[Storage] Blob items are now shared between blobs. Ready for disk swap.

storage/browser/blob/shareable_blob_data_item.h

+// Copyright (c) 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef STORAGE_BROWSER_BLOB_SHAREABLE_BLOB_DATA_ITEM_H_
+#define STORAGE_BROWSER_BLOB_SHAREABLE_BLOB_DATA_ITEM_H_
+
+#include "base/containers/hash_tables.h"
+#include "base/hash.h"
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "storage/common/data_element.h"
+
+namespace storage {
+class BlobDataItem;
+
+// This class allows blob items to be shared between blobs, and is only used by
+// BlobStorageContext. This class contains both the blob data item and the uuids
+// of all the blobs using this item.
+// The data in this class (the item) is immutable, but the item itself can be
+// swapped out with an item with the same data but a different backing (think
+// RAM vs file backed).
+class ShareableBlobDataItem : public base::RefCounted<ShareableBlobDataItem> {
+ public:
+  ShareableBlobDataItem(const std::string& blob_uuid,
+                        const scoped_refptr<BlobDataItem>& item);
+
+  const scoped_refptr<BlobDataItem>& item();
+
+  base::hash_set<std::string>& referencing_blobs() {
+    return referencing_blobs_;
+  }
+
+ private:
+  friend class base::RefCounted<ShareableBlobDataItem>;
+  ~ShareableBlobDataItem();
+  scoped_refptr<BlobDataItem> item_;
+
+  base::hash_set<std::string> referencing_blobs_;
+
+  DISALLOW_COPY_AND_ASSIGN(ShareableBlobDataItem);
+};
+}  // namespace storage
+#endif  // STORAGE_BROWSER_BLOB_SHAREABLE_BLOB_DATA_ITEM_H_