1 files changed, 576 insertions, 0 deletions
diff --git a/dom/media/systemservices/CamerasChild.cpp b/dom/media/systemservices/CamerasChild.cpp
new file mode 100644
index 0000000000..dcafdf12a0
--- /dev/null
+++ b/dom/media/systemservices/CamerasChild.cpp
@@ -0,0 +1,576 @@
+/* -*- 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/. */
+
+#include "CamerasChild.h"
+
+#undef FF
+
+#include "mozilla/Assertions.h"
+#include "mozilla/ipc/BackgroundChild.h"
+#include "mozilla/ipc/PBackgroundChild.h"
+#include "mozilla/Logging.h"
+#include "mozilla/SyncRunnable.h"
+#include "mozilla/WeakPtr.h"
+#include "mozilla/Unused.h"
+#include "MediaUtils.h"
+#include "nsThreadUtils.h"
+
+#undef LOG
+#undef LOG_ENABLED
+mozilla::LazyLogModule gCamerasChildLog("CamerasChild");
+#define LOG(args) MOZ_LOG(gCamerasChildLog, mozilla::LogLevel::Debug, args)
+#define LOG_ENABLED() MOZ_LOG_TEST(gCamerasChildLog, mozilla::LogLevel::Debug)
+
+namespace mozilla::camera {
+
+CamerasSingleton::CamerasSingleton()
+    : mCamerasMutex("CamerasSingleton::mCamerasMutex"),
+      mCameras(nullptr),
+      mCamerasChildThread(nullptr),
+      mInShutdown(false) {
+  LOG(("CamerasSingleton: %p", this));
+}
+
+CamerasSingleton::~CamerasSingleton() { LOG(("~CamerasSingleton: %p", this)); }
+
+class InitializeIPCThread : public Runnable {
+ public:
+  InitializeIPCThread()
+      : Runnable("camera::InitializeIPCThread"), mCamerasChild(nullptr) {}
+
+  NS_IMETHOD Run() override {
+    // Try to get the PBackground handle
+    ipc::PBackgroundChild* existingBackgroundChild =
+        ipc::BackgroundChild::GetForCurrentThread();
+    // If it's not spun up yet, block until it is, and retry
+    if (!existingBackgroundChild) {
+      LOG(("No existingBackgroundChild"));
+      existingBackgroundChild =
+          ipc::BackgroundChild::GetOrCreateForCurrentThread();
+      LOG(("BackgroundChild: %p", existingBackgroundChild));
+      if (!existingBackgroundChild) {
+        return NS_ERROR_FAILURE;
+      }
+    }
+
+    // Create CamerasChild
+    // We will be returning the resulting pointer (synchronously) to our caller.
+    mCamerasChild = static_cast<mozilla::camera::CamerasChild*>(
+        existingBackgroundChild->SendPCamerasConstructor());
+
+    return NS_OK;
+  }
+
+  CamerasChild* GetCamerasChild() { return mCamerasChild; }
+
+ private:
+  CamerasChild* mCamerasChild;
+};
+
+CamerasChild* GetCamerasChild() {
+  CamerasSingleton::Mutex().AssertCurrentThreadOwns();
+  if (!CamerasSingleton::Child()) {
+    MOZ_ASSERT(!NS_IsMainThread(), "Should not be on the main Thread");
+    MOZ_ASSERT(!CamerasSingleton::Thread());
+    LOG(("No sCameras, setting up IPC Thread"));
+    nsresult rv = NS_NewNamedThread("Cameras IPC",
+                                    getter_AddRefs(CamerasSingleton::Thread()));
+    if (NS_FAILED(rv)) {
+      LOG(("Error launching IPC Thread"));
+      return nullptr;
+    }
+
+    // At this point we are in the MediaManager thread, and the thread we are
+    // dispatching to is the specific Cameras IPC thread that was just made
+    // above, so now we will fire off a runnable to run
+    // BackgroundChild::GetOrCreateForCurrentThread there, while we
+    // block in this thread.
+    // We block until the following happens in the Cameras IPC thread:
+    // 1) Creation of PBackground finishes
+    // 2) Creation of PCameras finishes by sending a message to the parent
+    RefPtr<InitializeIPCThread> runnable = new InitializeIPCThread();
+    RefPtr<SyncRunnable> sr = new SyncRunnable(runnable);
+    sr->DispatchToThread(CamerasSingleton::Thread());
+    CamerasSingleton::Child() = runnable->GetCamerasChild();
+  }
+  if (!CamerasSingleton::Child()) {
+    LOG(("Failed to set up CamerasChild, are we in shutdown?"));
+  }
+  return CamerasSingleton::Child();
+}
+
+CamerasChild* GetCamerasChildIfExists() {
+  OffTheBooksMutexAutoLock lock(CamerasSingleton::Mutex());
+  return CamerasSingleton::Child();
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvReplyFailure(void) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  MonitorAutoLock monitor(mReplyMonitor);
+  mReceivedReply = true;
+  mReplySuccess = false;
+  monitor.Notify();
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvReplySuccess(void) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  MonitorAutoLock monitor(mReplyMonitor);
+  mReceivedReply = true;
+  mReplySuccess = true;
+  monitor.Notify();
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvReplyNumberOfCapabilities(
+    const int& numdev) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  MonitorAutoLock monitor(mReplyMonitor);
+  mReceivedReply = true;
+  mReplySuccess = true;
+  mReplyInteger = numdev;
+  monitor.Notify();
+  return IPC_OK();
+}
+
+// Helper function to dispatch calls to the IPC Thread and
+// CamerasChild object. Takes the needed locks and dispatches.
+// Takes a "failed" value and a reference to the output variable
+// as parameters, will return the right one depending on whether
+// dispatching succeeded.
+template <class T = int>
+class LockAndDispatch {
+ public:
+  LockAndDispatch(CamerasChild* aCamerasChild, const char* aRequestingFunc,
+                  nsIRunnable* aRunnable, const T& aFailureValue,
+                  const T& aSuccessValue)
+      : mCamerasChild(aCamerasChild),
+        mRequestingFunc(aRequestingFunc),
+        mRunnable(aRunnable),
+        mReplyLock(aCamerasChild->mReplyMonitor),
+        mRequestLock(aCamerasChild->mRequestMutex),
+        mSuccess(true),
+        mFailureValue(aFailureValue),
+        mSuccessValue(aSuccessValue) {
+    Dispatch();
+  }
+
+  T ReturnValue() const {
+    if (mSuccess) {
+      return mSuccessValue;
+    } else {
+      return mFailureValue;
+    }
+  }
+
+  const bool& Success() const { return mSuccess; }
+
+ private:
+  void Dispatch() {
+    if (!mCamerasChild->DispatchToParent(mRunnable, mReplyLock)) {
+      LOG(("Cameras dispatch for IPC failed in %s", mRequestingFunc));
+      mSuccess = false;
+    }
+  }
+
+  CamerasChild* mCamerasChild;
+  const char* mRequestingFunc;
+  nsIRunnable* mRunnable;
+  // Prevent concurrent use of the reply variables by holding
+  // the mReplyMonitor. Note that this is unlocked while waiting for
+  // the reply to be filled in, necessitating the additional mRequestLock/Mutex;
+  MonitorAutoLock mReplyLock;
+  MutexAutoLock mRequestLock;
+  bool mSuccess;
+  const T mFailureValue;
+  const T& mSuccessValue;
+};
+
+bool CamerasChild::DispatchToParent(nsIRunnable* aRunnable,
+                                    MonitorAutoLock& aMonitor) {
+  CamerasSingleton::Mutex().AssertCurrentThreadOwns();
+  CamerasSingleton::Thread()->Dispatch(aRunnable, NS_DISPATCH_NORMAL);
+  // We can't see if the send worked, so we need to be able to bail
+  // out on shutdown (when it failed and we won't get a reply).
+  if (!mIPCIsAlive) {
+    return false;
+  }
+  // Guard against spurious wakeups.
+  mReceivedReply = false;
+  // Wait for a reply
+  do {
+    aMonitor.Wait();
+  } while (!mReceivedReply && mIPCIsAlive);
+  if (!mReplySuccess) {
+    return false;
+  }
+  return true;
+}
+
+int CamerasChild::NumberOfCapabilities(CaptureEngine aCapEngine,
+                                       const char* deviceUniqueIdUTF8) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  LOG(("NumberOfCapabilities for %s", deviceUniqueIdUTF8));
+  nsCString unique_id(deviceUniqueIdUTF8);
+  nsCOMPtr<nsIRunnable> runnable =
+      mozilla::NewRunnableMethod<CaptureEngine, nsCString>(
+          "camera::PCamerasChild::SendNumberOfCapabilities", this,
+          &CamerasChild::SendNumberOfCapabilities, aCapEngine, unique_id);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, 0, mReplyInteger);
+  LOG(("Capture capability count: %d", dispatcher.ReturnValue()));
+  return dispatcher.ReturnValue();
+}
+
+int CamerasChild::NumberOfCaptureDevices(CaptureEngine aCapEngine) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  nsCOMPtr<nsIRunnable> runnable = mozilla::NewRunnableMethod<CaptureEngine>(
+      "camera::PCamerasChild::SendNumberOfCaptureDevices", this,
+      &CamerasChild::SendNumberOfCaptureDevices, aCapEngine);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, 0, mReplyInteger);
+  LOG(("Capture Devices: %d", dispatcher.ReturnValue()));
+  return dispatcher.ReturnValue();
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvReplyNumberOfCaptureDevices(
+    const int& numdev) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  MonitorAutoLock monitor(mReplyMonitor);
+  mReceivedReply = true;
+  mReplySuccess = true;
+  mReplyInteger = numdev;
+  monitor.Notify();
+  return IPC_OK();
+}
+
+int CamerasChild::EnsureInitialized(CaptureEngine aCapEngine) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  nsCOMPtr<nsIRunnable> runnable = mozilla::NewRunnableMethod<CaptureEngine>(
+      "camera::PCamerasChild::SendEnsureInitialized", this,
+      &CamerasChild::SendEnsureInitialized, aCapEngine);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, 0, mReplyInteger);
+  LOG(("Capture Devices: %d", dispatcher.ReturnValue()));
+  return dispatcher.ReturnValue();
+}
+
+int CamerasChild::GetCaptureCapability(
+    CaptureEngine aCapEngine, const char* unique_idUTF8,
+    const unsigned int capability_number,
+    webrtc::VideoCaptureCapability& capability) {
+  LOG(("GetCaptureCapability: %s %d", unique_idUTF8, capability_number));
+  nsCString unique_id(unique_idUTF8);
+  nsCOMPtr<nsIRunnable> runnable =
+      mozilla::NewRunnableMethod<CaptureEngine, nsCString, unsigned int>(
+          "camera::PCamerasChild::SendGetCaptureCapability", this,
+          &CamerasChild::SendGetCaptureCapability, aCapEngine, unique_id,
+          capability_number);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, -1, mZero);
+  if (dispatcher.Success()) {
+    capability = mReplyCapability;
+  }
+  return dispatcher.ReturnValue();
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvReplyGetCaptureCapability(
+    const VideoCaptureCapability& ipcCapability) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  MonitorAutoLock monitor(mReplyMonitor);
+  mReceivedReply = true;
+  mReplySuccess = true;
+  mReplyCapability.width = ipcCapability.width();
+  mReplyCapability.height = ipcCapability.height();
+  mReplyCapability.maxFPS = ipcCapability.maxFPS();
+  mReplyCapability.videoType =
+      static_cast<webrtc::VideoType>(ipcCapability.videoType());
+  mReplyCapability.interlaced = ipcCapability.interlaced();
+  monitor.Notify();
+  return IPC_OK();
+}
+
+int CamerasChild::GetCaptureDevice(
+    CaptureEngine aCapEngine, unsigned int list_number, char* device_nameUTF8,
+    const unsigned int device_nameUTF8Length, char* unique_idUTF8,
+    const unsigned int unique_idUTF8Length, bool* scary) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  nsCOMPtr<nsIRunnable> runnable =
+      mozilla::NewRunnableMethod<CaptureEngine, unsigned int>(
+          "camera::PCamerasChild::SendGetCaptureDevice", this,
+          &CamerasChild::SendGetCaptureDevice, aCapEngine, list_number);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, -1, mZero);
+  if (dispatcher.Success()) {
+    base::strlcpy(device_nameUTF8, mReplyDeviceName.get(),
+                  device_nameUTF8Length);
+    base::strlcpy(unique_idUTF8, mReplyDeviceID.get(), unique_idUTF8Length);
+    if (scary) {
+      *scary = mReplyScary;
+    }
+    LOG(("Got %s name %s id", device_nameUTF8, unique_idUTF8));
+  }
+  return dispatcher.ReturnValue();
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvReplyGetCaptureDevice(
+    const nsCString& device_name, const nsCString& device_id,
+    const bool& scary) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  MonitorAutoLock monitor(mReplyMonitor);
+  mReceivedReply = true;
+  mReplySuccess = true;
+  mReplyDeviceName = device_name;
+  mReplyDeviceID = device_id;
+  mReplyScary = scary;
+  monitor.Notify();
+  return IPC_OK();
+}
+
+int CamerasChild::AllocateCaptureDevice(CaptureEngine aCapEngine,
+                                        const char* unique_idUTF8,
+                                        const unsigned int unique_idUTF8Length,
+                                        int& aStreamId, uint64_t aWindowID) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  nsCString unique_id(unique_idUTF8);
+  nsCOMPtr<nsIRunnable> runnable =
+      mozilla::NewRunnableMethod<CaptureEngine, nsCString, const uint64_t&>(
+          "camera::PCamerasChild::SendAllocateCaptureDevice", this,
+          &CamerasChild::SendAllocateCaptureDevice, aCapEngine, unique_id,
+          aWindowID);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, -1, mZero);
+  if (dispatcher.Success()) {
+    LOG(("Capture Device allocated: %d", mReplyInteger));
+    aStreamId = mReplyInteger;
+  }
+  return dispatcher.ReturnValue();
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvReplyAllocateCaptureDevice(
+    const int& numdev) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  MonitorAutoLock monitor(mReplyMonitor);
+  mReceivedReply = true;
+  mReplySuccess = true;
+  mReplyInteger = numdev;
+  monitor.Notify();
+  return IPC_OK();
+}
+
+int CamerasChild::ReleaseCaptureDevice(CaptureEngine aCapEngine,
+                                       const int capture_id) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  nsCOMPtr<nsIRunnable> runnable =
+      mozilla::NewRunnableMethod<CaptureEngine, int>(
+          "camera::PCamerasChild::SendReleaseCaptureDevice", this,
+          &CamerasChild::SendReleaseCaptureDevice, aCapEngine, capture_id);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, -1, mZero);
+  return dispatcher.ReturnValue();
+}
+
+void CamerasChild::AddCallback(const CaptureEngine aCapEngine,
+                               const int capture_id, FrameRelay* render) {
+  MutexAutoLock lock(mCallbackMutex);
+  CapturerElement ce;
+  ce.engine = aCapEngine;
+  ce.id = capture_id;
+  ce.callback = render;
+  mCallbacks.AppendElement(ce);
+}
+
+void CamerasChild::RemoveCallback(const CaptureEngine aCapEngine,
+                                  const int capture_id) {
+  MutexAutoLock lock(mCallbackMutex);
+  for (unsigned int i = 0; i < mCallbacks.Length(); i++) {
+    CapturerElement ce = mCallbacks[i];
+    if (ce.engine == aCapEngine && ce.id == capture_id) {
+      mCallbacks.RemoveElementAt(i);
+      break;
+    }
+  }
+}
+
+int CamerasChild::StartCapture(CaptureEngine aCapEngine, const int capture_id,
+                               webrtc::VideoCaptureCapability& webrtcCaps,
+                               FrameRelay* cb) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  AddCallback(aCapEngine, capture_id, cb);
+  VideoCaptureCapability capCap(
+      webrtcCaps.width, webrtcCaps.height, webrtcCaps.maxFPS,
+      static_cast<int>(webrtcCaps.videoType), webrtcCaps.interlaced);
+  nsCOMPtr<nsIRunnable> runnable =
+      mozilla::NewRunnableMethod<CaptureEngine, int, VideoCaptureCapability>(
+          "camera::PCamerasChild::SendStartCapture", this,
+          &CamerasChild::SendStartCapture, aCapEngine, capture_id, capCap);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, -1, mZero);
+  return dispatcher.ReturnValue();
+}
+
+int CamerasChild::FocusOnSelectedSource(CaptureEngine aCapEngine,
+                                        const int aCaptureId) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  nsCOMPtr<nsIRunnable> runnable =
+      mozilla::NewRunnableMethod<CaptureEngine, int>(
+          "camera::PCamerasChild::SendFocusOnSelectedSource", this,
+          &CamerasChild::SendFocusOnSelectedSource, aCapEngine, aCaptureId);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, -1, mZero);
+  return dispatcher.ReturnValue();
+}
+
+int CamerasChild::StopCapture(CaptureEngine aCapEngine, const int capture_id) {
+  LOG(("%s", __PRETTY_FUNCTION__));
+  nsCOMPtr<nsIRunnable> runnable =
+      mozilla::NewRunnableMethod<CaptureEngine, int>(
+          "camera::PCamerasChild::SendStopCapture", this,
+          &CamerasChild::SendStopCapture, aCapEngine, capture_id);
+  LockAndDispatch<> dispatcher(this, __func__, runnable, -1, mZero);
+  if (dispatcher.Success()) {
+    RemoveCallback(aCapEngine, capture_id);
+  }
+  return dispatcher.ReturnValue();
+}
+
+void Shutdown(void) {
+  OffTheBooksMutexAutoLock lock(CamerasSingleton::Mutex());
+
+  CamerasSingleton::StartShutdown();
+
+  CamerasChild* child = CamerasSingleton::Child();
+  if (!child) {
+    // We don't want to cause everything to get fired up if we're
+    // really already shut down.
+    LOG(("Shutdown when already shut down"));
+    return;
+  }
+  child->ShutdownAll();
+}
+
+class ShutdownRunnable : public Runnable {
+ public:
+  explicit ShutdownRunnable(already_AddRefed<Runnable>&& aReplyEvent)
+      : Runnable("camera::ShutdownRunnable"), mReplyEvent(aReplyEvent){};
+
+  NS_IMETHOD Run() override {
+    LOG(("Closing BackgroundChild"));
+    ipc::BackgroundChild::CloseForCurrentThread();
+
+    NS_DispatchToMainThread(mReplyEvent.forget());
+
+    return NS_OK;
+  }
+
+ private:
+  RefPtr<Runnable> mReplyEvent;
+};
+
+void CamerasChild::ShutdownAll() {
+  // Called with CamerasSingleton::Mutex() held
+  ShutdownParent();
+  ShutdownChild();
+}
+
+void CamerasChild::ShutdownParent() {
+  // Called with CamerasSingleton::Mutex() held
+  {
+    MonitorAutoLock monitor(mReplyMonitor);
+    mIPCIsAlive = false;
+    monitor.NotifyAll();
+  }
+  if (CamerasSingleton::Thread()) {
+    LOG(("Dispatching actor deletion"));
+    // Delete the parent actor.
+    // CamerasChild (this) will remain alive and is only deleted by the
+    // IPC layer when SendAllDone returns.
+    nsCOMPtr<nsIRunnable> deleteRunnable = mozilla::NewRunnableMethod(
+        "camera::PCamerasChild::SendAllDone", this, &CamerasChild::SendAllDone);
+    CamerasSingleton::Thread()->Dispatch(deleteRunnable, NS_DISPATCH_NORMAL);
+  } else {
+    LOG(("ShutdownParent called without PBackground thread"));
+  }
+}
+
+void CamerasChild::ShutdownChild() {
+  // Called with CamerasSingleton::Mutex() held
+  if (CamerasSingleton::Thread()) {
+    LOG(("PBackground thread exists, dispatching close"));
+    // Dispatch closing the IPC thread back to us when the
+    // BackgroundChild is closed.
+    RefPtr<ShutdownRunnable> runnable = new ShutdownRunnable(
+        NewRunnableMethod("nsIThread::Shutdown", CamerasSingleton::Thread(),
+                          &nsIThread::Shutdown));
+    CamerasSingleton::Thread()->Dispatch(runnable.forget(), NS_DISPATCH_NORMAL);
+  } else {
+    LOG(("Shutdown called without PBackground thread"));
+  }
+  LOG(("Erasing sCameras & thread refs (original thread)"));
+  CamerasSingleton::Child() = nullptr;
+  CamerasSingleton::Thread() = nullptr;
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvDeliverFrame(
+    const CaptureEngine& capEngine, const int& capId,
+    mozilla::ipc::Shmem&& shmem, const VideoFrameProperties& prop) {
+  MutexAutoLock lock(mCallbackMutex);
+  if (Callback(capEngine, capId)) {
+    unsigned char* image = shmem.get<unsigned char>();
+    Callback(capEngine, capId)->DeliverFrame(image, prop);
+  } else {
+    LOG(("DeliverFrame called with dead callback"));
+  }
+  SendReleaseFrame(std::move(shmem));
+  return IPC_OK();
+}
+
+mozilla::ipc::IPCResult CamerasChild::RecvDeviceChange() {
+  mDeviceListChangeEvent.Notify();
+  return IPC_OK();
+}
+
+void CamerasChild::ActorDestroy(ActorDestroyReason aWhy) {
+  MonitorAutoLock monitor(mReplyMonitor);
+  mIPCIsAlive = false;
+  // Hopefully prevent us from getting stuck
+  // on replies that'll never come.
+  monitor.NotifyAll();
+}
+
+CamerasChild::CamerasChild()
+    : mCallbackMutex("mozilla::cameras::CamerasChild::mCallbackMutex"),
+      mIPCIsAlive(true),
+      mRequestMutex("mozilla::cameras::CamerasChild::mRequestMutex"),
+      mReplyMonitor("mozilla::cameras::CamerasChild::mReplyMonitor"),
+      mReceivedReply(false),
+      mReplySuccess(false),
+      mZero(0),
+      mReplyInteger(0),
+      mReplyScary(false) {
+  LOG(("CamerasChild: %p", this));
+
+  MOZ_COUNT_CTOR(CamerasChild);
+}
+
+CamerasChild::~CamerasChild() {
+  LOG(("~CamerasChild: %p", this));
+
+  if (!CamerasSingleton::InShutdown()) {
+    OffTheBooksMutexAutoLock lock(CamerasSingleton::Mutex());
+    // In normal circumstances we've already shut down and the
+    // following does nothing. But on fatal IPC errors we will
+    // get destructed immediately, and should not try to reach
+    // the parent.
+    ShutdownChild();
+  }
+
+  MOZ_COUNT_DTOR(CamerasChild);
+}
+
+FrameRelay* CamerasChild::Callback(CaptureEngine aCapEngine, int capture_id) {
+  for (unsigned int i = 0; i < mCallbacks.Length(); i++) {
+    CapturerElement ce = mCallbacks[i];
+    if (ce.engine == aCapEngine && ce.id == capture_id) {
+      return ce.callback;
+    }
+  }
+
+  return nullptr;
+}
+
+}  // namespace mozilla::camera