1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
|
// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_SEQUENCED_TASKRUNNER_H_
#define BASE_SEQUENCED_TASKRUNNER_H_
#pragma once
#include "base/base_export.h"
#include "base/sequenced_task_runner_helpers.h"
#include "base/task_runner.h"
namespace base {
// A SequencedTaskRunner is a subclass of TaskRunner that provides
// additional guarantees on the order that tasks are started, as well
// as guarantees on when tasks are in sequence, i.e. one task finishes
// before the other one starts.
//
// Summary
// -------
// Barring delayed/non-nestable tasks, tasks posted will run one by
// one in FIFO order.
//
// Detailed guarantees
// -------------------
//
// SequencedTaskRunner also adds additional methods for posting
// non-nestable tasks. In general, an implementation of TaskRunner
// may expose task-running methods which are themselves callable from
// within tasks. A non-nestable task is one that is guaranteed to not
// be run from within an already-running task. Conversely, a nestable
// task (the default) is a task that can be run from within an
// already-running task.
//
// The guarantees of SequencedTaskRunner are as follows:
//
// - Given two tasks T2 and T1, T2 will start after T1 starts if:
//
// * T2 is posted after T1;
// * T2 has equal or higher delay than T1; and
// * T2 is non-nestable or T1 is nestable.
//
// - If T2 will start after T1 starts by the above guarantee, then
// T2 will start after T1 finishes if:
//
// * T2 is non-nestable, or
// * T1 doesn't call any task-running methods.
//
// - If T2 will start after T1 finishes by the above guarantee, then
// all memory changes in T1 will be visible to T2.
//
// - If T2 runs nested within T1 via a call to the task-running
// method M, then all memory changes in T1 up to the call to M
// will be visible to T2, and all memory changes in T2 will be
// visible to T1 from the return from M.
//
// Note that SequencedTaskRunner does not guarantee that tasks are run
// on a single dedicated thread, although the above guarantees provide
// most (but not all) of the same guarantees. If you do need to
// guarantee that tasks are run on a single dedicated thread, see
// SingleThreadTaskRunner (in single_thread_task_runner.h).
//
// Some corollaries to the above guarantees, assuming the tasks in
// question don't call any task-running methods:
//
// - Tasks posted via PostTask are run in FIFO order.
//
// - Tasks posted via PostNonNestableTask are run in FIFO order.
//
// - Tasks posted with the same delay and the same nestable state
// are run in FIFO order.
//
// - A list of tasks with the same nestable state posted in order of
// non-decreasing delay is run in FIFO order.
//
// - A list of tasks posted in order of non-decreasing delay with at
// most a single change in nestable state from nestable to
// non-nestable is run in FIFO order. (This is equivalent to the
// statement of the first guarantee above.)
//
// Some theoretical implementations of SequencedTaskRunner:
//
// - A SequencedTaskRunner that wraps a regular TaskRunner but makes
// sure that only one task at a time is posted to the TaskRunner,
// with appropriate memory barriers in between tasks.
//
// - A SequencedTaskRunner that, for each task, spawns a joinable
// thread to run that task and immediately quit, and then
// immediately joins that thread.
//
// - A SequencedTaskRunner that stores the list of posted tasks and
// has a method Run() that runs each runnable task in FIFO order
// that can be called from any thread, but only if another
// (non-nested) Run() call isn't already happening.
class BASE_EXPORT SequencedTaskRunner : public TaskRunner {
public:
// The two PostNonNestable*Task methods below are like their
// nestable equivalents in TaskRunner, but they guarantee that the
// posted task will not run nested within an already-running task.
//
// A simple corollary is that posting a task as non-nestable can
// only delay when the task gets run. That is, posting a task as
// non-nestable may not affect when the task gets run, or it could
// make it run later than it normally would, but it won't make it
// run earlier than it normally would.
// TODO(akalin): Get rid of the boolean return value for the methods
// below.
bool PostNonNestableTask(const tracked_objects::Location& from_here,
const Closure& task);
virtual bool PostNonNestableDelayedTask(
const tracked_objects::Location& from_here,
const Closure& task,
base::TimeDelta delay) = 0;
// Submits a non-nestable task to delete the given object. Returns
// true if the object may be deleted at some point in the future,
// and false if the object definitely will not be deleted.
template <class T>
bool DeleteSoon(const tracked_objects::Location& from_here,
const T* object) {
return
subtle::DeleteHelperInternal<T, bool>::DeleteViaSequencedTaskRunner(
this, from_here, object);
}
// Submits a non-nestable task to release the given object. Returns
// true if the object may be released at some point in the future,
// and false if the object definitely will not be released.
template <class T>
bool ReleaseSoon(const tracked_objects::Location& from_here,
T* object) {
return
subtle::ReleaseHelperInternal<T, bool>::ReleaseViaSequencedTaskRunner(
this, from_here, object);
}
protected:
virtual ~SequencedTaskRunner() {}
private:
template <class T, class R> friend class subtle::DeleteHelperInternal;
template <class T, class R> friend class subtle::ReleaseHelperInternal;
bool DeleteSoonInternal(const tracked_objects::Location& from_here,
void(*deleter)(const void*),
const void* object);
bool ReleaseSoonInternal(const tracked_objects::Location& from_here,
void(*releaser)(const void*),
const void* object);
};
} // namespace base
#endif // BASE_SEQUENCED_TASKRUNNER_H_
|
