Motown: ffmpeg implementations of framework 'parts'

services/media/framework_ffmpeg/ffmpeg_init.cc

+// Copyright 2016 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 "services/media/framework_ffmpeg/ffmpeg_init.h"
+
+extern "C" {
+#include "third_party/ffmpeg/libavformat/avformat.h"
+}
+
+namespace mojo {
+namespace media {
+
+void InitFfmpeg() {
+  static bool initialized_ = false;

[johngro, 2016/03/01 01:31:39]

While idempotent in a purely single threaded world, this is not thread safe.

I think that it is perfectly reasonable to say that it needs to be neither
idempotent nor thread-safe as this code should probably be called only once very
early in an application's main.  By this argument, you could remove the
initialized flag entirely and just call av_register_all().

If you want some protection against an accidental double call, you could use
std::atomic to do a compare and exchange, and then DCHECK that the method was
not already called.  This enforces the call-once requirement, and is thread-safe
for practically no cost (even though it should not need to, since the rule is
probably call-once only).

If it should be possible from an API perspective to call this multiple times,
then this becomes more complicated to make thread safe.  One would need to both
avoid the double init, but also use a synchronization primitive of some form to
ensure that the post-condition (FFmpeg has been fully initialized) has been met
before the second call to init exits.

[dalesat, 2016/03/01 20:43:02]

Added a lock.

+  if (!initialized_) {
+    initialized_ = true;
+    av_register_all();
+  }
+}
+
+} // namespace media
+} // namespace mojo