base: Support setting thread priorities generically.

base/threading/platform_thread.h

Index: base/threading/platform_thread.h
diff --git a/base/threading/platform_thread.h b/base/threading/platform_thread.h
index 576695a4e7b5f8028f47cc5e02921b551ffb0a39..2f7f3fe1617fc582cbd25794be8f27672cd8f702 100644
--- a/base/threading/platform_thread.h
+++ b/base/threading/platform_thread.h
@@ -43,7 +43,9 @@ const PlatformThreadId kInvalidThreadId = 0;
 enum ThreadPriority{
   kThreadPriority_Normal,
   // Suitable for low-latency, glitch-resistant audio.
-  kThreadPriority_RealtimeAudio
+  kThreadPriority_RealtimeAudio,
+  kThreadPriority_Display,

[jar (doing other things), 2013/04/24 01:04:57]

Although I can guess what Normal means, vs Background, I don't know how to fit
in Display and RealtimeAudio.

Can you at a minimum add comments, and probably better would be to also lengthen
the name so that its significance would be clear at point of use (without
reading the comments).

[epenner, 2013/04/30 04:37:08]

Done.

+  kThreadPriority_Background
 };
 
 // A namespace for low-level thread functions.
@@ -106,11 +108,11 @@ class BASE_EXPORT PlatformThread {
   // |thread_handle|.
   static void Join(PlatformThreadHandle thread_handle);
 
-  // Sets the priority of the thread specified in |handle| to |priority|.
-  // This does not work on Linux, use CreateWithPriority() instead.
+  // Only one of the handle/id is required depending on the
+  // platform. However, for cross platform code, pass both.
   static void SetThreadPriority(PlatformThreadHandle handle,
+                                PlatformThreadId id,

[jar (doing other things), 2013/04/24 01:04:57]

Rather than having both, can you unify this in a class some how?  What is the
significance of listing both in call??

[epenner, 2013/04/30 04:37:08]

See also my previous general comment on the patch.

The closest I think would be trying to store the ID inside the Handle itself, at
thread creation time. Perhaps the ID could then be removed entirely from the
PlatformThread abstraction. The trouble is that this imposes limits on when
priorities can be set on certain platforms.

Specifically, I think you would need to have created the thread yourself in
order to set it's priority (on all platforms), even though this is only a real
limitation on windows (IIUC).

                                 ThreadPriority priority);
-

[jar (doing other things), 2013/04/24 01:04:57]

nit: preserve the white-space line before the private keyword.  I couldn't find
explicit support in the style guide, but *all* the examples they provide had a
blank line before it.... so you should probably at least keep it here.

[epenner, 2013/04/30 04:37:08]

Done.

  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PlatformThread);
 };