Bug 1517252 - p1: don't lock HLSResourceCallbacksSupport mutex unnecessarily. r=jya

HLSResourceCallbacksSupport::mDecoder is cleared on main thread too, so
the nullness check already ensures decoder methods won't be called after
shut down. In fact, for OnError() the lock is harmful because the task calls
MediaDecoder::NetworkError(), which triggers decoder shutdown and eventually
HLSResourceCallbacksSupport::Detach(), which tries to lock the mutex again
while the mutex is still locked.

Differential Revision: https://phabricator.services.mozilla.com/D16709

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

dom/media/hls/HLSDecoder.cpp

@@ -52,6 +52,7 @@ HLSResourceCallbacksSupport::HLSResourceCallbacksSupport(HLSDecoder* aDecoder)
 }
 
 void HLSResourceCallbacksSupport::Detach() {
+  MOZ_ASSERT(NS_IsMainThread());
   MutexAutoLock lock(mMutex);
   mDecoder = nullptr;
 }
@@ -65,7 +66,6 @@ void HLSResourceCallbacksSupport::OnDataArrived() {
   RefPtr<HLSResourceCallbacksSupport> self = this;
   NS_DispatchToMainThread(NS_NewRunnableFunction(
       "HLSResourceCallbacksSupport::OnDataArrived", [self]() -> void {
-        MutexAutoLock lock(self->mMutex);
         if (self->mDecoder) {
           self->mDecoder->NotifyDataArrived();
         }
@@ -80,8 +80,7 @@ void HLSResourceCallbacksSupport::OnError(int aErrorCode) {
   }
   RefPtr<HLSResourceCallbacksSupport> self = this;
   NS_DispatchToMainThread(NS_NewRunnableFunction(
-      "HLSResourceCallbacksSupport::OnDataArrived", [self]() -> void {
-        MutexAutoLock lock(self->mMutex);
+      "HLSResourceCallbacksSupport::OnError", [self]() -> void {
         if (self->mDecoder) {
           // Since HLS source should be from the Internet, we treat all resource
           // errors from GeckoHlsPlayer as network errors.