Bug 1568372 - add mutex in RemoteDecoderModule::LaunchRDDProcessIfNeeded. r=jya

This stops a potential race when multiple media playback threads attempt to
launch the RDD process and setup the IPC connections between
RemoteDecoderManager{Child|Parent} at the same time.

Differential Revision: https://phabricator.services.mozilla.com/D39120

--HG--
extra : moz-landing-system : lando

dom/media/ipc/RemoteDecoderModule.cpp

@@ -29,6 +29,8 @@ using dom::ContentChild;
 using namespace ipc;
 using namespace layers;
 
+StaticMutex RemoteDecoderModule::sLaunchMonitor;
+
 RemoteDecoderModule::RemoteDecoderModule()
     : mManagerThread(RemoteDecoderManagerChild::GetManagerThread()) {}
 
@@ -62,6 +64,8 @@ void RemoteDecoderModule::LaunchRDDProcessIfNeeded() {
     return;
   }
 
+  StaticMutexAutoLock mon(sLaunchMonitor);
+
   // We have a couple possible states here.  We are in a content process
   // and:
   // 1) the RDD process has never been launched.  RDD should be launched

dom/media/ipc/RemoteDecoderModule.h

@@ -7,6 +7,8 @@
 #define include_dom_media_ipc_RemoteDecoderModule_h
 #include "PlatformDecoderModule.h"
 
+#include "mozilla/StaticMutex.h"
+
 namespace mozilla {
 
 // A PDM implementation that creates a RemoteMediaDataDecoder (a
@@ -31,6 +33,7 @@ class RemoteDecoderModule : public PlatformDecoderModule {
 
  private:
   RefPtr<nsIThread> mManagerThread;
+  static StaticMutex sLaunchMonitor;
 };
 
 }  // namespace mozilla