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diff --git a/gfx/layers/SourceSurfaceSharedData.h b/gfx/layers/SourceSurfaceSharedData.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef MOZILLA_GFX_SOURCESURFACESHAREDDATA_H_
+#define MOZILLA_GFX_SOURCESURFACESHAREDDATA_H_
+
+#include "mozilla/gfx/2D.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/ipc/SharedMemoryBasic.h"
+#include "nsExpirationTracker.h"
+
+namespace mozilla {
+namespace gfx {
+
+class SourceSurfaceSharedData;
+
+/**
+ * This class is used to wrap shared (as in process) data buffers allocated by
+ * a SourceSurfaceSharedData object. It may live in the same process or a
+ * different process from the actual SourceSurfaceSharedData object.
+ *
+ * If it is in the same process, mBuf is the same object as that in the surface.
+ * It is a useful abstraction over just using the surface directly, because it
+ * can have a different lifetime from the surface; if the surface gets freed,
+ * consumers may continue accessing the data in the buffer. Releasing the
+ * original surface is a signal which feeds into SharedSurfacesParent to decide
+ * to release the SourceSurfaceSharedDataWrapper.
+ *
+ * If it is in a different process, mBuf is a new SharedMemoryBasic object which
+ * mapped in the given shared memory handle as read only memory.
+ */
+class SourceSurfaceSharedDataWrapper final : public DataSourceSurface {
+  typedef mozilla::ipc::SharedMemoryBasic SharedMemoryBasic;
+
+ public:
+  MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(SourceSurfaceSharedDataWrapper,
+                                          override)
+
+  SourceSurfaceSharedDataWrapper()
+      : mStride(0),
+        mConsumers(0),
+        mFormat(SurfaceFormat::UNKNOWN),
+        mCreatorPid(0),
+        mCreatorRef(true) {}
+
+  void Init(const IntSize& aSize, int32_t aStride, SurfaceFormat aFormat,
+            SharedMemoryBasic::Handle aHandle, base::ProcessId aCreatorPid);
+
+  void Init(SourceSurfaceSharedData* aSurface);
+
+  base::ProcessId GetCreatorPid() const { return mCreatorPid; }
+
+  int32_t Stride() override { return mStride; }
+
+  SurfaceType GetType() const override {
+    return SurfaceType::DATA_SHARED_WRAPPER;
+  }
+  IntSize GetSize() const override { return mSize; }
+  SurfaceFormat GetFormat() const override { return mFormat; }
+
+  uint8_t* GetData() override { return static_cast<uint8_t*>(mBuf->memory()); }
+
+  bool OnHeap() const override { return false; }
+
+  bool Map(MapType aMapType, MappedSurface* aMappedSurface) final;
+
+  void Unmap() final;
+
+  void ExpireMap();
+
+  bool AddConsumer() { return ++mConsumers == 1; }
+
+  bool RemoveConsumer(bool aForCreator) {
+    MOZ_ASSERT(mConsumers > 0);
+    if (aForCreator) {
+      if (!mCreatorRef) {
+        MOZ_ASSERT_UNREACHABLE("Already released creator reference!");
+        return false;
+      }
+      mCreatorRef = false;
+    }
+    return --mConsumers == 0;
+  }
+
+  uint32_t GetConsumers() const {
+    MOZ_ASSERT(mConsumers > 0);
+    return mConsumers;
+  }
+
+  bool HasCreatorRef() const { return mCreatorRef; }
+
+  nsExpirationState* GetExpirationState() { return &mExpirationState; }
+
+ private:
+  size_t GetDataLength() const {
+    return static_cast<size_t>(mStride) * mSize.height;
+  }
+
+  size_t GetAlignedDataLength() const {
+    return mozilla::ipc::SharedMemory::PageAlignedSize(GetDataLength());
+  }
+
+  bool EnsureMapped(size_t aLength);
+
+  // Protects mapping and unmapping of mBuf.
+  Maybe<Mutex> mHandleLock;
+  nsExpirationState mExpirationState;
+  int32_t mStride;
+  uint32_t mConsumers;
+  IntSize mSize;
+  RefPtr<SharedMemoryBasic> mBuf;
+  SurfaceFormat mFormat;
+  base::ProcessId mCreatorPid;
+  bool mCreatorRef;
+};
+
+/**
+ * This class is used to wrap shared (as in process) data buffers used by a
+ * source surface.
+ */
+class SourceSurfaceSharedData : public DataSourceSurface {
+  typedef mozilla::ipc::SharedMemoryBasic SharedMemoryBasic;
+
+ public:
+  MOZ_DECLARE_REFCOUNTED_VIRTUAL_TYPENAME(SourceSurfaceSharedData, override)
+
+  SourceSurfaceSharedData()
+      : mMutex("SourceSurfaceSharedData"),
+        mStride(0),
+        mHandleCount(0),
+        mFormat(SurfaceFormat::UNKNOWN),
+        mClosed(false),
+        mFinalized(false),
+        mShared(false) {}
+
+  /**
+   * Initialize the surface by creating a shared memory buffer with a size
+   * determined by aSize, aStride and aFormat. If aShare is true, it will also
+   * immediately attempt to share the surface with the GPU process via
+   * SharedSurfacesChild.
+   */
+  bool Init(const IntSize& aSize, int32_t aStride, SurfaceFormat aFormat,
+            bool aShare = true);
+
+  uint8_t* GetData() final {
+    MutexAutoLock lock(mMutex);
+    return GetDataInternal();
+  }
+
+  int32_t Stride() final { return mStride; }
+
+  SurfaceType GetType() const override { return SurfaceType::DATA_SHARED; }
+  IntSize GetSize() const final { return mSize; }
+  SurfaceFormat GetFormat() const final { return mFormat; }
+
+  void SizeOfExcludingThis(MallocSizeOf aMallocSizeOf,
+                           SizeOfInfo& aInfo) const final;
+
+  bool OnHeap() const final { return false; }
+
+  /**
+   * Although Map (and Moz2D in general) isn't normally threadsafe,
+   * we want to allow it for SourceSurfaceSharedData since it should
+   * always be fine (for reading at least).
+   *
+   * This is the same as the base class implementation except using
+   * mMapCount instead of mIsMapped since that breaks for multithread.
+   *
+   * Additionally if a reallocation happened while there were active
+   * mappings, then we guarantee that GetData will continue to return
+   * the same data pointer by retaining the old shared buffer until
+   * the last mapping is freed via Unmap.
+   */
+  bool Map(MapType aMapType, MappedSurface* aMappedSurface) final {
+    MutexAutoLock lock(mMutex);
+    if (mFinalized && aMapType != MapType::READ) {
+      // Once finalized the data may be write-protected
+      return false;
+    }
+    ++mMapCount;
+    aMappedSurface->mData = GetDataInternal();
+    aMappedSurface->mStride = mStride;
+    return true;
+  }
+
+  void Unmap() final {
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(mMapCount > 0);
+    if (--mMapCount == 0) {
+      mOldBuf = nullptr;
+    }
+  }
+
+  /**
+   * Get a handle to share to another process for this buffer. Returns:
+   *   NS_OK -- success, aHandle is valid.
+   *   NS_ERROR_NOT_AVAILABLE -- handle was closed, need to reallocate.
+   *   NS_ERROR_FAILURE -- failed to create a handle to share.
+   */
+  nsresult CloneHandle(SharedMemoryBasic::Handle& aHandle);
+
+  /**
+   * Indicates the buffer is not expected to be shared with any more processes.
+   * May release the handle if possible (see CloseHandleInternal).
+   */
+  void FinishedSharing() {
+    MutexAutoLock lock(mMutex);
+    mShared = true;
+    CloseHandleInternal();
+  }
+
+  /**
+   * Indicates whether or not the buffer can be shared with another process
+   * without reallocating. Note that this is racy and should only be used for
+   * informational/reporting purposes.
+   */
+  bool CanShare() const {
+    MutexAutoLock lock(mMutex);
+    return !mClosed;
+  }
+
+  /**
+   * Allocate a new shared memory buffer so that we can get a new handle for
+   * sharing to new processes. CloneHandle must have failed with
+   * NS_ERROR_NOT_AVAILABLE in order for this to be safe to call. Returns true
+   * if the operation succeeds. If it fails, there is no state change.
+   */
+  bool ReallocHandle();
+
+  /**
+   * Signals we have finished writing to the buffer and it may be marked as
+   * read only.
+   */
+  void Finalize();
+
+  /**
+   * Indicates whether or not the buffer can change. If this returns true, it is
+   * guaranteed to continue to do so for the remainder of the surface's life.
+   */
+  bool IsFinalized() const {
+    MutexAutoLock lock(mMutex);
+    return mFinalized;
+  }
+
+  /**
+   * Yields a dirty rect of what has changed since it was last called.
+   */
+  Maybe<IntRect> TakeDirtyRect() final {
+    MutexAutoLock lock(mMutex);
+    if (mDirtyRect) {
+      Maybe<IntRect> ret = std::move(mDirtyRect);
+      return ret;
+    }
+    return Nothing();
+  }
+
+  /**
+   * Increment the invalidation counter.
+   */
+  void Invalidate(const IntRect& aDirtyRect) final {
+    MutexAutoLock lock(mMutex);
+    if (!aDirtyRect.IsEmpty()) {
+      if (mDirtyRect) {
+        mDirtyRect->UnionRect(mDirtyRect.ref(), aDirtyRect);
+      } else {
+        mDirtyRect = Some(aDirtyRect);
+      }
+    } else {
+      mDirtyRect = Some(IntRect(IntPoint(0, 0), mSize));
+    }
+    MOZ_ASSERT_IF(mDirtyRect, !mDirtyRect->IsEmpty());
+  }
+
+  /**
+   * While a HandleLock exists for the given surface, the shared memory handle
+   * cannot be released.
+   */
+  class MOZ_STACK_CLASS HandleLock final {
+   public:
+    explicit HandleLock(SourceSurfaceSharedData* aSurface)
+        : mSurface(aSurface) {
+      mSurface->LockHandle();
+    }
+
+    ~HandleLock() { mSurface->UnlockHandle(); }
+
+   private:
+    RefPtr<SourceSurfaceSharedData> mSurface;
+  };
+
+ protected:
+  virtual ~SourceSurfaceSharedData() = default;
+
+ private:
+  friend class SourceSurfaceSharedDataWrapper;
+
+  void LockHandle() {
+    MutexAutoLock lock(mMutex);
+    ++mHandleCount;
+  }
+
+  void UnlockHandle() {
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(mHandleCount > 0);
+    --mHandleCount;
+    mShared = true;
+    CloseHandleInternal();
+  }
+
+  uint8_t* GetDataInternal() const;
+
+  size_t GetDataLength() const {
+    return static_cast<size_t>(mStride) * mSize.height;
+  }
+
+  size_t GetAlignedDataLength() const {
+    return mozilla::ipc::SharedMemory::PageAlignedSize(GetDataLength());
+  }
+
+  /**
+   * Attempt to close the handle. Only if the buffer has been both finalized
+   * and we have completed sharing will it be released.
+   */
+  void CloseHandleInternal();
+
+  mutable Mutex mMutex MOZ_UNANNOTATED;
+  int32_t mStride;
+  int32_t mHandleCount;
+  Maybe<IntRect> mDirtyRect;
+  IntSize mSize;
+  RefPtr<SharedMemoryBasic> mBuf;
+  RefPtr<SharedMemoryBasic> mOldBuf;
+  SurfaceFormat mFormat;
+  bool mClosed : 1;
+  bool mFinalized : 1;
+  bool mShared : 1;
+};
+
+}  // namespace gfx
+}  // namespace mozilla
+
+#endif /* MOZILLA_GFX_SOURCESURFACESHAREDDATA_H_ */