1// Copyright 2016 Google Inc. All Rights Reserved.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7//      http://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14
15package pubsub
16
17import (
18	"sync"
19	"time"
20
21	"golang.org/x/net/context"
22)
23
24// keepAlive keeps track of which Messages need to have their deadline extended, and
25// periodically extends them.
26// Messages are tracked by Ack ID.
27type keepAlive struct {
28	s             service
29	Ctx           context.Context  // The context to use when extending deadlines.
30	Sub           string           // The full name of the subscription.
31	ExtensionTick <-chan time.Time // ExtenstionTick supplies the frequency with which to make extension requests.
32	Deadline      time.Duration    // How long to extend messages for each time they are extended. Should be greater than ExtensionTick frequency.
33	MaxExtension  time.Duration    // How long to keep extending each message's ack deadline before automatically removing it.
34
35	mu sync.Mutex
36	// key: ackID; value: time at which ack deadline extension should cease.
37	items map[string]time.Time
38	dr    drain
39
40	wg sync.WaitGroup
41}
42
43// Start initiates the deadline extension loop.  Stop must be called once keepAlive is no longer needed.
44func (ka *keepAlive) Start() {
45	ka.items = make(map[string]time.Time)
46	ka.dr = drain{Drained: make(chan struct{})}
47	ka.wg.Add(1)
48	go func() {
49		defer ka.wg.Done()
50		for {
51			select {
52			case <-ka.Ctx.Done():
53				// Don't bother waiting for items to be removed: we can't extend them any more.
54				return
55			case <-ka.dr.Drained:
56				return
57			case <-ka.ExtensionTick:
58				live, expired := ka.getAckIDs()
59				ka.wg.Add(1)
60				go func() {
61					defer ka.wg.Done()
62					ka.extendDeadlines(live)
63				}()
64
65				for _, id := range expired {
66					ka.Remove(id)
67				}
68			}
69		}
70	}()
71}
72
73// Add adds an ack id to be kept alive.
74// It should not be called after Stop.
75func (ka *keepAlive) Add(ackID string) {
76	ka.mu.Lock()
77	defer ka.mu.Unlock()
78
79	ka.items[ackID] = time.Now().Add(ka.MaxExtension)
80	ka.dr.SetPending(true)
81}
82
83// Remove removes ackID from the list to be kept alive.
84func (ka *keepAlive) Remove(ackID string) {
85	ka.mu.Lock()
86	defer ka.mu.Unlock()
87
88	// Note: If users NACKs a message after it has been removed due to
89	// expiring, Remove will be called twice with same ack id.  This is OK.
90	delete(ka.items, ackID)
91	ka.dr.SetPending(len(ka.items) != 0)
92}
93
94// Stop waits until all added ackIDs have been removed, and cleans up resources.
95// Stop may only be called once.
96func (ka *keepAlive) Stop() {
97	ka.mu.Lock()
98	ka.dr.Drain()
99	ka.mu.Unlock()
100
101	ka.wg.Wait()
102}
103
104// getAckIDs returns the set of ackIDs that are being kept alive.
105// The set is divided into two lists: one with IDs that should continue to be kept alive,
106// and the other with IDs that should be dropped.
107func (ka *keepAlive) getAckIDs() (live, expired []string) {
108	ka.mu.Lock()
109	defer ka.mu.Unlock()
110
111	now := time.Now()
112	for id, expiry := range ka.items {
113		if expiry.Before(now) {
114			expired = append(expired, id)
115		} else {
116			live = append(live, id)
117		}
118	}
119	return live, expired
120}
121
122const maxExtensionAttempts = 2
123
124func (ka *keepAlive) extendDeadlines(ackIDs []string) {
125	head, tail := ka.s.splitAckIDs(ackIDs)
126	for len(head) > 0 {
127		for i := 0; i < maxExtensionAttempts; i++ {
128			if ka.s.modifyAckDeadline(ka.Ctx, ka.Sub, ka.Deadline, head) == nil {
129				break
130			}
131		}
132		// NOTE: Messages whose deadlines we fail to extend will
133		// eventually be redelivered and this is a documented behaviour
134		// of the API.
135		//
136		// NOTE: If we fail to extend deadlines here, this
137		// implementation will continue to attempt extending the
138		// deadlines for those ack IDs the next time the extension
139		// ticker ticks.  By then the deadline will have expired.
140		// Re-extending them is harmless, however.
141		//
142		// TODO: call Remove for ids which fail to be extended.
143
144		head, tail = ka.s.splitAckIDs(tail)
145	}
146}
147
148// A drain (once started) indicates via a channel when there is no work pending.
149type drain struct {
150	started bool
151	pending bool
152
153	// Drained is closed once there are no items outstanding if Drain has been called.
154	Drained chan struct{}
155}
156
157// Drain starts the drain process. This cannot be undone.
158func (d *drain) Drain() {
159	d.started = true
160	d.closeIfDrained()
161}
162
163// SetPending sets whether there is work pending or not. It may be called multiple times before or after Drain.
164func (d *drain) SetPending(pending bool) {
165	d.pending = pending
166	d.closeIfDrained()
167}
168
169func (d *drain) closeIfDrained() {
170	if !d.pending && d.started {
171		// Check to see if d.Drained is closed before closing it.
172		// This allows SetPending(false) to be safely called multiple times.
173		select {
174		case <-d.Drained:
175		default:
176			close(d.Drained)
177		}
178	}
179}
180