1// Copyright 2016 The Chromium Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5#include "base/threading/thread_task_runner_handle.h"
6
7#include <utility>
8
9#include "base/bind.h"
10#include "base/lazy_instance.h"
11#include "base/logging.h"
12#include "base/memory/ptr_util.h"
13#include "base/threading/sequenced_task_runner_handle.h"
14#include "base/threading/thread_local.h"
15
16namespace base {
17
18namespace {
19
20base::LazyInstance<base::ThreadLocalPointer<ThreadTaskRunnerHandle>>::Leaky
21    lazy_tls_ptr = LAZY_INSTANCE_INITIALIZER;
22
23}  // namespace
24
25// static
26scoped_refptr<SingleThreadTaskRunner> ThreadTaskRunnerHandle::Get() {
27  ThreadTaskRunnerHandle* current = lazy_tls_ptr.Pointer()->Get();
28  DCHECK(current);
29  return current->task_runner_;
30}
31
32// static
33bool ThreadTaskRunnerHandle::IsSet() {
34  return !!lazy_tls_ptr.Pointer()->Get();
35}
36
37// static
38ScopedClosureRunner ThreadTaskRunnerHandle::OverrideForTesting(
39    scoped_refptr<SingleThreadTaskRunner> overriding_task_runner) {
40  // OverrideForTesting() is not compatible with a SequencedTaskRunnerHandle
41  // being set (but SequencedTaskRunnerHandle::IsSet() includes
42  // ThreadTaskRunnerHandle::IsSet() so that's discounted as the only valid
43  // excuse for it to be true). Sadly this means that tests that merely need a
44  // SequencedTaskRunnerHandle on their main thread can be forced to use a
45  // ThreadTaskRunnerHandle if they're also using test task runners (that
46  // OverrideForTesting() when running their tasks from said main thread). To
47  // solve this: sequence_task_runner_handle.cc and thread_task_runner_handle.cc
48  // would have to be merged into a single impl file and share TLS state. This
49  // was deemed unecessary for now as most tests should use higher level
50  // constructs and not have to instantiate task runner handles on their own.
51  DCHECK(!SequencedTaskRunnerHandle::IsSet() || IsSet());
52
53  if (!IsSet()) {
54    std::unique_ptr<ThreadTaskRunnerHandle> top_level_ttrh =
55        MakeUnique<ThreadTaskRunnerHandle>(std::move(overriding_task_runner));
56    return ScopedClosureRunner(base::Bind(
57        [](std::unique_ptr<ThreadTaskRunnerHandle> ttrh_to_release) {},
58        base::Passed(&top_level_ttrh)));
59  }
60
61  ThreadTaskRunnerHandle* ttrh = lazy_tls_ptr.Pointer()->Get();
62  // Swap the two (and below bind |overriding_task_runner|, which is now the
63  // previous one, as the |task_runner_to_restore|).
64  ttrh->task_runner_.swap(overriding_task_runner);
65
66  return ScopedClosureRunner(base::Bind(
67      [](scoped_refptr<SingleThreadTaskRunner> task_runner_to_restore,
68         SingleThreadTaskRunner* expected_task_runner_before_restore) {
69        ThreadTaskRunnerHandle* ttrh = lazy_tls_ptr.Pointer()->Get();
70
71        DCHECK_EQ(expected_task_runner_before_restore, ttrh->task_runner_.get())
72            << "Nested overrides must expire their ScopedClosureRunners "
73               "in LIFO order.";
74
75        ttrh->task_runner_.swap(task_runner_to_restore);
76      },
77      base::Passed(&overriding_task_runner),
78      base::Unretained(ttrh->task_runner_.get())));
79}
80
81ThreadTaskRunnerHandle::ThreadTaskRunnerHandle(
82    scoped_refptr<SingleThreadTaskRunner> task_runner)
83    : task_runner_(std::move(task_runner)) {
84  DCHECK(task_runner_->BelongsToCurrentThread());
85  // No SequencedTaskRunnerHandle (which includes ThreadTaskRunnerHandles)
86  // should already be set for this thread.
87  DCHECK(!SequencedTaskRunnerHandle::IsSet());
88  lazy_tls_ptr.Pointer()->Set(this);
89}
90
91ThreadTaskRunnerHandle::~ThreadTaskRunnerHandle() {
92  DCHECK(task_runner_->BelongsToCurrentThread());
93  DCHECK_EQ(lazy_tls_ptr.Pointer()->Get(), this);
94  lazy_tls_ptr.Pointer()->Set(nullptr);
95}
96
97}  // namespace base
98