resp.ID),
	TTL: resp.TTL,
	GrantedTTL: resp.GrantedTTL,
	Keys: resp.Keys,
	}
	return gresp, nil
	}
	return nil, toErr(ctx, err)
	}

	func (l lessor) Leases(ctx context.Context) (LeaseLeasesResponse, error) {
	resp, err := l.remote.LeaseLeases(ctx, &pb.LeaseLeasesRequest{}, l.callOpts...)
	if err == nil {
	leases := make([]LeaseStatus, len(resp.Leases))
	for i := range resp.Leases {
	leases[i] = LeaseStatus{ID: LeaseID(resp.Leases[i].ID)}
	}
	return &LeaseLeasesResponse{ResponseHeader: resp.GetHeader(), Leases: leases}, nil
	}
	return nil, toErr(ctx, err)
	}

	func (l lessor) KeepAlive(ctx context.Context, id LeaseID) (<-chan LeaseKeepAliveResponse, error) {
	ch := make(chan *LeaseKeepAliveResponse, LeaseResponseChSize)

	l.mu.Lock()
	// ensure that recvKeepAliveLoop is still running
	select {
	case <-l.donec:
	err := l.loopErr
	l.mu.Unlock()
	close(ch)
	return ch, ErrKeepAliveHalted{Reason: err}
	default:
	}
	ka, ok := l.keepAlives[id]
	if !ok {
	// create fresh keep alive
	ka = &keepAlive{
	chs: []chan<- *LeaseKeepAliveResponse{ch},
	ctxs: []context.Context{ctx},
	deadline: time.Now().Add(l.firstKeepAliveTimeout),
	nextKeepAlive: time.Now(),
	donec: make(chan struct{}),
	}
	l.keepAlives[id] = ka
	} else {
	// add channel and context to existing keep alive
	ka.ctxs = append(ka.ctxs, ctx)
	ka.chs = append(ka.chs, ch)
	}
	l.mu.Unlock()

	go l.keepAliveCtxCloser(id, ctx, ka.donec)
	l.firstKeepAliveOnce.Do(func() {
	go l.recvKeepAliveLoop()
	go l.deadlineLoop()
	})

	return ch, nil
	}

	func (l lessor) KeepAliveOnce(ctx context.Context, id LeaseID) (LeaseKeepAliveResponse, error) {
	for {
	resp, err := l.keepAliveOnce(ctx, id)
	if err == nil {
	if resp.TTL <= 0 {
	err = rpctypes.ErrLeaseNotFound
	}
	return resp, err
	}
	if isHaltErr(ctx, err) {
	return nil, toErr(ctx, err)
	}
	}
	}

	func (l *lessor) Close() error {
	l.stopCancel()
	// close for synchronous teardown if stream goroutines never launched
	l.firstKeepAliveOnce.Do(func() { close(l.donec) })
	<-l.donec
	return nil
	}

	func (l *lessor) keepAliveCtxCloser(id LeaseID, ctx context.Context, donec <-chan struct{}) {
	select {
	case <-donec:
	return
	case <-l.donec:
	return
	case <-ctx.Done():
	}

	l.mu.Lock()
	defer l.mu.Unlock()

	ka, ok := l.keepAlives[id]
	if !ok {
	return
	}

	// close channel and remove context if still associated with keep alive
	for i, c := range ka.ctxs {
	if c == ctx {
	close(ka.chs[i])
	ka.ctxs = append(ka.ctxs[:i], ka.ctxs[i+1:]...)
	ka.chs = append(ka.chs[:i], ka.chs[i+1:]...)
	break
	}
	}
	// remove if no one more listeners
	if len(ka.chs) == 0 {
	delete(l.keepAlives, id)
	}
	}

	// closeRequireLeader scans keepAlives for ctxs that have require leader
	// and closes the associated channels.
	func (l *lessor) closeRequireLeader() {
	l.mu.Lock()
	defer l.mu.Unlock()
	for _, ka := range l.keepAlives {
	reqIdxs := 0
	// find all required leader channels, close, mark as nil
	for i, ctx := range ka.ctxs {
	md, ok := metadata.FromOutgoingContext(ctx)
	if !ok {
	continue
	}
	ks := md[rpctypes.MetadataRequireLeaderKey]
	if len(ks) < 1 \|\| ks[0] != rpctypes.MetadataHasLeader {
	continue
	}
	close(ka.chs[i])
	ka.chs[i] = nil
	reqIdxs++
	}
	if reqIdxs == 0 {
	continue
	}
	// remove all channels that required a leader from keepalive
	newChs := make([]chan<- *LeaseKeepAliveResponse, len(ka.chs)-reqIdxs)
	newCtxs := make([]context.Context, len(newChs))
	newIdx := 0
	for i := range ka.chs {
	if ka.chs[i] == nil {
	continue
	}
	newChs[newIdx], newCtxs[newIdx] = ka.chs[i], ka.ctxs[newIdx]
	newIdx++
	}
	ka.chs, ka.ctxs = newChs, newCtxs
	}
	}

	func (l lessor) keepAliveOnce(ctx context.Context, id LeaseID) (LeaseKeepAliveResponse, error) {
	cctx, cancel := context.WithCancel(ctx)
	defer cancel()

	stream, err := l.remote.LeaseKeepAlive(cctx, l.callOpts...)
	if err != nil {
	return nil, toErr(ctx, err)
	}

	err = stream.Send(&pb.LeaseKeepAliveRequest{ID: int64(id)})
	if err != nil {
	return nil, toErr(ctx, err)
	}

	resp, rerr := stream.Recv()
	if rerr != nil {
	return nil, toErr(ctx, rerr)
	}

	karesp := &LeaseKeepAliveResponse{
	ResponseHeader: resp.GetHeader(),
	ID: LeaseID(resp.ID),
	TTL: resp.TTL,
	}
	return karesp, nil
	}

	func (l *lessor) recvKeepAliveLoop() (gerr error) {
	defer func() {
	l.mu.Lock()
	close(l.donec)
	l.loopErr = gerr
	for _, ka := range l.keepAlives {
	ka.close()
	}
	l.keepAlives = make(map[LeaseID]*keepAlive)
	l.mu.Unlock()
	}()

	for {
	stream, err := l.resetRecv()
	if err != nil {
	if canceledByCaller(l.stopCtx, err) {
	return err
	}
	} else {
	for {
	resp, err := stream.Recv()
	if err != nil {
	if canceledByCaller(l.stopCtx, err) {
	return err
	}

	if toErr(l.stopCtx, err) == rpctypes.ErrNoLeader {
	l.closeRequireLeader()
	}
	break
	}

	l.recvKeepAlive(resp)
	}
	}

	select {
	case <-time.After(retryConnWait):
	continue
	case <-l.stopCtx.Done():
	return l.stopCtx.Err()
	}
	}
	}

	// resetRecv opens a new lease stream and starts sending keep alive requests.
	func (l *lessor) resetRecv() (pb.Lease_LeaseKeepAliveClient, error) {
	sctx, cancel := context.WithCancel(l.stopCtx)
	stream, err := l.remote.LeaseKeepAlive(sctx, l.callOpts...)
	if err != nil {
	cancel()
	return nil, err
	}

	l.mu.Lock()
	defer l.mu.Unlock()
	if l.stream != nil && l.streamCancel != nil {
	l.streamCancel()
	}

	l.streamCancel = cancel
	l.stream = stream

	go l.sendKeepAliveLoop(stream)
	return stream, nil
	}

	// recvKeepAlive updates a lease based on its LeaseKeepAliveResponse
	func (l lessor) recvKeepAlive(resp pb.LeaseKeepAliveResponse) {
	karesp := &LeaseKeepAliveResponse{
	ResponseHeader: resp.GetHeader(),
	ID: LeaseID(resp.ID),
	TTL: resp.TTL,
	}

	l.mu.Lock()
	defer l.mu.Unlock()

	ka, ok := l.keepAlives[karesp.ID]
	if !ok {
	return
	}

	if karesp.TTL <= 0 {
	// lease expired; close all keep alive channels
	delete(l.keepAlives, karesp.ID)
	ka.close()
	return
	}

	// send update to all channels
	nextKeepAlive := time.Now().Add((time.Duration(karesp.TTL) * time.Second) / 3.0)
	ka.deadline = time.Now().Add(time.Duration(karesp.TTL) * time.Second)
	for _, ch := range ka.chs {
	select {
	case ch <- karesp:
	default:
	}
	// still advance in order to rate-limit keep-alive sends
	ka.nextKeepAlive = nextKeepAlive
	}
	}

	// deadlineLoop reaps any keep alive channels that have not received a response
	// within the lease TTL
	func (l *lessor) deadlineLoop() {
	for {
	select {
	case <-time.After(time.Second):
	case <-l.donec:
	return
	}
	now := time.Now()
	l.mu.Lock()
	for id, ka := range l.keepAlives {
	if ka.deadline.Before(now) {
	// waited too long for response; lease may be expired
	ka.close()
	delete(l.keepAlives, id)
	}
	}
	l.mu.Unlock()
	}
	}

	// sendKeepAliveLoop sends keep alive requests for the lifetime of the given stream.
	func (l *lessor) sendKeepAliveLoop(stream pb.Lease_LeaseKeepAliveClient) {
	for {
	var tosend []LeaseID

	now := time.Now()
	l.mu.Lock()
	for id, ka := range l.keepAlives {
	if ka.nextKeepAlive.Before(now) {
	tosend = append(tosend, id)
	}
	}
	l.mu.Unlock()

	for _, id := range tosend {
	r := &pb.LeaseKeepAliveRequest{ID: int64(id)}
	if err := stream.Send(r); err != nil {
	// TODO do something with this error?
	return
	}
	}

	select {
	case <-time.After(500 * time.Millisecond):
	case <-stream.Context().Done():
	return
	case <-l.donec:
	return
	case <-l.stopCtx.Done():
	return
	}
	}
	}

	func (ka *keepAlive) close() {
	close(ka.donec)
	for _, ch := range ka.chs {
	close(ch)
	}
	}