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diff --git a/dom/media/webrtc/MediaEngineRemoteVideoSource.h b/dom/media/webrtc/MediaEngineRemoteVideoSource.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set sw=2 ts=8 et ft=cpp : */
+/* 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 MEDIAENGINE_REMOTE_VIDEO_SOURCE_H_
+#define MEDIAENGINE_REMOTE_VIDEO_SOURCE_H_
+
+#include "prcvar.h"
+#include "prthread.h"
+
+#include "mozilla/Mutex.h"
+#include "nsCOMPtr.h"
+#include "nsThreadUtils.h"
+#include "DOMMediaStream.h"
+#include "nsDirectoryServiceDefs.h"
+#include "nsComponentManagerUtils.h"
+
+// Avoid warnings about redefinition of WARN_UNUSED_RESULT
+#include "ipc/IPCMessageUtils.h"
+#include "VideoUtils.h"
+#include "MediaEngineSource.h"
+#include "VideoSegment.h"
+#include "AudioSegment.h"
+#include "MediaTrackGraph.h"
+
+#include "MediaEngineWrapper.h"
+#include "mozilla/dom/MediaStreamTrackBinding.h"
+
+// Camera Access via IPC
+#include "CamerasChild.h"
+
+#include "NullTransport.h"
+
+// WebRTC includes
+#include "common_video/include/video_frame_buffer_pool.h"
+#include "modules/video_capture/video_capture_defines.h"
+
+namespace webrtc {
+using CaptureCapability = VideoCaptureCapability;
+}
+
+namespace mozilla {
+
+// Fitness distance is defined in
+// https://w3c.github.io/mediacapture-main/getusermedia.html#dfn-selectsettings
+
+// The main difference of feasibility and fitness distance is that if the
+// constraint is required ('max', or 'exact'), and the settings dictionary's
+// value for the constraint does not satisfy the constraint, the fitness
+// distance is positive infinity. Given a continuous space of settings
+// dictionaries comprising all discrete combinations of dimension and frame-rate
+// related properties, the feasibility distance is still in keeping with the
+// constraints algorithm.
+enum DistanceCalculation { kFitness, kFeasibility };
+
+/**
+ * The WebRTC implementation of the MediaEngine interface.
+ */
+class MediaEngineRemoteVideoSource : public MediaEngineSource,
+                                     public camera::FrameRelay {
+  ~MediaEngineRemoteVideoSource();
+
+  struct CapabilityCandidate {
+    explicit CapabilityCandidate(webrtc::CaptureCapability aCapability,
+                                 uint32_t aDistance = 0)
+        : mCapability(aCapability), mDistance(aDistance) {}
+
+    const webrtc::CaptureCapability mCapability;
+    uint32_t mDistance;
+  };
+
+  class CapabilityComparator {
+   public:
+    bool Equals(const CapabilityCandidate& aCandidate,
+                const webrtc::CaptureCapability& aCapability) const {
+      return aCandidate.mCapability == aCapability;
+    }
+  };
+
+  bool ChooseCapability(const NormalizedConstraints& aConstraints,
+                        const MediaEnginePrefs& aPrefs,
+                        webrtc::CaptureCapability& aCapability,
+                        const DistanceCalculation aCalculate);
+
+  uint32_t GetDistance(const webrtc::CaptureCapability& aCandidate,
+                       const NormalizedConstraintSet& aConstraints,
+                       const DistanceCalculation aCalculate) const;
+
+  uint32_t GetFitnessDistance(
+      const webrtc::CaptureCapability& aCandidate,
+      const NormalizedConstraintSet& aConstraints) const;
+
+  uint32_t GetFeasibilityDistance(
+      const webrtc::CaptureCapability& aCandidate,
+      const NormalizedConstraintSet& aConstraints) const;
+
+  static void TrimLessFitCandidates(nsTArray<CapabilityCandidate>& aSet);
+
+ public:
+  explicit MediaEngineRemoteVideoSource(const MediaDevice* aMediaDevice);
+
+  // ExternalRenderer
+  int DeliverFrame(uint8_t* aBuffer,
+                   const camera::VideoFrameProperties& aProps) override;
+
+  // MediaEngineSource
+  nsresult Allocate(const dom::MediaTrackConstraints& aConstraints,
+                    const MediaEnginePrefs& aPrefs, uint64_t aWindowID,
+                    const char** aOutBadConstraint) override;
+  nsresult Deallocate() override;
+  void SetTrack(const RefPtr<MediaTrack>& aTrack,
+                const PrincipalHandle& aPrincipal) override;
+  nsresult Start() override;
+  nsresult Reconfigure(const dom::MediaTrackConstraints& aConstraints,
+                       const MediaEnginePrefs& aPrefs,
+                       const char** aOutBadConstraint) override;
+  nsresult FocusOnSelectedSource() override;
+  nsresult Stop() override;
+
+  uint32_t GetBestFitnessDistance(
+      const nsTArray<const NormalizedConstraintSet*>& aConstraintSets)
+      const override;
+  void GetSettings(dom::MediaTrackSettings& aOutSettings) const override;
+
+  RefPtr<GenericNonExclusivePromise> GetFirstFramePromise() const override {
+    return mFirstFramePromise;
+  }
+
+  const TrackingId& GetTrackingId() const override;
+
+  static camera::CaptureEngine CaptureEngine(dom::MediaSourceEnum aMediaSource);
+
+ private:
+  /**
+   * Returns the number of capabilities for the underlying device.
+   *
+   * Guaranteed to return at least one capability.
+   */
+  size_t NumCapabilities() const;
+
+  /**
+   * Returns the capability with index `aIndex` for our assigned device.
+   *
+   * It is an error to call this with `aIndex >= NumCapabilities()`.
+   *
+   * The lifetime of the returned capability is the same as for this source.
+   */
+  webrtc::CaptureCapability& GetCapability(size_t aIndex) const;
+
+  int mCaptureId = -1;
+  const camera::CaptureEngine mCapEngine;  // source of media (cam, screen etc)
+
+  // A tracking id used to uniquely identify the source of video frames.
+  // Set under mMutex on the owning thread. Accessed under one of the two.
+  TrackingId mTrackingId;
+
+  // Mirror of mTrackingId on the frame-delivering thread (Cameras IPC).
+  Maybe<TrackingId> mFrameDeliveringTrackingId;
+
+  // mMutex protects certain members on 3 threads:
+  // MediaManager, Cameras IPC and MediaTrackGraph.
+  Mutex mMutex MOZ_UNANNOTATED;
+
+  // Current state of this source.
+  // Set under mMutex on the owning thread. Accessed under one of the two.
+  MediaEngineSourceState mState = kReleased;
+
+  // The source track that we feed video data to.
+  // Set under mMutex on the owning thread. Accessed under one of the two.
+  RefPtr<SourceMediaTrack> mTrack;
+
+  // The PrincipalHandle that gets attached to the frames we feed to mTrack.
+  // Set under mMutex on the owning thread. Accessed under one of the two.
+  PrincipalHandle mPrincipal = PRINCIPAL_HANDLE_NONE;
+
+  // Set in Start() and Deallocate() on the owning thread.
+  // Accessed in DeliverFrame() on the camera IPC thread, guaranteed to happen
+  // after Start() and before the end of Stop().
+  RefPtr<layers::ImageContainer> mImageContainer;
+
+  // A buffer pool used to manage the temporary buffer used when rescaling
+  // incoming images. Cameras IPC thread only.
+  webrtc::VideoFrameBufferPool mRescalingBufferPool;
+
+  // The intrinsic size of the latest captured image, so we can feed black
+  // images of the same size while stopped.
+  // Set under mMutex on the Cameras IPC thread. Accessed under one of the two.
+  gfx::IntSize mImageSize = gfx::IntSize(0, 0);
+
+  struct AtomicBool {
+    Atomic<bool> mValue;
+  };
+
+  // True when resolution settings have been updated from a real frame's
+  // resolution. Threadsafe.
+  // TODO: This can be removed in bug 1453269.
+  const RefPtr<media::Refcountable<AtomicBool>> mSettingsUpdatedByFrame;
+
+  // The current settings of this source.
+  // Note that these may be different from the settings of the underlying device
+  // since we scale frames to avoid fingerprinting.
+  // Members are main thread only.
+  const RefPtr<media::Refcountable<dom::MediaTrackSettings>> mSettings;
+  MozPromiseHolder<GenericNonExclusivePromise> mFirstFramePromiseHolder;
+  RefPtr<GenericNonExclusivePromise> mFirstFramePromise;
+
+  // The capability currently chosen by constraints of the user of this source.
+  // Set under mMutex on the owning thread. Accessed under one of the two.
+  webrtc::CaptureCapability mCapability;
+
+  /**
+   * Capabilities that we choose between when applying constraints.
+   *
+   * This allows for memoization of capabilities as they're requested from the
+   * parent process.
+   *
+   * This is mutable so that the const methods NumCapabilities() and
+   * GetCapability() can reset it. Owning thread only.
+   */
+  mutable nsTArray<UniquePtr<webrtc::CaptureCapability>> mCapabilities;
+
+  /**
+   * True if mCapabilities only contains hardcoded capabilities. This can happen
+   * if the underlying device is not reporting any capabilities. These can be
+   * affected by constraints, so they're evaluated in ChooseCapability() rather
+   * than GetCapability().
+   *
+   * This is mutable so that the const methods NumCapabilities() and
+   * GetCapability() can reset it. Owning thread only.
+   */
+  mutable bool mCapabilitiesAreHardcoded = false;
+
+  const RefPtr<const MediaDevice> mMediaDevice;
+  const nsCString mDeviceUUID;
+  Maybe<nsString> mFacingMode;
+};
+
+}  // namespace mozilla
+
+#endif /* MEDIAENGINE_REMOTE_VIDEO_SOURCE_H_ */