[BlobAsync] Disk support for blob storage

storage/browser/blob/shareable_blob_data_item.h

 // Copyright 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 <string>
 
 #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;
 class InternalBlobData;
 
 // 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> {
+class ShareableBlobDataItem
+    : public base::RefCountedThreadSafe<ShareableBlobDataItem> {

[michaeln, 2016/07/07 20:05:22]

Why does this need to be a threadsafe refcount? If this really is needed, seems
like BlobDataItem might also need a threadsafe refcount.

  public:
-  ShareableBlobDataItem(const std::string& blob_uuid,
-                        const scoped_refptr<BlobDataItem>& item);
+  ShareableBlobDataItem(uint64_t item_id, scoped_refptr<BlobDataItem> item);
 
   const scoped_refptr<BlobDataItem>& item();
 
+  uint64_t item_id() { return item_id_; }
+
   base::hash_set<std::string>& referencing_blobs() {
     return referencing_blobs_;
   }
 
  private:
-  friend class base::RefCounted<ShareableBlobDataItem>;
+  friend class base::RefCountedThreadSafe<ShareableBlobDataItem>;
   friend class InternalBlobData;
+  friend class BlobMemoryController;
   ~ShareableBlobDataItem();
 
+  // This is a unique identifier for this ShareableBlobDataItem.
+  uint64_t item_id_;
   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_