[Storage] Blob Storage Refactoring pt 1

storage/browser/blob/blob_data_handle.h

 // Copyright (c) 2013 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_BLOB_DATA_HANDLE_H_
 #define STORAGE_BROWSER_BLOB_BLOB_DATA_HANDLE_H_
 
 #include <string>
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/supports_user_data.h"
 #include "storage/browser/storage_browser_export.h"
 
 namespace base {
 class SequencedTaskRunner;
 }
 
 namespace storage {
 
-class BlobData;
+class BlobDataSnapshot;
 class BlobStorageContext;
 
-// A scoper object for use in chrome's main browser process, ensures
-// the underlying BlobData and its uuid remain in BlobStorageContext's
-// collection for the duration. This object has delete semantics and
-// maybe deleted on any thread.
+// BlobDataHandle ensures that the underlying blob (keyed by the uuid) remains
+// in the BlobStorageContext's collection while this object is alive. Anything
+// that needs to keep a blob alive needs to store this handle.
+// When the blob data itself is needed, clients must call the CreateSnapshot()
+// method on the IO thread to create a snapshot of the blob data.  This snapshot
+// is not intended to be persisted, and serves to ensure that the backing
+// resources remain around for the duration of reading the blob.  This snapshot
+// can be read on any thread, but it must be destructed on the IO thread.
+// This object has delete semantics and may be deleted on any thread.
 class STORAGE_EXPORT BlobDataHandle
     : public base::SupportsUserData::Data {
  public:
   BlobDataHandle(const BlobDataHandle& other);  // May be copied on any thread.
-  ~BlobDataHandle() override;  // Maybe be deleted on any thread.
-  BlobData* data() const;  // May only be accessed on the IO thread.
+  ~BlobDataHandle() override;                   // May be deleted on any thread.
 
-  std::string uuid() const;  // May be accessed on any thread.
+  // A BlobDataSnapshot is used to read the data from the blob.  This object is
+  // intended to be transient and should not be stored for any extended period
+  // of time.
+  // This call and the destruction of the returned snapshot must be called
+  // on the IO thread.
+  scoped_ptr<BlobDataSnapshot> CreateSnapshot() const;
+
+  const std::string& uuid() const;  // May be accessed on any thread.
 
  private:
+  // Internal class whose destructor is guarenteed to be called on the IO
+  // thread.
   class BlobDataHandleShared
       : public base::RefCountedThreadSafe<BlobDataHandleShared> {
    public:
-    BlobDataHandleShared(BlobData* blob_data,
+    BlobDataHandleShared(const std::string& uuid,
                          BlobStorageContext* context,
                          base::SequencedTaskRunner* task_runner);
 
-    BlobData* data() const;
+    scoped_ptr<BlobDataSnapshot> CreateSnapshot() const;
     const std::string& uuid() const;
 
    private:
     friend class base::DeleteHelper<BlobDataHandleShared>;
     friend class base::RefCountedThreadSafe<BlobDataHandleShared>;
     friend class BlobDataHandle;
 
     virtual ~BlobDataHandleShared();
 
-    scoped_refptr<BlobData> blob_data_;
+    const std::string uuid_;
     base::WeakPtr<BlobStorageContext> context_;
 
     DISALLOW_COPY_AND_ASSIGN(BlobDataHandleShared);
   };
 
   friend class BlobStorageContext;
-  BlobDataHandle(BlobData* blob_data, BlobStorageContext* context,
+  BlobDataHandle(const std::string& uuid,
+                 BlobStorageContext* context,
                  base::SequencedTaskRunner* task_runner);
 
   scoped_refptr<base::SequencedTaskRunner> io_task_runner_;
   scoped_refptr<BlobDataHandleShared> shared_;
 };
 
 }  // namespace storage
 
 #endif  // STORAGE_BROWSER_BLOB_BLOB_DATA_HANDLE_H_