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diff --git a/dom/media/webrtc/libwebrtcglue/FrameTransformerProxy.h b/dom/media/webrtc/libwebrtcglue/FrameTransformerProxy.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+#ifndef MOZILLA_DOM_MEDIA_WEBRTC_LIBWEBRTCGLUE_FRAMETRANSFORMERPROXY_H_
+#define MOZILLA_DOM_MEDIA_WEBRTC_LIBWEBRTCGLUE_FRAMETRANSFORMERPROXY_H_
+
+#include "nsISupportsImpl.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/Maybe.h"
+#include <list>
+#include <memory>
+
+class nsIEventTarget;
+
+namespace webrtc {
+class TransformableFrameInterface;
+class VideoReceiveStreamInterface;
+}  // namespace webrtc
+
+namespace mozilla {
+
+class FrameTransformer;
+class WebrtcVideoConduit;
+class CopyableErrorResult;
+
+namespace dom {
+class RTCRtpScriptTransformer;
+class RTCRtpSender;
+class RTCRtpReceiver;
+}  // namespace dom
+
+// This corresponds to a single RTCRtpScriptTransform (and its
+// RTCRtpScriptTransformer, once that is created on the worker thread). This
+// is intended to decouple threading/lifecycle/include-dependencies between
+// FrameTransformer (on the libwebrtc side of things), RTCRtpScriptTransformer
+// (on the worker side of things), RTCRtpScriptTransform and
+// RTCRtpSender/Receiver (on the main thread), and prevents frames from being
+// lost while we're setting things up on the worker. In other words, this
+// handles the inconvenient stuff.
+class FrameTransformerProxy {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(FrameTransformerProxy);
+
+  FrameTransformerProxy();
+  FrameTransformerProxy(const FrameTransformerProxy& aRhs) = delete;
+  FrameTransformerProxy(FrameTransformerProxy&& aRhs) = delete;
+  FrameTransformerProxy& operator=(const FrameTransformerProxy& aRhs) = delete;
+  FrameTransformerProxy& operator=(FrameTransformerProxy&& aRhs) = delete;
+
+  // Called at most once (might not be called if the worker is shutting down),
+  // on the worker thread.
+  void SetScriptTransformer(dom::RTCRtpScriptTransformer& aTransformer);
+
+  // Can be called from the worker thread (if the worker is shutting down), or
+  // main (if RTCRtpSender/RTCRtpReceiver is done with us).
+  void ReleaseScriptTransformer();
+
+  // RTCRtpScriptTransformer calls this when it is done transforming a frame.
+  void OnTransformedFrame(
+      std::unique_ptr<webrtc::TransformableFrameInterface> aFrame);
+
+  Maybe<bool> IsVideo() const;
+
+  // Called by FrameTransformer, on main. Only one FrameTransformer will ever
+  // be registered over the lifetime of this object. This is where we route
+  // transformed frames. If this is set, we can also expect to receive calls to
+  // Transform.
+  void SetLibwebrtcTransformer(FrameTransformer* aLibwebrtcTransformer);
+
+  // FrameTransformer calls this while we're registered with it (by
+  // SetLibwebrtcTransformer)
+  void Transform(std::unique_ptr<webrtc::TransformableFrameInterface> aFrame);
+
+  void SetSender(dom::RTCRtpSender* aSender);
+  void SetReceiver(dom::RTCRtpReceiver* aReceiver);
+
+  // Called on worker thread
+  bool RequestKeyFrame();
+  // Called on call thread
+  void KeyFrameRequestDone(bool aSuccess);
+
+  bool GenerateKeyFrame(const Maybe<std::string>& aRid);
+  void GenerateKeyFrameError(const Maybe<std::string>& aRid,
+                             const CopyableErrorResult& aResult);
+
+ private:
+  virtual ~FrameTransformerProxy();
+
+  // Worker thread only. Set at most once.
+  // Does not need any mutex protection.
+  RefPtr<dom::RTCRtpScriptTransformer> mScriptTransformer;
+
+  mutable Mutex mMutex;
+  // Written on the worker thread. Read on libwebrtc threads, mainthread, and
+  // the worker thread.
+  RefPtr<nsIEventTarget> mWorkerThread MOZ_GUARDED_BY(mMutex);
+  // We need a flag for this in case the ReleaseScriptTransformer call comes
+  // _before_ the script transformer is set, to disable SetScriptTransformer.
+  // Could be written on main or the worker thread. Read on main, worker, and
+  // libwebrtc threads.
+  bool mReleaseScriptTransformerCalled MOZ_GUARDED_BY(mMutex) = false;
+  // Used when frames arrive before the script transformer is created, which
+  // should be pretty rare. Accessed on worker and libwebrtc threads.
+  std::list<std::unique_ptr<webrtc::TransformableFrameInterface>> mQueue
+      MOZ_GUARDED_BY(mMutex);
+  // Written on main, read on the worker thread.
+  FrameTransformer* mLibwebrtcTransformer MOZ_GUARDED_BY(mMutex) = nullptr;
+
+  // TODO: Will be used to route GenerateKeyFrame. Details TBD.
+  RefPtr<dom::RTCRtpSender> mSender MOZ_GUARDED_BY(mMutex);
+  // Set on mainthread. This is where we route RequestKeyFrame calls from the
+  // worker thread. Mutex protected because spec wants sync errors if the
+  // receiver is not set (or the right type). If spec drops this requirement,
+  // this could be mainthread only and non-mutex-protected.
+  RefPtr<dom::RTCRtpReceiver> mReceiver MOZ_GUARDED_BY(mMutex);
+  Maybe<bool> mVideo MOZ_GUARDED_BY(mMutex);
+};
+
+}  // namespace mozilla
+
+#endif  // MOZILLA_DOM_MEDIA_WEBRTC_LIBWEBRTCGLUE_FRAMETRANSFORMERPROXY_H_