Bug 1599799 - p2: discard frame when callback is invalid. r=jya

Client of WebrtcMediaDataEncoder unregisters callback before calling
Shutdown(). When a pending encode promise gets resolved after that,
it will cause null pointer error.

Differential Revision: https://phabricator.services.mozilla.com/D55760

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

media/webrtc/signaling/src/media-conduit/WebrtcMediaDataEncoderCodec.cpp

@@ -157,17 +157,25 @@ bool WebrtcMediaDataEncoder::InitEncoder() {
 
 int32_t WebrtcMediaDataEncoder::RegisterEncodeCompleteCallback(
     webrtc::EncodedImageCallback* aCallback) {
-  mCallback = aCallback;
+  OwnerThread()->Dispatch(NS_NewRunnableFunction(
+      "WebrtcMediaDataEncoder::RegisterEncodeCompleteCallback",
+      [self = RefPtr<WebrtcMediaDataEncoder>(this), aCallback]() {
+        self->mCallback = aCallback;
+      }));
   return WEBRTC_VIDEO_CODEC_OK;
 }
 
 int32_t WebrtcMediaDataEncoder::Shutdown() {
   LOG("Release encoder");
-  auto rv = media::Await(do_AddRef(mThreadPool), mEncoder->Shutdown());
-  mEncoder = nullptr;
-  mCallback = nullptr;
-  mError = NS_OK;
-  return rv.IsResolve() ? WEBRTC_VIDEO_CODEC_OK : WEBRTC_VIDEO_CODEC_ERROR;
+  OwnerThread()->Dispatch(NS_NewRunnableFunction(
+      "WebrtcMediaDataEncoder::Shutdown",
+      [self = RefPtr<WebrtcMediaDataEncoder>(this),
+       encoder = RefPtr<MediaDataEncoder>(mEncoder.forget())]() {
+        self->mCallback = nullptr;
+        self->mError = NS_OK;
+        encoder->Shutdown();
+      }));
+  return WEBRTC_VIDEO_CODEC_OK;
 }
 
 RefPtr<MediaData> WebrtcMediaDataEncoder::CreateVideoDataFromWebrtcVideoFrame(
@@ -242,6 +250,10 @@ void WebrtcMediaDataEncoder::ProcessEncode(
           [display, self = RefPtr<WebrtcMediaDataEncoder>(this),
            // capture this for printing address in LOG.
            this](const MediaDataEncoder::EncodedData& aData) {
+            // Callback has been unregistered.
+            if (!mCallback) {
+              return;
+            }
             // The encoder haven't finished encoding yet.
             if (aData.IsEmpty()) {
               return;