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diff --git a/dom/media/ExternalEngineStateMachine.h b/dom/media/ExternalEngineStateMachine.h
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+++ b/dom/media/ExternalEngineStateMachine.h
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+/* 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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef DOM_MEDIA_EXTERNALENGINESTATEMACHINE_H_
+#define DOM_MEDIA_EXTERNALENGINESTATEMACHINE_H_
+
+#include "MediaDecoderStateMachineBase.h"
+#include "SeekJob.h"
+#include "mozilla/Variant.h"
+
+namespace mozilla {
+
+/**
+ * ExternalPlaybackEngine represents a media engine which is responsible for
+ * decoding and playback, which are not controlled by Gecko.
+ */
+class ExternalPlaybackEngine;
+
+enum class ExternalEngineEvent {
+  LoadedMetaData,
+  LoadedFirstFrame,
+  LoadedData,
+  Waiting,
+  Playing,
+  Seeked,
+  BufferingStarted,
+  BufferingEnded,
+  Timeupdate,
+  Ended,
+  RequestForAudio,
+  RequestForVideo,
+  AudioEnough,
+  VideoEnough,
+};
+const char* ExternalEngineEventToStr(ExternalEngineEvent aEvent);
+
+/**
+ * When using ExternalEngineStateMachine, that means we use an external engine
+ * to control decoding and playback (including A/V sync). Eg. Media Foundation
+ * Media Engine on Windows.
+ *
+ * The external engine does most of playback works, and uses ExternalEngineEvent
+ * to tell us its internal state. Therefore, this state machine is responsible
+ * to address those events from the engine and coordinate the format reader in
+ * order to provide data to the engine correctly.
+ */
+DDLoggedTypeDeclName(ExternalEngineStateMachine);
+
+class ExternalEngineStateMachine final
+    : public MediaDecoderStateMachineBase,
+      public DecoderDoctorLifeLogger<ExternalEngineStateMachine> {
+ public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(ExternalEngineStateMachine, override)
+
+  ExternalEngineStateMachine(MediaDecoder* aDecoder,
+                             MediaFormatReader* aReader);
+
+  RefPtr<GenericPromise> InvokeSetSink(
+      const RefPtr<AudioDeviceInfo>& aSink) override;
+
+  // The media sample would be managed by the external engine so we won't store
+  // any samples in our side.
+  size_t SizeOfVideoQueue() const override { return 0; }
+  size_t SizeOfAudioQueue() const override { return 0; }
+
+  // Not supported.
+  void SetVideoDecodeMode(VideoDecodeMode aMode) override {}
+  void InvokeSuspendMediaSink() override {}
+  void InvokeResumeMediaSink() override {}
+  RefPtr<GenericPromise> RequestDebugInfo(
+      dom::MediaDecoderStateMachineDebugInfo& aInfo) override {
+    // This debug info doesn't fit in this scenario because most decoding
+    // details are only visible inside the external engine.
+    return GenericPromise::CreateAndResolve(true, __func__);
+  }
+
+  void NotifyEvent(ExternalEngineEvent aEvent) {
+    // On the engine manager thread.
+    Unused << OwnerThread()->Dispatch(NS_NewRunnableFunction(
+        "ExternalEngineStateMachine::NotifyEvent",
+        [self = RefPtr{this}, aEvent] { self->NotifyEventInternal(aEvent); }));
+  }
+  void NotifyError(const MediaResult& aError) {
+    // On the engine manager thread.
+    Unused << OwnerThread()->Dispatch(NS_NewRunnableFunction(
+        "ExternalEngineStateMachine::NotifyError",
+        [self = RefPtr{this}, aError] { self->NotifyErrorInternal(aError); }));
+  }
+
+  const char* GetStateStr() const;
+
+  RefPtr<SetCDMPromise> SetCDMProxy(CDMProxy* aProxy) override;
+
+  bool IsCDMProxySupported(CDMProxy* aProxy) override;
+
+ private:
+  ~ExternalEngineStateMachine() = default;
+
+  void AssertOnTaskQueue() const { MOZ_ASSERT(OnTaskQueue()); }
+
+  // A light-weight state object that helps to store some variables which would
+  // only be used in a certain state. Also be able to do the cleaning for the
+  // state transition. Only modify on the task queue.
+  struct StateObject final {
+    enum class State {
+      InitEngine,
+      ReadingMetadata,
+      RunningEngine,
+      SeekingData,
+      ShutdownEngine,
+      RecoverEngine,
+    };
+    struct InitEngine {
+      InitEngine() = default;
+      ~InitEngine() { mEngineInitRequest.DisconnectIfExists(); }
+      MozPromiseRequestHolder<GenericNonExclusivePromise> mEngineInitRequest;
+      RefPtr<GenericNonExclusivePromise> mInitPromise;
+    };
+    struct ReadingMetadata {
+      ReadingMetadata() = default;
+      ~ReadingMetadata() { mMetadataRequest.DisconnectIfExists(); }
+      MozPromiseRequestHolder<MediaFormatReader::MetadataPromise>
+          mMetadataRequest;
+    };
+    struct RunningEngine {};
+    struct SeekingData {
+      SeekingData() = default;
+      SeekingData(SeekingData&&) = default;
+      SeekingData(const SeekingData&) = delete;
+      SeekingData& operator=(const SeekingData&) = delete;
+      ~SeekingData() {
+        mSeekJob.RejectIfExists(__func__);
+        mSeekRequest.DisconnectIfExists();
+      }
+      void SetTarget(const SeekTarget& aTarget) {
+        // If there is any promise for previous seeking, reject it first.
+        mSeekJob.RejectIfExists(__func__);
+        mSeekRequest.DisconnectIfExists();
+        // Then create a new seek job.
+        mSeekJob = SeekJob();
+        mSeekJob.mTarget = Some(aTarget);
+      }
+      void Resolve(const char* aCallSite) {
+        MOZ_ASSERT(mSeekJob.Exists());
+        mSeekJob.Resolve(aCallSite);
+        mSeekJob = SeekJob();
+      }
+      void RejectIfExists(const char* aCallSite) {
+        mSeekJob.RejectIfExists(aCallSite);
+      }
+      bool IsSeeking() const { return mSeekRequest.Exists(); }
+      media::TimeUnit GetTargetTime() const {
+        return mSeekJob.mTarget ? mSeekJob.mTarget->GetTime()
+                                : media::TimeUnit::Invalid();
+      }
+      // Set it to true when starting seeking, and would be set to false after
+      // receiving engine's `seeked` event. Used on thhe task queue only.
+      bool mWaitingEngineSeeked = false;
+      bool mWaitingReaderSeeked = false;
+      MozPromiseRequestHolder<MediaFormatReader::SeekPromise> mSeekRequest;
+      SeekJob mSeekJob;
+    };
+    struct ShutdownEngine {
+      RefPtr<ShutdownPromise> mShutdown;
+    };
+    // This state is used to recover the media engine after the MF CDM process
+    // crashes.
+    struct RecoverEngine : public InitEngine {};
+
+    StateObject() : mData(InitEngine()), mName(State::InitEngine){};
+    explicit StateObject(ReadingMetadata&& aArg)
+        : mData(std::move(aArg)), mName(State::ReadingMetadata){};
+    explicit StateObject(RunningEngine&& aArg)
+        : mData(std::move(aArg)), mName(State::RunningEngine){};
+    explicit StateObject(SeekingData&& aArg)
+        : mData(std::move(aArg)), mName(State::SeekingData){};
+    explicit StateObject(ShutdownEngine&& aArg)
+        : mData(std::move(aArg)), mName(State::ShutdownEngine){};
+    explicit StateObject(RecoverEngine&& aArg)
+        : mData(std::move(aArg)), mName(State::RecoverEngine){};
+
+    bool IsInitEngine() const { return mData.is<InitEngine>(); }
+    bool IsReadingMetadata() const { return mData.is<ReadingMetadata>(); }
+    bool IsRunningEngine() const { return mData.is<RunningEngine>(); }
+    bool IsSeekingData() const { return mData.is<SeekingData>(); }
+    bool IsShutdownEngine() const { return mData.is<ShutdownEngine>(); }
+    bool IsRecoverEngine() const { return mData.is<RecoverEngine>(); }
+
+    InitEngine* AsInitEngine() {
+      if (IsInitEngine()) {
+        return &mData.as<InitEngine>();
+      }
+      if (IsRecoverEngine()) {
+        return &mData.as<RecoverEngine>();
+      }
+      return nullptr;
+    }
+    ReadingMetadata* AsReadingMetadata() {
+      return IsReadingMetadata() ? &mData.as<ReadingMetadata>() : nullptr;
+    }
+    SeekingData* AsSeekingData() {
+      return IsSeekingData() ? &mData.as<SeekingData>() : nullptr;
+    }
+    ShutdownEngine* AsShutdownEngine() {
+      return IsShutdownEngine() ? &mData.as<ShutdownEngine>() : nullptr;
+    }
+
+    Variant<InitEngine, ReadingMetadata, RunningEngine, SeekingData,
+            ShutdownEngine, RecoverEngine>
+        mData;
+    State mName;
+  } mState;
+  using State = StateObject::State;
+
+  void NotifyEventInternal(ExternalEngineEvent aEvent);
+  void NotifyErrorInternal(const MediaResult& aError);
+
+  RefPtr<ShutdownPromise> Shutdown() override;
+
+  void SetPlaybackRate(double aPlaybackRate) override;
+  void BufferedRangeUpdated() override;
+  void VolumeChanged() override;
+  void PreservesPitchChanged() override;
+  void PlayStateChanged() override;
+  void LoopingChanged() override;
+
+  // Not supported.
+  void SetIsLiveStream(bool aIsLiveStream) override {}
+  void SetCanPlayThrough(bool aCanPlayThrough) override {}
+  void SetFragmentEndTime(const media::TimeUnit& aFragmentEndTime) override {}
+
+  void InitEngine();
+  void OnEngineInitSuccess();
+  void OnEngineInitFailure();
+
+  void ReadMetadata();
+  void OnMetadataRead(MetadataHolder&& aMetadata);
+  void OnMetadataNotRead(const MediaResult& aError);
+  bool IsFormatSupportedByExternalEngine(const MediaInfo& aInfo);
+
+  // Functions for handling external engine event.
+  void OnLoadedFirstFrame();
+  void OnLoadedData();
+  void OnWaiting();
+  void OnPlaying();
+  void OnSeeked();
+  void OnBufferingStarted();
+  void OnBufferingEnded();
+  void OnTimeupdate();
+  void OnEnded();
+  void OnRequestAudio();
+  void OnRequestVideo();
+
+  void ResetDecode();
+
+  void EndOfStream(MediaData::Type aType);
+  void WaitForData(MediaData::Type aType);
+
+  void StartRunningEngine();
+  void RunningEngineUpdate(MediaData::Type aType);
+
+  void ChangeStateTo(State aNextState);
+  static const char* StateToStr(State aState);
+
+  RefPtr<MediaDecoder::SeekPromise> Seek(const SeekTarget& aTarget) override;
+  void SeekReader();
+  void OnSeekResolved(const media::TimeUnit& aUnit);
+  void OnSeekRejected(const SeekRejectValue& aReject);
+  bool IsSeeking();
+  void CheckIfSeekCompleted();
+
+  void MaybeFinishWaitForData();
+
+  void SetBlankVideoToVideoContainer();
+
+  media::TimeUnit GetVideoThreshold();
+
+  bool ShouldRunEngineUpdateForRequest();
+
+  void UpdateSecondaryVideoContainer() override;
+
+  void RecoverFromCDMProcessCrashIfNeeded();
+
+  UniquePtr<ExternalPlaybackEngine> mEngine;
+
+  bool mHasEnoughAudio = false;
+  bool mHasEnoughVideo = false;
+  bool mSentPlaybackEndedEvent = false;
+  bool mHasReceivedFirstDecodedVideoFrame = false;
+
+  // Only used if setting CDM happens before the engine finishes initialization.
+  MozPromiseHolder<SetCDMPromise> mSetCDMProxyPromise;
+  MozPromiseRequestHolder<SetCDMPromise> mSetCDMProxyRequest;
+};
+
+class ExternalPlaybackEngine {
+ public:
+  explicit ExternalPlaybackEngine(ExternalEngineStateMachine* aOwner)
+      : mOwner(aOwner) {}
+
+  virtual ~ExternalPlaybackEngine() = default;
+
+  // Init the engine and specify the preload request.
+  virtual RefPtr<GenericNonExclusivePromise> Init(bool aShouldPreload) = 0;
+  virtual void Shutdown() = 0;
+  virtual uint64_t Id() const = 0;
+
+  // Following methods should only be called after successfully initialize the
+  // external engine.
+  virtual void Play() = 0;
+  virtual void Pause() = 0;
+  virtual void Seek(const media::TimeUnit& aTargetTime) = 0;
+  virtual void SetPlaybackRate(double aPlaybackRate) = 0;
+  virtual void SetVolume(double aVolume) = 0;
+  virtual void SetLooping(bool aLooping) = 0;
+  virtual void SetPreservesPitch(bool aPreservesPitch) = 0;
+  virtual media::TimeUnit GetCurrentPosition() = 0;
+  virtual void NotifyEndOfStream(TrackInfo::TrackType aType) = 0;
+  virtual void SetMediaInfo(const MediaInfo& aInfo) = 0;
+  virtual bool SetCDMProxy(CDMProxy* aProxy) = 0;
+
+  ExternalEngineStateMachine* const MOZ_NON_OWNING_REF mOwner;
+};
+
+}  // namespace mozilla
+
+#endif  // DOM_MEDIA_EXTERNALENGINESTATEMACHINE_H_