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base/deferred_sequenced_task_runner.cc

Index: base/deferred_sequenced_task_runner.cc
diff --git a/base/deferred_sequenced_task_runner.cc b/base/deferred_sequenced_task_runner.cc
new file mode 100644
index 0000000000000000000000000000000000000000..d32c39a0bc24ff4540faa04de8297308d7f6b7eb
--- /dev/null
+++ b/base/deferred_sequenced_task_runner.cc
@@ -0,0 +1,93 @@
+// 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.
+
+#include "base/deferred_sequenced_task_runner.h"
+
+#include "base/bind.h"
+#include "base/threading/thread_checker.h"
+
+namespace base {
+
+DeferredSequencedTaskRunner::DeferredTask::DeferredTask() {
+}
+
+DeferredSequencedTaskRunner::DeferredTask::~DeferredTask() {
+}
+
+DeferredSequencedTaskRunner::DeferredSequencedTaskRunner(
+    const scoped_refptr<SequencedTaskRunner>& target_task_runner)
+    : target_task_runner_(target_task_runner) {
+}
+
+DeferredSequencedTaskRunner::~DeferredSequencedTaskRunner() {
+}
+
+bool DeferredSequencedTaskRunner::PostDelayedTask(
+    const tracked_objects::Location& from_here,
+    const Closure& task,
+    TimeDelta delay) {
+  AutoLock lock(lock_);
+  if (started_) {
+    DCHECK(deferred_tasks_queue_.empty());
+    return target_task_runner_->PostDelayedTask(from_here, task, delay);
+  }
+
+  QueueDeferredTask(from_here, task, delay, false /* is_non_nestable */);
+  return true;
+}
+
+bool DeferredSequencedTaskRunner::RunsTasksOnCurrentThread() const {
+  return target_task_runner_->RunsTasksOnCurrentThread();
+}
+
+bool DeferredSequencedTaskRunner::PostNonNestableDelayedTask(
+    const tracked_objects::Location& from_here,
+    const Closure& task,
+    TimeDelta delay) {
+  AutoLock lock(lock_);
+  if (started_) {
+    DCHECK(deferred_tasks_queue_.empty());
+    return target_task_runner_->PostDelayedTask(from_here, task, delay);
+  }
+  QueueDeferredTask(from_here, task, delay, true /* is_non_nestable */);
+  return true;
+}
+
+void DeferredSequencedTaskRunner::QueueDeferredTask(
+    const tracked_objects::Location& from_here,
+    const Closure& task,
+    TimeDelta delay,
+    bool is_non_nestable) {
+  DeferredTask deferred_task;
+  deferred_task.posted_from = from_here;
+  deferred_task.task = task;
+  deferred_task.delay = delay;
+  deferred_task.is_non_nestable = true;
+  deferred_tasks_queue_.push_back(deferred_task);
+}
+
+
+void DeferredSequencedTaskRunner::Start() {
+  AutoLock lock(lock_);
+  if (started_)
+    return;
+
+  started_ = true;
+  for (std::vector<DeferredTask>::iterator i = deferred_tasks_queue_.begin();
+      i != deferred_tasks_queue_.end(); ++i) {
+    const DeferredTask& task = *i;
+    if (task.is_non_nestable) {
+      target_task_runner_->PostNonNestableDelayedTask(task.posted_from,

[Jeffrey Yasskin, 2013/03/27 18:07:37]

Calling user-controlled functions while |lock_| is held risks deadlock, although
I expect that there will be a natural order of the relevant locks that'll
prevent it in this case.

It would be safer to have this function move (swap+clear) the
deferred_tasks_queue_ into a local variable under the lock, and then release the
lock and iterate that variable into the target_task_runner_. You'd loop to do
this dump possibly several times until you see the deferred_tasks_queue_ empty.
Only then would you set started_ to true to let other Posters take their fast
path.

[msarda, 2013/03/29 14:40:03]

With this design, can we guarantee that if Start is called twice from different
threads at the same time, the tasks are posted in sequence? Or we don't care
about this case?


On 2013/03/27 18:07:37, Jeffrey Yasskin wrote:

[Jeffrey Yasskin, 2013/04/03 16:25:35]

Good catch. You can guarantee that if you use a condition variable to have the
second Start() wait for the first to finish. I suspect Start() will only be
called from the UI thread though, in which case it can't be called twice
concurrently.

+                                                      task.task,
+                                                      task.delay);
+    } else {
+      target_task_runner_->PostDelayedTask(task.posted_from,
+                                           task.task,
+                                           task.delay);
+    }
+  }
+  deferred_tasks_queue_.clear();
+}
+
+}  // namespace base