diff -r 000000000000 -r 6474c204b198 content/media/SharedBuffer.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/media/SharedBuffer.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,77 @@
+/* -*- 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/. */
+
+#ifndef MOZILLA_SHAREDBUFFER_H_
+#define MOZILLA_SHAREDBUFFER_H_
+
+#include "mozilla/CheckedInt.h"
+#include "mozilla/mozalloc.h"
+#include "nsCOMPtr.h"
+#include "nsAutoPtr.h"
+
+namespace mozilla {
+
+/**
+ * Base class for objects with a thread-safe refcount and a virtual
+ * destructor.
+ */
+class ThreadSharedObject {
+public:
+  NS_INLINE_DECL_THREADSAFE_REFCOUNTING(ThreadSharedObject)
+
+  bool IsShared() { return mRefCnt.get() > 1; }
+
+  virtual size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const
+  {
+    return 0;
+  }
+
+  virtual size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
+  {
+    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
+  }
+protected:
+  // Protected destructor, to discourage deletion outside of Release():
+  virtual ~ThreadSharedObject() {}
+};
+
+/**
+ * Heap-allocated chunk of arbitrary data with threadsafe refcounting.
+ * Typically you would allocate one of these, fill it in, and then treat it as
+ * immutable while it's shared.
+ * This only guarantees 4-byte alignment of the data. For alignment we
+ * simply assume that the refcount is at least 4-byte aligned and its size
+ * is divisible by 4.
+ */
+class SharedBuffer : public ThreadSharedObject {
+public:
+  void* Data() { return this + 1; }
+
+  static already_AddRefed<SharedBuffer> Create(size_t aSize)
+  {
+    CheckedInt<size_t> size = sizeof(SharedBuffer);
+    size += aSize;
+    if (!size.isValid()) {
+      MOZ_CRASH();
+    }
+    void* m = moz_xmalloc(size.value());
+    nsRefPtr<SharedBuffer> p = new (m) SharedBuffer();
+    NS_ASSERTION((reinterpret_cast<char*>(p.get() + 1) - reinterpret_cast<char*>(p.get())) % 4 == 0,
+                 "SharedBuffers should be at least 4-byte aligned");
+    return p.forget();
+  }
+
+  virtual size_t SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const MOZ_OVERRIDE
+  {
+    return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
+  }
+
+private:
+  SharedBuffer() {}
+};
+
+}
+
+#endif /* MOZILLA_SHAREDBUFFER_H_ */