content: Add an overridable task runner to ChildThreadImpl

content/child/child_thread_impl.h

Index: content/child/child_thread_impl.h
diff --git a/content/child/child_thread_impl.h b/content/child/child_thread_impl.h
index 529fb0823b3884958bab9d91e71f970985658fc9..e5d4c790130e5e20768e3ae7202fd6e03ecc1966 100644
--- a/content/child/child_thread_impl.h
+++ b/content/child/child_thread_impl.h
@@ -80,6 +80,7 @@ class CONTENT_EXPORT ChildThreadImpl
   bool Send(IPC::Message* msg) override;
 
   // ChildThread implementation:
+  scoped_refptr<base::SingleThreadTaskRunner> GetTaskRunner() override;

[no sievers, 2015/03/04 19:15:02]

Is there a way (at least longer term) how we could make it more clear how to get
the right task runner for the different call sites? Esp. since
ChildThreadImpl::message_loop() is also public, and MessageLoop also has
GetTaskRunner().

Could you also explain in the cl description what exactly the impact/bug was of
using the wrong task runner and bypassing the scheduler?

[Sami, 2015/03/04 19:39:28]

I agree that it's a mess :( To add on top of the things you mentioned, there's
base::MessageLoop::current(), base::MessageLoopProxy::current() and
ThreadTaskRunnerHandle::Get().

I tried removing message_loop() here but couldn't because some callers want to
add task observers to it, which you can't do with a SingleThreadTaskRunner.
Maybe one way to clean this up would be to separate out task observing into a
new interface (or possibly into SingleThreadTaskRunner) so that fewer things
need to care about the concrete message loop.

Longer term I'd like to remove most if not all global ways to get to a task
runner and require everyone to plumb the task runners to where they are needed.
That's a little ways off though :)

I'll add some instructions here on how to use this.
Done.

 #if defined(OS_WIN)
   void PreCacheFont(const LOGFONT& log_font) override;
   void ReleaseCachedFonts() override;
@@ -253,6 +254,7 @@ class CONTENT_EXPORT ChildThreadImpl
   bool on_channel_error_called_;
 
   base::MessageLoop* message_loop_;
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   scoped_ptr<FileSystemDispatcher> file_system_dispatcher_;
 
@@ -304,6 +306,7 @@ struct ChildThreadImpl::Options {
   bool use_mojo_channel;
   bool in_browser_process;
   std::vector<IPC::MessageFilter*> startup_filters;
+  scoped_refptr<base::SingleThreadTaskRunner> task_runner;
 
  private:
   Options();
@@ -317,6 +320,8 @@ class ChildThreadImpl::Options::Builder {
   Builder& UseMojoChannel(bool use_mojo_channel);
   Builder& WithChannelName(const std::string& channel_name);
   Builder& AddStartupFilter(IPC::MessageFilter* filter);
+  Builder& WithTaskRunner(
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner);
 
   Options Build();