Remove unnecessary lock on setClient().

media/blink/webaudiosourceprovider_impl.cc

Index: media/blink/webaudiosourceprovider_impl.cc
diff --git a/media/blink/webaudiosourceprovider_impl.cc b/media/blink/webaudiosourceprovider_impl.cc
index e54dbc81aa1f4e9d37989d16385b47f8707dae10..9b135356f3514a78e704f6869bb66291b0a1a270 100644
--- a/media/blink/webaudiosourceprovider_impl.cc
+++ b/media/blink/webaudiosourceprovider_impl.cc
@@ -105,8 +105,13 @@ WebAudioSourceProviderImpl::~WebAudioSourceProviderImpl() {
 
 void WebAudioSourceProviderImpl::setClient(
     blink::WebAudioSourceProviderClient* client) {
+  // Skip taking the lock if unnecessary. This function is the only setter for
+  // |client_| so it's safe to check |client_| outside of the lock.
+  if (client_ == client)

[o1ka, 2016/11/24 13:21:16]

It looks like a data race if setClient() is called concurrently from two
different threads:
|client_| may be accessed here and modified at l.119 at the same time.

As far as I see it is never called concurrently thought: HTMLMediaElement always
calls setClient() under |provideInputLock|, so it should hopefully be fine if we
have a comment for setClient() that it cannot be called concurrently.

[DaleCurtis, 2016/11/24 18:06:53]

The lock in HTMLME is for the same reason the lock exists internally here; to
prevent the client from being null'd during provideInput / Render calls, but
otherwise all methods on this interface are always called on the render thread.

I think instead of adding a comment to this interface, which no caller would
really ever see, we should add one to
https://cs.chromium.org/chromium/src/third_party/WebKit/public/platform/WebAu...

WDYT? I can do that in a followup patch set if so. I don't have access to make
changes today (Turkey time!)

+    return;
+
   base::AutoLock auto_lock(sink_lock_);
-  if (client && client != client_) {
+  if (client) {
     // Detach the audio renderer from normal playback.
     sink_->Stop();
 
@@ -122,15 +127,16 @@ void WebAudioSourceProviderImpl::setClient(
     // ensures we have the same locking order when calling into |client_|.
     if (tee_filter_->IsInitialized())
       base::ResetAndReturn(&set_format_cb_).Run();
-  } else if (!client && client_) {
-    // Restore normal playback.
-    client_ = nullptr;
-    sink_->SetVolume(volume_);
-    if (state_ >= kStarted)
-      sink_->Start();
-    if (state_ >= kPlaying)
-      sink_->Play();
+    return;
   }
+
+  // Restore normal playback.
+  client_ = nullptr;
+  sink_->SetVolume(volume_);
+  if (state_ >= kStarted)
+    sink_->Start();
+  if (state_ >= kPlaying)
+    sink_->Play();
 }
 
 void WebAudioSourceProviderImpl::provideInput(