diff -r 000000000000 -r 6474c204b198 gfx/thebes/gfxReusableSurfaceWrapper.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/thebes/gfxReusableSurfaceWrapper.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,89 @@
+/* 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 GFXCOWSURFACEWRAPPER
+#define GFXCOWSURFACEWRAPPER
+
+#include "gfxImageSurface.h"
+#include "nsISupportsImpl.h"
+#include "nsAutoPtr.h"
+
+
+/**
+ * Provides an interface to implement a cross thread/process wrapper for a
+ * gfxImageSurface that has copy-on-write semantics.
+ *
+ * Only the owner thread can write to the surface and acquire
+ * read locks. Destroying a gfxReusableSurfaceWrapper releases
+ * a read lock.
+ *
+ * OMTC Usage:
+ * 1) Content creates a writable copy of this surface
+ *    wrapper which will be optimized to the same wrapper if there
+ *    are no readers.
+ * 2) The surface is sent from content to the compositor once
+ *    or potentially many times, each increasing a read lock.
+ * 3) When the compositor receives the surface, it adopts the
+ *    read lock.
+ * 4) Once the compositor has processed the surface and uploaded
+ *    the content, it then releases the read lock by dereferencing
+ *    its wrapper.
+ */
+class gfxReusableSurfaceWrapper {
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(gfxReusableSurfaceWrapper)
+public:
+
+  /**
+   * Returns a read-only pointer to the image data.
+   */
+  virtual const unsigned char* GetReadOnlyData() const = 0;
+
+  /**
+   * Returns the image surface format.
+   */
+  virtual gfxImageFormat Format() = 0;
+
+  /**
+   * Returns a writable copy of the image.
+   * If necessary this will copy the wrapper. If there are no contention
+   * the same wrapper will be returned. A ReadLock must be held when
+   * calling this function, and calling it will give up this lock.
+   */
+  virtual gfxReusableSurfaceWrapper* GetWritable(gfxImageSurface** aSurface) = 0;
+
+  /**
+   * A read only lock count is recorded, any attempts to
+   * call GetWritable() while this count is greater than one will
+   * create a new surface/wrapper pair.
+   *
+   * When a surface's read count falls to zero, its memory will be
+   * deallocated. It is the responsibility of the user to make sure
+   * that all locks are matched with an equal number of unlocks.
+   */
+  virtual void ReadLock() = 0;
+  virtual void ReadUnlock() = 0;
+
+  /**
+   * Types for each implementation of gfxReusableSurfaceWrapper.
+   */
+  enum Type {
+    TYPE_SHARED_IMAGE,
+    TYPE_IMAGE,
+
+    TYPE_MAX
+  };
+
+  /**
+   * Returns a unique ID for each implementation of gfxReusableSurfaceWrapper.
+   */
+  virtual Type GetType() = 0;
+
+protected:
+  // Protected destructor, to discourage deletion outside of Release():
+  virtual ~gfxReusableSurfaceWrapper() {}
+
+  NS_DECL_OWNINGTHREAD
+};
+
+#endif // GFXCOWSURFACEWRAPPER