1 files changed, 713 insertions, 0 deletions
diff --git a/widget/windows/RemoteBackbuffer.cpp b/widget/windows/RemoteBackbuffer.cpp
new file mode 100644
index 0000000000..56a8bae0fe
--- /dev/null
+++ b/widget/windows/RemoteBackbuffer.cpp
@@ -0,0 +1,713 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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 "RemoteBackbuffer.h"
+#include "GeckoProfiler.h"
+#include "nsThreadUtils.h"
+#include "mozilla/Span.h"
+#include "mozilla/gfx/Point.h"
+#include "WinUtils.h"
+#include <algorithm>
+#include <type_traits>
+
+namespace mozilla {
+namespace widget {
+namespace remote_backbuffer {
+
+// This number can be adjusted as a time-memory tradeoff
+constexpr uint8_t kMaxDirtyRects = 8;
+
+struct IpcSafeRect {
+  explicit IpcSafeRect(const gfx::IntRect& aRect)
+      : x(aRect.x), y(aRect.y), width(aRect.width), height(aRect.height) {}
+  int32_t x;
+  int32_t y;
+  int32_t width;
+  int32_t height;
+};
+
+enum class ResponseResult {
+  Unknown,
+  Error,
+  BorrowSuccess,
+  BorrowSameBuffer,
+  PresentSuccess
+};
+
+enum class SharedDataType {
+  BorrowRequest,
+  BorrowRequestAllowSameBuffer,
+  BorrowResponse,
+  PresentRequest,
+  PresentResponse
+};
+
+struct BorrowResponseData {
+  ResponseResult result;
+  int32_t width;
+  int32_t height;
+  HANDLE fileMapping;
+};
+
+struct PresentRequestData {
+  uint8_t lenDirtyRects;
+  IpcSafeRect dirtyRects[kMaxDirtyRects];
+};
+
+struct PresentResponseData {
+  ResponseResult result;
+};
+
+struct SharedData {
+  SharedDataType dataType;
+  union {
+    BorrowResponseData borrowResponse;
+    PresentRequestData presentRequest;
+    PresentResponseData presentResponse;
+  } data;
+};
+
+static_assert(std::is_trivially_copyable<SharedData>::value &&
+                  std::is_standard_layout<SharedData>::value,
+              "SharedData must be safe to pass over IPC boundaries");
+
+class SharedImage {
+ public:
+  SharedImage()
+      : mWidth(0), mHeight(0), mFileMapping(nullptr), mPixelData(nullptr) {}
+
+  ~SharedImage() {
+    if (mPixelData) {
+      MOZ_ALWAYS_TRUE(::UnmapViewOfFile(mPixelData));
+    }
+
+    if (mFileMapping) {
+      MOZ_ALWAYS_TRUE(::CloseHandle(mFileMapping));
+    }
+  }
+
+  bool Initialize(int32_t aWidth, int32_t aHeight) {
+    MOZ_ASSERT(aWidth > 0);
+    MOZ_ASSERT(aHeight > 0);
+
+    mWidth = aWidth;
+    mHeight = aHeight;
+
+    DWORD bufferSize = static_cast<DWORD>(mHeight * GetStride());
+
+    mFileMapping = ::CreateFileMappingW(
+        INVALID_HANDLE_VALUE, nullptr /*secattr*/, PAGE_READWRITE,
+        0 /*sizeHigh*/, bufferSize, nullptr /*name*/);
+    if (!mFileMapping) {
+      return false;
+    }
+
+    void* mappedFilePtr =
+        ::MapViewOfFile(mFileMapping, FILE_MAP_ALL_ACCESS, 0 /*offsetHigh*/,
+                        0 /*offsetLow*/, 0 /*bytesToMap*/);
+    if (!mappedFilePtr) {
+      return false;
+    }
+
+    mPixelData = reinterpret_cast<unsigned char*>(mappedFilePtr);
+
+    return true;
+  }
+
+  bool InitializeRemote(int32_t aWidth, int32_t aHeight, HANDLE aFileMapping) {
+    MOZ_ASSERT(aWidth > 0);
+    MOZ_ASSERT(aHeight > 0);
+    MOZ_ASSERT(aFileMapping);
+
+    mWidth = aWidth;
+    mHeight = aHeight;
+    mFileMapping = aFileMapping;
+
+    void* mappedFilePtr =
+        ::MapViewOfFile(mFileMapping, FILE_MAP_ALL_ACCESS, 0 /*offsetHigh*/,
+                        0 /*offsetLow*/, 0 /*bytesToMap*/);
+    if (!mappedFilePtr) {
+      return false;
+    }
+
+    mPixelData = reinterpret_cast<unsigned char*>(mappedFilePtr);
+
+    return true;
+  }
+
+  HBITMAP CreateDIBSection() {
+    BITMAPINFO bitmapInfo = {};
+    bitmapInfo.bmiHeader.biSize = sizeof(bitmapInfo.bmiHeader);
+    bitmapInfo.bmiHeader.biWidth = mWidth;
+    bitmapInfo.bmiHeader.biHeight = -mHeight;
+    bitmapInfo.bmiHeader.biPlanes = 1;
+    bitmapInfo.bmiHeader.biBitCount = 32;
+    bitmapInfo.bmiHeader.biCompression = BI_RGB;
+    void* dummy = nullptr;
+    return ::CreateDIBSection(nullptr /*paletteDC*/, &bitmapInfo,
+                              DIB_RGB_COLORS, &dummy, mFileMapping,
+                              0 /*offset*/);
+  }
+
+  HANDLE CreateRemoteFileMapping(HANDLE aTargetProcess) {
+    MOZ_ASSERT(aTargetProcess);
+
+    HANDLE fileMapping = nullptr;
+    if (!::DuplicateHandle(GetCurrentProcess(), mFileMapping, aTargetProcess,
+                           &fileMapping, 0 /*desiredAccess*/,
+                           FALSE /*inheritHandle*/, DUPLICATE_SAME_ACCESS)) {
+      return nullptr;
+    }
+    return fileMapping;
+  }
+
+  already_AddRefed<gfx::DrawTarget> CreateDrawTarget() {
+    return gfx::Factory::CreateDrawTargetForData(
+        gfx::BackendType::CAIRO, mPixelData, gfx::IntSize(mWidth, mHeight),
+        GetStride(), gfx::SurfaceFormat::B8G8R8A8);
+  }
+
+  void CopyPixelsFrom(const SharedImage& other) {
+    const unsigned char* src = other.mPixelData;
+    unsigned char* dst = mPixelData;
+
+    int32_t width = std::min(mWidth, other.mWidth);
+    int32_t height = std::min(mHeight, other.mHeight);
+
+    for (int32_t row = 0; row < height; ++row) {
+      memcpy(dst, src, static_cast<uint32_t>(width * kBytesPerPixel));
+      src += other.GetStride();
+      dst += GetStride();
+    }
+  }
+
+  int32_t GetWidth() { return mWidth; }
+
+  int32_t GetHeight() { return mHeight; }
+
+  SharedImage(const SharedImage&) = delete;
+  SharedImage(SharedImage&&) = delete;
+  SharedImage& operator=(const SharedImage&) = delete;
+  SharedImage& operator=(SharedImage&&) = delete;
+
+ private:
+  static constexpr int32_t kBytesPerPixel = 4;
+
+  int32_t GetStride() const {
+    // DIB requires 32-bit row alignment
+    return (((mWidth * kBytesPerPixel) + 3) / 4) * 4;
+  }
+
+  int32_t mWidth;
+  int32_t mHeight;
+  HANDLE mFileMapping;
+  unsigned char* mPixelData;
+};
+
+class PresentableSharedImage {
+ public:
+  PresentableSharedImage()
+      : mSharedImage(),
+        mDeviceContext(nullptr),
+        mDIBSection(nullptr),
+        mSavedObject(nullptr) {}
+
+  ~PresentableSharedImage() {
+    if (mSavedObject) {
+      MOZ_ALWAYS_TRUE(::SelectObject(mDeviceContext, mSavedObject));
+    }
+
+    if (mDIBSection) {
+      MOZ_ALWAYS_TRUE(::DeleteObject(mDIBSection));
+    }
+
+    if (mDeviceContext) {
+      MOZ_ALWAYS_TRUE(::DeleteDC(mDeviceContext));
+    }
+  }
+
+  bool Initialize(int32_t aWidth, int32_t aHeight) {
+    if (!mSharedImage.Initialize(aWidth, aHeight)) {
+      return false;
+    }
+
+    mDeviceContext = ::CreateCompatibleDC(nullptr);
+    if (!mDeviceContext) {
+      return false;
+    }
+
+    mDIBSection = mSharedImage.CreateDIBSection();
+    if (!mDIBSection) {
+      return false;
+    }
+
+    mSavedObject = ::SelectObject(mDeviceContext, mDIBSection);
+    if (!mSavedObject) {
+      return false;
+    }
+
+    return true;
+  }
+
+  bool PresentToWindow(HWND aWindowHandle, TransparencyMode aTransparencyMode,
+                       Span<const IpcSafeRect> aDirtyRects) {
+    if (aTransparencyMode == TransparencyMode::Transparent) {
+      // If our window is a child window or a child-of-a-child, the window
+      // that needs to be updated is the top level ancestor of the tree
+      HWND topLevelWindow = WinUtils::GetTopLevelHWND(aWindowHandle, true);
+      MOZ_ASSERT(::GetWindowLongPtr(topLevelWindow, GWL_EXSTYLE) &
+                 WS_EX_LAYERED);
+
+      BLENDFUNCTION bf = {AC_SRC_OVER, 0, 255, AC_SRC_ALPHA};
+      POINT srcPos = {0, 0};
+      RECT clientRect = {};
+      if (!::GetClientRect(aWindowHandle, &clientRect)) {
+        return false;
+      }
+      MOZ_ASSERT(clientRect.left == 0);
+      MOZ_ASSERT(clientRect.top == 0);
+      int32_t width = clientRect.right;
+      int32_t height = clientRect.bottom;
+      SIZE winSize = {width, height};
+      // Window resize could cause the client area to be different than
+      // mSharedImage's size. If the client area doesn't match,
+      // PresentToWindow() returns false without calling UpdateLayeredWindow().
+      // Another call to UpdateLayeredWindow() will follow shortly, since the
+      // resize will eventually force the backbuffer to repaint itself again.
+      // When client area is larger than mSharedImage's size,
+      // UpdateLayeredWindow() draws the window completely invisible. But it
+      // does not return false.
+      if (width != mSharedImage.GetWidth() ||
+          height != mSharedImage.GetHeight()) {
+        return false;
+      }
+
+      return !!::UpdateLayeredWindow(
+          topLevelWindow, nullptr /*paletteDC*/, nullptr /*newPos*/, &winSize,
+          mDeviceContext, &srcPos, 0 /*colorKey*/, &bf, ULW_ALPHA);
+    }
+
+    gfx::IntRect sharedImageRect{0, 0, mSharedImage.GetWidth(),
+                                 mSharedImage.GetHeight()};
+
+    bool result = true;
+
+    HDC windowDC = ::GetDC(aWindowHandle);
+    if (!windowDC) {
+      return false;
+    }
+
+    for (auto& ipcDirtyRect : aDirtyRects) {
+      gfx::IntRect dirtyRect{ipcDirtyRect.x, ipcDirtyRect.y, ipcDirtyRect.width,
+                             ipcDirtyRect.height};
+      gfx::IntRect bltRect = dirtyRect.Intersect(sharedImageRect);
+
+      if (!::BitBlt(windowDC, bltRect.x /*dstX*/, bltRect.y /*dstY*/,
+                    bltRect.width, bltRect.height, mDeviceContext,
+                    bltRect.x /*srcX*/, bltRect.y /*srcY*/, SRCCOPY)) {
+        result = false;
+        break;
+      }
+    }
+
+    MOZ_ALWAYS_TRUE(::ReleaseDC(aWindowHandle, windowDC));
+
+    return result;
+  }
+
+  HANDLE CreateRemoteFileMapping(HANDLE aTargetProcess) {
+    return mSharedImage.CreateRemoteFileMapping(aTargetProcess);
+  }
+
+  already_AddRefed<gfx::DrawTarget> CreateDrawTarget() {
+    return mSharedImage.CreateDrawTarget();
+  }
+
+  void CopyPixelsFrom(const PresentableSharedImage& other) {
+    mSharedImage.CopyPixelsFrom(other.mSharedImage);
+  }
+
+  int32_t GetWidth() { return mSharedImage.GetWidth(); }
+
+  int32_t GetHeight() { return mSharedImage.GetHeight(); }
+
+  PresentableSharedImage(const PresentableSharedImage&) = delete;
+  PresentableSharedImage(PresentableSharedImage&&) = delete;
+  PresentableSharedImage& operator=(const PresentableSharedImage&) = delete;
+  PresentableSharedImage& operator=(PresentableSharedImage&&) = delete;
+
+ private:
+  SharedImage mSharedImage;
+  HDC mDeviceContext;
+  HBITMAP mDIBSection;
+  HGDIOBJ mSavedObject;
+};
+
+Provider::Provider()
+    : mWindowHandle(nullptr),
+      mTargetProcess(nullptr),
+      mFileMapping(nullptr),
+      mRequestReadyEvent(nullptr),
+      mResponseReadyEvent(nullptr),
+      mSharedDataPtr(nullptr),
+      mStopServiceThread(false),
+      mServiceThread(nullptr),
+      mBackbuffer() {}
+
+Provider::~Provider() {
+  mBackbuffer.reset();
+
+  if (mServiceThread) {
+    mStopServiceThread = true;
+    MOZ_ALWAYS_TRUE(::SetEvent(mRequestReadyEvent));
+    MOZ_ALWAYS_TRUE(PR_JoinThread(mServiceThread) == PR_SUCCESS);
+  }
+
+  if (mSharedDataPtr) {
+    MOZ_ALWAYS_TRUE(::UnmapViewOfFile(mSharedDataPtr));
+  }
+
+  if (mResponseReadyEvent) {
+    MOZ_ALWAYS_TRUE(::CloseHandle(mResponseReadyEvent));
+  }
+
+  if (mRequestReadyEvent) {
+    MOZ_ALWAYS_TRUE(::CloseHandle(mRequestReadyEvent));
+  }
+
+  if (mFileMapping) {
+    MOZ_ALWAYS_TRUE(::CloseHandle(mFileMapping));
+  }
+
+  if (mTargetProcess) {
+    MOZ_ALWAYS_TRUE(::CloseHandle(mTargetProcess));
+  }
+}
+
+bool Provider::Initialize(HWND aWindowHandle, DWORD aTargetProcessId,
+                          TransparencyMode aTransparencyMode) {
+  MOZ_ASSERT(aWindowHandle);
+  MOZ_ASSERT(aTargetProcessId);
+
+  mWindowHandle = aWindowHandle;
+
+  mTargetProcess = ::OpenProcess(PROCESS_DUP_HANDLE, FALSE /*inheritHandle*/,
+                                 aTargetProcessId);
+  if (!mTargetProcess) {
+    return false;
+  }
+
+  mFileMapping = ::CreateFileMappingW(
+      INVALID_HANDLE_VALUE, nullptr /*secattr*/, PAGE_READWRITE, 0 /*sizeHigh*/,
+      static_cast<DWORD>(sizeof(SharedData)), nullptr /*name*/);
+  if (!mFileMapping) {
+    return false;
+  }
+
+  mRequestReadyEvent =
+      ::CreateEventW(nullptr /*secattr*/, FALSE /*manualReset*/,
+                     FALSE /*initialState*/, nullptr /*name*/);
+  if (!mRequestReadyEvent) {
+    return false;
+  }
+
+  mResponseReadyEvent =
+      ::CreateEventW(nullptr /*secattr*/, FALSE /*manualReset*/,
+                     FALSE /*initialState*/, nullptr /*name*/);
+  if (!mResponseReadyEvent) {
+    return false;
+  }
+
+  void* mappedFilePtr =
+      ::MapViewOfFile(mFileMapping, FILE_MAP_ALL_ACCESS, 0 /*offsetHigh*/,
+                      0 /*offsetLow*/, 0 /*bytesToMap*/);
+  if (!mappedFilePtr) {
+    return false;
+  }
+
+  mSharedDataPtr = reinterpret_cast<SharedData*>(mappedFilePtr);
+
+  mStopServiceThread = false;
+
+  // Use a raw NSPR OS-level thread here instead of nsThread because we are
+  // performing low-level synchronization across processes using Win32 Events,
+  // and nsThread is designed around an incompatible "in-process task queue"
+  // model
+  mServiceThread = PR_CreateThread(
+      PR_USER_THREAD, [](void* p) { static_cast<Provider*>(p)->ThreadMain(); },
+      this, PR_PRIORITY_NORMAL, PR_GLOBAL_THREAD, PR_JOINABLE_THREAD,
+      0 /*default stack size*/);
+  if (!mServiceThread) {
+    return false;
+  }
+
+  mTransparencyMode = uint32_t(aTransparencyMode);
+
+  return true;
+}
+
+Maybe<RemoteBackbufferHandles> Provider::CreateRemoteHandles() {
+  return Some(
+      RemoteBackbufferHandles(ipc::FileDescriptor(mFileMapping),
+                              ipc::FileDescriptor(mRequestReadyEvent),
+                              ipc::FileDescriptor(mResponseReadyEvent)));
+}
+
+void Provider::UpdateTransparencyMode(TransparencyMode aTransparencyMode) {
+  mTransparencyMode = uint32_t(aTransparencyMode);
+}
+
+void Provider::ThreadMain() {
+  AUTO_PROFILER_REGISTER_THREAD("RemoteBackbuffer");
+  NS_SetCurrentThreadName("RemoteBackbuffer");
+
+  while (true) {
+    {
+      AUTO_PROFILER_THREAD_SLEEP;
+      MOZ_ALWAYS_TRUE(::WaitForSingleObject(mRequestReadyEvent, INFINITE) ==
+                      WAIT_OBJECT_0);
+    }
+
+    if (mStopServiceThread) {
+      break;
+    }
+
+    switch (mSharedDataPtr->dataType) {
+      case SharedDataType::BorrowRequest:
+      case SharedDataType::BorrowRequestAllowSameBuffer: {
+        BorrowResponseData responseData = {};
+
+        HandleBorrowRequest(&responseData,
+                            mSharedDataPtr->dataType ==
+                                SharedDataType::BorrowRequestAllowSameBuffer);
+
+        mSharedDataPtr->dataType = SharedDataType::BorrowResponse;
+        mSharedDataPtr->data.borrowResponse = responseData;
+
+        MOZ_ALWAYS_TRUE(::SetEvent(mResponseReadyEvent));
+
+        break;
+      }
+      case SharedDataType::PresentRequest: {
+        PresentRequestData requestData = mSharedDataPtr->data.presentRequest;
+        PresentResponseData responseData = {};
+
+        HandlePresentRequest(requestData, &responseData);
+
+        mSharedDataPtr->dataType = SharedDataType::PresentResponse;
+        mSharedDataPtr->data.presentResponse = responseData;
+
+        MOZ_ALWAYS_TRUE(::SetEvent(mResponseReadyEvent));
+
+        break;
+      }
+      default:
+        break;
+    };
+  }
+}
+
+void Provider::HandleBorrowRequest(BorrowResponseData* aResponseData,
+                                   bool aAllowSameBuffer) {
+  MOZ_ASSERT(aResponseData);
+
+  aResponseData->result = ResponseResult::Error;
+
+  RECT clientRect = {};
+  if (!::GetClientRect(mWindowHandle, &clientRect)) {
+    return;
+  }
+
+  MOZ_ASSERT(clientRect.left == 0);
+  MOZ_ASSERT(clientRect.top == 0);
+
+  int32_t width = clientRect.right ? clientRect.right : 1;
+  int32_t height = clientRect.bottom ? clientRect.bottom : 1;
+
+  bool needNewBackbuffer = !aAllowSameBuffer || !mBackbuffer ||
+                           (mBackbuffer->GetWidth() != width) ||
+                           (mBackbuffer->GetHeight() != height);
+
+  if (!needNewBackbuffer) {
+    aResponseData->result = ResponseResult::BorrowSameBuffer;
+    return;
+  }
+
+  auto newBackbuffer = std::make_unique<PresentableSharedImage>();
+  if (!newBackbuffer->Initialize(width, height)) {
+    return;
+  }
+
+  // Preserve the contents of the old backbuffer (if it exists)
+  if (mBackbuffer) {
+    newBackbuffer->CopyPixelsFrom(*mBackbuffer);
+    mBackbuffer.reset();
+  }
+
+  HANDLE remoteFileMapping =
+      newBackbuffer->CreateRemoteFileMapping(mTargetProcess);
+  if (!remoteFileMapping) {
+    return;
+  }
+
+  aResponseData->result = ResponseResult::BorrowSuccess;
+  aResponseData->width = width;
+  aResponseData->height = height;
+  aResponseData->fileMapping = remoteFileMapping;
+
+  mBackbuffer = std::move(newBackbuffer);
+}
+
+void Provider::HandlePresentRequest(const PresentRequestData& aRequestData,
+                                    PresentResponseData* aResponseData) {
+  MOZ_ASSERT(aResponseData);
+
+  Span rectSpan(aRequestData.dirtyRects, kMaxDirtyRects);
+
+  aResponseData->result = ResponseResult::Error;
+
+  if (!mBackbuffer) {
+    return;
+  }
+
+  if (!mBackbuffer->PresentToWindow(
+          mWindowHandle, GetTransparencyMode(),
+          rectSpan.First(aRequestData.lenDirtyRects))) {
+    return;
+  }
+
+  aResponseData->result = ResponseResult::PresentSuccess;
+}
+
+Client::Client()
+    : mFileMapping(nullptr),
+      mRequestReadyEvent(nullptr),
+      mResponseReadyEvent(nullptr),
+      mSharedDataPtr(nullptr),
+      mBackbuffer() {}
+
+Client::~Client() {
+  mBackbuffer.reset();
+
+  if (mSharedDataPtr) {
+    MOZ_ALWAYS_TRUE(::UnmapViewOfFile(mSharedDataPtr));
+  }
+
+  if (mResponseReadyEvent) {
+    MOZ_ALWAYS_TRUE(::CloseHandle(mResponseReadyEvent));
+  }
+
+  if (mRequestReadyEvent) {
+    MOZ_ALWAYS_TRUE(::CloseHandle(mRequestReadyEvent));
+  }
+
+  if (mFileMapping) {
+    MOZ_ALWAYS_TRUE(::CloseHandle(mFileMapping));
+  }
+}
+
+bool Client::Initialize(const RemoteBackbufferHandles& aRemoteHandles) {
+  MOZ_ASSERT(aRemoteHandles.fileMapping().IsValid());
+  MOZ_ASSERT(aRemoteHandles.requestReadyEvent().IsValid());
+  MOZ_ASSERT(aRemoteHandles.responseReadyEvent().IsValid());
+
+  // FIXME: Due to PCompositorWidget using virtual Recv methods,
+  // RemoteBackbufferHandles is passed by const reference, and cannot have its
+  // signature customized, meaning that we need to clone the handles here.
+  //
+  // Once PCompositorWidget is migrated to use direct call semantics, the
+  // signature can be changed to accept RemoteBackbufferHandles by rvalue
+  // reference or value, and the DuplicateHandle calls here can be avoided.
+  mFileMapping = aRemoteHandles.fileMapping().ClonePlatformHandle().release();
+  mRequestReadyEvent =
+      aRemoteHandles.requestReadyEvent().ClonePlatformHandle().release();
+  mResponseReadyEvent =
+      aRemoteHandles.responseReadyEvent().ClonePlatformHandle().release();
+
+  void* mappedFilePtr =
+      ::MapViewOfFile(mFileMapping, FILE_MAP_ALL_ACCESS, 0 /*offsetHigh*/,
+                      0 /*offsetLow*/, 0 /*bytesToMap*/);
+  if (!mappedFilePtr) {
+    return false;
+  }
+
+  mSharedDataPtr = reinterpret_cast<SharedData*>(mappedFilePtr);
+
+  return true;
+}
+
+already_AddRefed<gfx::DrawTarget> Client::BorrowDrawTarget() {
+  mSharedDataPtr->dataType = mBackbuffer
+                                 ? SharedDataType::BorrowRequestAllowSameBuffer
+                                 : SharedDataType::BorrowRequest;
+
+  MOZ_ALWAYS_TRUE(::SetEvent(mRequestReadyEvent));
+  MOZ_ALWAYS_TRUE(::WaitForSingleObject(mResponseReadyEvent, INFINITE) ==
+                  WAIT_OBJECT_0);
+
+  if (mSharedDataPtr->dataType != SharedDataType::BorrowResponse) {
+    return nullptr;
+  }
+
+  BorrowResponseData responseData = mSharedDataPtr->data.borrowResponse;
+
+  if ((responseData.result != ResponseResult::BorrowSameBuffer) &&
+      (responseData.result != ResponseResult::BorrowSuccess)) {
+    return nullptr;
+  }
+
+  if (responseData.result == ResponseResult::BorrowSuccess) {
+    mBackbuffer.reset();
+
+    auto newBackbuffer = std::make_unique<SharedImage>();
+    if (!newBackbuffer->InitializeRemote(responseData.width,
+                                         responseData.height,
+                                         responseData.fileMapping)) {
+      return nullptr;
+    }
+
+    mBackbuffer = std::move(newBackbuffer);
+  }
+
+  MOZ_ASSERT(mBackbuffer);
+
+  return mBackbuffer->CreateDrawTarget();
+}
+
+bool Client::PresentDrawTarget(gfx::IntRegion aDirtyRegion) {
+  mSharedDataPtr->dataType = SharedDataType::PresentRequest;
+
+  // Simplify the region until it has <= kMaxDirtyRects
+  aDirtyRegion.SimplifyOutward(kMaxDirtyRects);
+
+  Span rectSpan(mSharedDataPtr->data.presentRequest.dirtyRects, kMaxDirtyRects);
+
+  uint8_t rectIndex = 0;
+  for (auto iter = aDirtyRegion.RectIter(); !iter.Done(); iter.Next()) {
+    rectSpan[rectIndex] = IpcSafeRect(iter.Get());
+    ++rectIndex;
+  }
+
+  mSharedDataPtr->data.presentRequest.lenDirtyRects = rectIndex;
+
+  MOZ_ALWAYS_TRUE(::SetEvent(mRequestReadyEvent));
+  MOZ_ALWAYS_TRUE(::WaitForSingleObject(mResponseReadyEvent, INFINITE) ==
+                  WAIT_OBJECT_0);
+
+  if (mSharedDataPtr->dataType != SharedDataType::PresentResponse) {
+    return false;
+  }
+
+  if (mSharedDataPtr->data.presentResponse.result !=
+      ResponseResult::PresentSuccess) {
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace remote_backbuffer
+}  // namespace widget
+}  // namespace mozilla