Bug 1436694 - Clarify that MediaEngineSources can be double-stopped. r=padenot

This is already true for the audio sources. It should be for all.

Crashtests showed that shutting down amidst the async init can lead to
double-stops. It is impossible to completely protect yourself from them without
waiting for all queued operations to resolve (results to become known) before
taking action. Doing that would require a refactor in MediaManager and cause
higher latency for device operations so it seems like the wrong way to go.

MozReview-Commit-ID: 5Cci6whzTL7

--HG--
extra : rebase_source : 18efef4e294ac2b02753721ca5813bcbf63e3111

dom/media/webrtc/MediaEngineDefault.cpp

@@ -238,6 +238,10 @@ MediaEngineDefaultVideoSource::Stop(const RefPtr<const AllocationHandle>& aHandl
 {
   AssertIsOnOwningThread();
 
+  if (mState == kStopped || mState == kAllocated) {
+    return NS_OK;
+  }
+
   MOZ_ASSERT(mState == kStarted);
   MOZ_ASSERT(mTimer);
   MOZ_ASSERT(mStream);
@@ -498,8 +502,11 @@ MediaEngineDefaultAudioSource::Stop(const RefPtr<const AllocationHandle>& aHandl
 {
   AssertIsOnOwningThread();
 
-  MOZ_ASSERT(mState == kStarted);
+  if (mState == kStopped || mState == kAllocated) {
+    return NS_OK;
+  }
 
+  MOZ_ASSERT(mState == kStarted);
 
   MutexAutoLock lock(mMutex);
   mState = kStopped;

dom/media/webrtc/MediaEngineRemoteVideoSource.cpp

@@ -324,6 +324,10 @@ MediaEngineRemoteVideoSource::Stop(const RefPtr<const AllocationHandle>& aHandle
   LOG((__PRETTY_FUNCTION__));
   AssertIsOnOwningThread();
 
+  if (mState == kStopped || mState == kAllocated) {
+    return NS_OK;
+  }
+
   MOZ_ASSERT(mState == kStarted);
 
   if (camera::GetChildAndCall(&camera::CamerasChild::StopCapture,

dom/media/webrtc/MediaEngineSource.h

@@ -171,6 +171,9 @@ public:
    *
    * If this was the last AllocationHandle that had been started,
    * the underlying device will be stopped.
+   *
+   * Double-stopping a given allocation handle is allowed and will return NS_OK.
+   * This is necessary sometimes during shutdown.
    */
   virtual nsresult Stop(const RefPtr<const AllocationHandle>& aHandle) = 0;
 

dom/media/webrtc/MediaEngineTabVideoSource.cpp

@@ -402,6 +402,11 @@ nsresult
 MediaEngineTabVideoSource::Stop(const RefPtr<const AllocationHandle>& aHandle)
 {
   AssertIsOnOwningThread();
+
+  if (mState == kStopped || mState == kAllocated) {
+    return NS_OK;
+  }
+
   MOZ_ASSERT(mState == kStarted);
 
   // If mBlackedoutWindow is true, we may be running