Added code for AVFoundation Finch Experiment enabling

media/video/capture/mac/avfoundation_glue.mm

 // Copyright 2013 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.
 
 #import "media/video/capture/mac/avfoundation_glue.h"
 
 #include <dlfcn.h>
 
 #include "base/command_line.h"
 #include "base/lazy_instance.h"
 #include "base/mac/mac_util.h"
+#include "base/metrics/field_trial.h"
 #include "media/base/media_switches.h"
 
 namespace {
 
 // This class is used to retrieve AVFoundation NSBundle and library handle. It
 // must be used as a LazyInstance so that it is initialised once and in a
 // thread-safe way. Normally no work is done in constructors: LazyInstance is
 // an exception.
 class AVFoundationInternal {
  public:
@@ skipping 24 matching lines @@
 namespace media {
 
 // TODO(mcasas):http://crbug.com/323536 cache the string pointers.
 static NSString* ReadNSStringPtr(const char* symbol) {
   NSString** string_pointer = reinterpret_cast<NSString**>(
       dlsym(AVFoundationGlue::AVFoundationLibraryHandle(), symbol));
   DCHECK(string_pointer) << dlerror();
   return *string_pointer;
 }
 
+bool UseAVFoundationExperiment() {
+  const std::string group_name =
+      base::FieldTrialList::FindFullName("AVFoundationMacVideoCapture");
+  return group_name == "Enabled";
+}
+
 }  // namespace media
 
 bool AVFoundationGlue::IsAVFoundationSupported() {
   const CommandLine* cmd_line = CommandLine::ForCurrentProcess();
-  return cmd_line->HasSwitch(switches::kEnableAVFoundation) &&
-      base::mac::IsOSLionOrLater() && [AVFoundationBundle() load];
+  return (cmd_line->HasSwitch(switches::kEnableAVFoundation) ||
+      media::UseAVFoundationExperiment()) && base::mac::IsOSLionOrLater() &&

[Alexei Svitkine (slow), 2014/01/09 15:50:39]

This is a bit tricky. One thing to understand is that querying the field trial
will mark it as "active", which in this case (since you're OS-version based and
also will have a forcing_flag) means that the order you do it in is important.

Here's the order I suggest:

bool UseAVFoundationExperiment() {
   if (base::mac::IsOSLionOrLater())
     return false;
   const std::string group_name =
       base::FieldTrialList::FindFullName("AVFoundationMacVideoCapture");
   const CommandLine* cmd_line = CommandLine::ForCurrentProcess();
   return cmd_line->HasSwitch(switches::kEnableAVFoundation) || group_name ==
"Enabled";
}

Why this order?

First, if you're on the wrong OS version, we shouldn't be marking you as being
in any experimental group (since it would skew results since the reality is this
feature wouldn't be being used there anyway). That's why that's done before
querying the field trial.

Next, we query the field trial API to mark it as active before checking the
command-line. That way, if a forcing flag is used, then the trial is still
marked as active (since there's no mechanism to mark a trial as active if a
given command-line flag is queried - so you have to to it in this order). Then,
if there's no forcing flag used, default to the experiment group.

Makes sense?

[Alexei Svitkine (slow), 2014/01/09 15:56:20]

Just to clarify, since I think I didn't give enough context. When a field trial
is "active", then it will be reported to UMA and its metrics can be analyzed. If
it's not marked as active, then it's as if it doesn't exist. Hence it's
important to not set it as active if it doesn't apply (i.e. wrong OS version).

[mcasas, 2014/01/09 16:20:35]

I think it makes sense: only use the experiment code for those platforms
where it can be used. I didn't know that every time the FileTrialList class is 
used, it already counts as a YES/NO experiment.

Only thing is that [AVFoundationBundle() load] should still be there
somewhere.

+      [AVFoundationBundle() load];

[Alexei Svitkine (slow), 2014/01/09 15:50:39]

Not super familiar with this code, but is it safe to call -load multiple times
if IsAVFoundationSupported() is called multiple times? If not, seems like the
API can result in an error in that case...

[mcasas, 2014/01/09 16:20:35]

NSBundle -load can be called safely several times (is a 
special beast).

 }
 
 NSBundle const* AVFoundationGlue::AVFoundationBundle() {
   return g_avfoundation_handle.Get().bundle();
 }
 
 void* AVFoundationGlue::AVFoundationLibraryHandle() {
   return g_avfoundation_handle.Get().library_handle();
 }
 
@@ skipping 80 matching lines @@
 @implementation AVCaptureDeviceInputGlue
 
 + (CrAVCaptureDeviceInput*)deviceInputWithDevice:(CrAVCaptureDevice*)device
                                            error:(NSError**)outError {
   return [[AVFoundationGlue::AVFoundationBundle()
       classNamed:@"AVCaptureDeviceInput"] deviceInputWithDevice:device
                                                           error:outError];
 }
 
 @end  // @implementation AVCaptureDeviceInputGlue