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      1 // Copyright 2015 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/sequenced_task_runner_handle.h"
      6 
      7 #include <utility>
      8 
      9 #include "base/lazy_instance.h"
     10 #include "base/logging.h"
     11 #include "base/threading/sequenced_worker_pool.h"
     12 #include "base/threading/thread_local.h"
     13 #include "base/threading/thread_task_runner_handle.h"
     14 
     15 namespace base {
     16 
     17 namespace {
     18 
     19 base::LazyInstance<base::ThreadLocalPointer<SequencedTaskRunnerHandle>>::Leaky
     20     lazy_tls_ptr = LAZY_INSTANCE_INITIALIZER;
     21 
     22 }  // namespace
     23 
     24 // static
     25 scoped_refptr<SequencedTaskRunner> SequencedTaskRunnerHandle::Get() {
     26   // Return the registered SequencedTaskRunner, if any.
     27   const SequencedTaskRunnerHandle* handle = lazy_tls_ptr.Pointer()->Get();
     28   if (handle) {
     29     // Various modes of setting SequencedTaskRunnerHandle don't combine.
     30     DCHECK(!base::ThreadTaskRunnerHandle::IsSet());
     31     DCHECK(!SequencedWorkerPool::GetSequencedTaskRunnerForCurrentThread());
     32     return handle->task_runner_;
     33   }
     34 
     35   // Return the SequencedTaskRunner obtained from SequencedWorkerPool, if any.
     36   scoped_refptr<base::SequencedTaskRunner> task_runner =
     37       SequencedWorkerPool::GetSequencedTaskRunnerForCurrentThread();
     38   if (task_runner) {
     39     DCHECK(!base::ThreadTaskRunnerHandle::IsSet());
     40     return task_runner;
     41   }
     42 
     43   // Return the SingleThreadTaskRunner for the current thread otherwise.
     44   return base::ThreadTaskRunnerHandle::Get();
     45 }
     46 
     47 // static
     48 bool SequencedTaskRunnerHandle::IsSet() {
     49   return lazy_tls_ptr.Pointer()->Get() ||
     50          SequencedWorkerPool::GetWorkerPoolForCurrentThread() ||
     51          base::ThreadTaskRunnerHandle::IsSet();
     52 }
     53 
     54 SequencedTaskRunnerHandle::SequencedTaskRunnerHandle(
     55     scoped_refptr<SequencedTaskRunner> task_runner)
     56     : task_runner_(std::move(task_runner)) {
     57   DCHECK(task_runner_->RunsTasksOnCurrentThread());
     58   DCHECK(!SequencedTaskRunnerHandle::IsSet());
     59   lazy_tls_ptr.Pointer()->Set(this);
     60 }
     61 
     62 SequencedTaskRunnerHandle::~SequencedTaskRunnerHandle() {
     63   DCHECK(task_runner_->RunsTasksOnCurrentThread());
     64   DCHECK_EQ(lazy_tls_ptr.Pointer()->Get(), this);
     65   lazy_tls_ptr.Pointer()->Set(nullptr);
     66 }
     67 
     68 }  // namespace base
     69