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// Copyright 2016 Google Inc. All Rights Reserved.
// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package pubsub
import (
// keepAlive keeps track of which Messages need to have their deadline extended, and
// periodically extends them.
// Messages are tracked by Ack ID.
type keepAlive struct {
s service
Ctx context.Context // The context to use when extending deadlines.
Sub string // The full name of the subscription.
ExtensionTick <-chan time.Time // ExtensionTick supplies the frequency with which to make extension requests.
Deadline time.Duration // How long to extend messages for each time they are extended. Should be greater than ExtensionTick frequency.
MaxExtension time.Duration // How long to keep extending each message's ack deadline before automatically removing it.
mu sync.Mutex
// key: ackID; value: time at which ack deadline extension should cease.
items map[string]time.Time
dr drain
wg sync.WaitGroup
// Start initiates the deadline extension loop. Stop must be called once keepAlive is no longer needed.
func (ka *keepAlive) Start() {
ka.items = make(map[string]time.Time)
ka.dr = drain{Drained: make(chan struct{})}
go func() {
defer ka.wg.Done()
for {
select {
case <-ka.Ctx.Done():
// Don't bother waiting for items to be removed: we can't extend them any more.
case <-ka.dr.Drained:
case <-ka.ExtensionTick:
live, expired := ka.getAckIDs()
go func() {
defer ka.wg.Done()
for _, id := range expired {
// Add adds an ack id to be kept alive.
// It should not be called after Stop.
func (ka *keepAlive) Add(ackID string) {
ka.items[ackID] = time.Now().Add(ka.MaxExtension)
// Remove removes ackID from the list to be kept alive.
func (ka *keepAlive) Remove(ackID string) {
// Note: If users NACKs a message after it has been removed due to
// expiring, Remove will be called twice with same ack id. This is OK.
delete(ka.items, ackID)
ka.dr.SetPending(len(ka.items) != 0)
// Stop waits until all added ackIDs have been removed, and cleans up resources.
// Stop may only be called once.
func (ka *keepAlive) Stop() {
// getAckIDs returns the set of ackIDs that are being kept alive.
// The set is divided into two lists: one with IDs that should continue to be kept alive,
// and the other with IDs that should be dropped.
func (ka *keepAlive) getAckIDs() (live, expired []string) {
return getKeepAliveAckIDs(ka.items)
func getKeepAliveAckIDs(items map[string]time.Time) (live, expired []string) {
now := time.Now()
for id, expiry := range items {
if expiry.Before(now) {
expired = append(expired, id)
} else {
live = append(live, id)
return live, expired
const maxExtensionAttempts = 2
func (ka *keepAlive) extendDeadlines(ackIDs []string) {
head, tail := ka.s.splitAckIDs(ackIDs)
for len(head) > 0 {
for i := 0; i < maxExtensionAttempts; i++ {
if ka.s.modifyAckDeadline(ka.Ctx, ka.Sub, ka.Deadline, head) == nil {
// NOTE: Messages whose deadlines we fail to extend will
// eventually be redelivered and this is a documented behaviour
// of the API.
// NOTE: If we fail to extend deadlines here, this
// implementation will continue to attempt extending the
// deadlines for those ack IDs the next time the extension
// ticker ticks. By then the deadline will have expired.
// Re-extending them is harmless, however.
// TODO: call Remove for ids which fail to be extended.
head, tail = ka.s.splitAckIDs(tail)
// A drain (once started) indicates via a channel when there is no work pending.
type drain struct {
started bool
pending bool
// Drained is closed once there are no items outstanding if Drain has been called.
Drained chan struct{}
// Drain starts the drain process. This cannot be undone.
func (d *drain) Drain() {
d.started = true
// SetPending sets whether there is work pending or not. It may be called multiple times before or after Drain.
func (d *drain) SetPending(pending bool) {
d.pending = pending
func (d *drain) closeIfDrained() {
if !d.pending && d.started {
// Check to see if d.Drained is closed before closing it.
// This allows SetPending(false) to be safely called multiple times.
select {
case <-d.Drained: