[Sync] Refactor AttachmentStore classes. Introduce concept of referrer.

sync/api/attachments/attachment_store.h

 // Copyright 2014 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 SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_H_
 #define SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_H_
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "sync/api/attachments/attachment.h"
 #include "sync/api/attachments/attachment_id.h"
 #include "sync/api/attachments/attachment_metadata.h"
 #include "sync/base/sync_export.h"
 
 namespace base {
 class FilePath;
-class RefCountedMemory;
 class SequencedTaskRunner;
 }  // namespace base
 
 namespace syncer {
 
-class Attachment;
-class AttachmentId;
+class AttachmentStoreFrontend;
+class AttachmentStoreBackend;
 
 // AttachmentStore is a place to locally store and access Attachments.
 //
 // AttachmentStore class is an interface exposed to data type and
 // AttachmentService code.
 // It also contains factory methods for default attachment store
 // implementations.
 // Destroying this object does not necessarily cancel outstanding async
 // operations. If you need cancel like semantics, use WeakPtr in the callbacks.
-class SYNC_EXPORT AttachmentStore
-    : public base::RefCountedThreadSafe<AttachmentStore> {
+class SYNC_EXPORT AttachmentStore {
  public:
   // TODO(maniscalco): Consider udpating Read and Write methods to support
   // resumable transfers (bug 353292).
 
   // The result status of an attachment store operation.
   // Do not re-order or delete these entries; they are used in a UMA histogram.
   enum Result {
     SUCCESS = 0,            // No error, all completed successfully.
     UNSPECIFIED_ERROR = 1,  // An unspecified error occurred for >= 1 item.
     STORE_INITIALIZATION_FAILED = 2,  // AttachmentStore initialization failed.
     // When adding a value here, you must increment RESULT_SIZE below.
   };
   static const int RESULT_SIZE =
       10;  // Size of the Result enum; used for histograms.
 
+  // Each attachment can have references from sync or model type. Tracking these
+  // references is needed for lifetime management of attachment, it can only be
+  // deleted from the store when it doesn't have references.
+  enum AttachmentReferrer {
+    MODEL_TYPE,
+    SYNC,
+  };
+
   typedef base::Callback<void(const Result&)> InitCallback;
   typedef base::Callback<void(const Result&,
                               scoped_ptr<AttachmentMap>,
                               scoped_ptr<AttachmentIdList>)> ReadCallback;
   typedef base::Callback<void(const Result&)> WriteCallback;
   typedef base::Callback<void(const Result&)> DropCallback;
   typedef base::Callback<void(const Result&,
                               scoped_ptr<AttachmentMetadataList>)>
       ReadMetadataCallback;
 
-  AttachmentStore();
-
-  // Asynchronously initializes attachment store.
-  //
-  // This method should not be called by consumer of this interface. It is
-  // called by factory methods in AttachmentStore class. When initialization is
-  // complete |callback| is invoked with result, in case of failure result is
-  // UNSPECIFIED_ERROR.
-  virtual void Init(const InitCallback& callback) = 0;
+  ~AttachmentStore();
 
   // Asynchronously reads the attachments identified by |ids|.
   //
   // |callback| will be invoked when finished. AttachmentStore will attempt to
   // read all attachments specified in ids. If any of the attachments do not
   // exist or could not be read, |callback|'s Result will be UNSPECIFIED_ERROR.
   // Callback's AttachmentMap will contain all attachments that were
   // successfully read, AttachmentIdList will contain attachment ids of
   // attachments that are unavailable in attachment store, these need to be
   // downloaded from server.
   //
   // Reads on individual attachments are treated atomically; |callback| will not
   // read only part of an attachment.
-  virtual void Read(const AttachmentIdList& ids,
-                    const ReadCallback& callback) = 0;
+  void Read(const AttachmentIdList& ids, const ReadCallback& callback);
 
   // Asynchronously writes |attachments| to the store.
   //
   // Will not overwrite stored attachments. Attempting to overwrite an
   // attachment that already exists is not an error.
   //
   // |callback| will be invoked when finished. If any of the attachments could
   // not be written |callback|'s Result will be UNSPECIFIED_ERROR. When this
   // happens, some or none of the attachments may have been written
   // successfully.
-  virtual void Write(const AttachmentList& attachments,
-                     const WriteCallback& callback) = 0;
+  void Write(const AttachmentList& attachments, const WriteCallback& callback);
 
   // Asynchronously drops |attchments| from this store.
   //
   // This does not remove attachments from the server.
   //
   // |callback| will be invoked when finished. Attempting to drop an attachment
   // that does not exist is not an error. If any of the existing attachment
   // could not be dropped, |callback|'s Result will be UNSPECIFIED_ERROR. When
   // this happens, some or none of the attachments may have been dropped
   // successfully.
-  virtual void Drop(const AttachmentIdList& ids,
-                    const DropCallback& callback) = 0;
+  void Drop(const AttachmentIdList& ids, const DropCallback& callback);
 
   // Asynchronously reads metadata for the attachments identified by |ids|.
   //
   // |callback| will be invoked when finished. AttachmentStore will attempt to
   // read metadata for all attachments specified in ids. If any of the
   // metadata entries do not exist or could not be read, |callback|'s Result
   // will be UNSPECIFIED_ERROR.
-  virtual void ReadMetadata(const AttachmentIdList& ids,
-                            const ReadMetadataCallback& callback) = 0;
+  void ReadMetadata(const AttachmentIdList& ids,
+                    const ReadMetadataCallback& callback);
 
   // Asynchronously reads metadata for all attachments in the store.
   //
   // |callback| will be invoked when finished. If any of the metadata entries
   // could not be read, |callback|'s Result will be UNSPECIFIED_ERROR.
-  virtual void ReadAllMetadata(const ReadMetadataCallback& callback) = 0;
+  void ReadAllMetadata(const ReadMetadataCallback& callback);
 
-  // Creates an AttachmentStoreHandle backed by in-memory implementation of
-  // attachment store. For now frontend lives on the same thread as backend.
-  static scoped_refptr<AttachmentStore> CreateInMemoryStore();
+  // Given current AttachmentStore (this) creates separate AttachmentStore that
+  // will be used by sync components (AttachmentService). Resulting
+  // AttachmentStore is backed by the same frontend/backend.
+  scoped_ptr<AttachmentStore> CreateAttachmentStoreForSync() const;
 
-  // Creates an AttachmentStoreHandle backed by on-disk implementation of
-  // attachment store. Opens corresponding leveldb database located at |path|.
-  // All backend operations are scheduled to |backend_task_runner|. Opening
-  // attachment store is asynchronous, once it finishes |callback| will be
-  // called on the thread that called CreateOnDiskStore. Calling Read/Write/Drop
-  // before initialization completed is allowed.  Later if initialization fails
-  // these operations will fail with STORE_INITIALIZATION_FAILED error.
-  static scoped_refptr<AttachmentStore> CreateOnDiskStore(
+  // Creates an AttachmentStore backed by in-memory implementation of attachment
+  // store. For now frontend lives on the same thread as backend.
+  static scoped_ptr<AttachmentStore> CreateInMemoryStore();
+
+  // Creates an AttachmentStore backed by on-disk implementation of attachment
+  // store. Opens corresponding leveldb database located at |path|. All backend
+  // operations are scheduled to |backend_task_runner|. Opening attachment store
+  // is asynchronous, once it finishes |callback| will be called on the thread
+  // that called CreateOnDiskStore. Calling Read/Write/Drop before
+  // initialization completed is allowed.  Later if initialization fails these
+  // operations will fail with STORE_INITIALIZATION_FAILED error.
+  static scoped_ptr<AttachmentStore> CreateOnDiskStore(
       const base::FilePath& path,
       const scoped_refptr<base::SequencedTaskRunner>& backend_task_runner,
       const InitCallback& callback);
 
- protected:
-  friend class base::RefCountedThreadSafe<AttachmentStore>;
-  virtual ~AttachmentStore();
-};
-
-// Interface for AttachmentStore backends.
-//
-// AttachmentStoreBackend provides interface for different backends (on-disk,
-// in-memory). Factory methods in AttachmentStore create corresponding backend
-// and pass reference to AttachmentStoreHandle.
-// All functions in AttachmentStoreBackend mirror corresponding functions in
-// AttachmentStore.
-// All callbacks and result codes are used directly from AttachmentStore.
-// AttachmentStoreHandle only passes callbacks and results, there is no need to
-// declare separate set.
-class SYNC_EXPORT AttachmentStoreBackend {
- public:
-  explicit AttachmentStoreBackend(
-      const scoped_refptr<base::SequencedTaskRunner>& callback_task_runner);
-  virtual ~AttachmentStoreBackend();
-  virtual void Init(const AttachmentStore::InitCallback& callback) = 0;
-  virtual void Read(const AttachmentIdList& ids,
-                    const AttachmentStore::ReadCallback& callback) = 0;
-  virtual void Write(const AttachmentList& attachments,
-                     const AttachmentStore::WriteCallback& callback) = 0;
-  virtual void Drop(const AttachmentIdList& ids,
-                    const AttachmentStore::DropCallback& callback) = 0;
-  virtual void ReadMetadata(
-      const AttachmentIdList& ids,
-      const AttachmentStore::ReadMetadataCallback& callback) = 0;
-  virtual void ReadAllMetadata(
-      const AttachmentStore::ReadMetadataCallback& callback) = 0;
-
- protected:
-  // Helper function to post callback on callback_task_runner.
-  void PostCallback(const base::Closure& callback);
+  // Creates set of AttachmentStore/AttachmentStoreFrontend instances for tests
+  // that provide their own implementation of AttachmentstoreBackend for
+  // mocking.
+  static scoped_ptr<AttachmentStore> CreateMockStoreForTest(
+      scoped_ptr<AttachmentStoreBackend> backend);
 
  private:
-  scoped_refptr<base::SequencedTaskRunner> callback_task_runner_;
+  AttachmentStore(const scoped_refptr<AttachmentStoreFrontend>& frontend,
+                  AttachmentReferrer referrer);
 
-  DISALLOW_COPY_AND_ASSIGN(AttachmentStoreBackend);
+  scoped_refptr<AttachmentStoreFrontend> frontend_;
+  // Modification operations with attachment store will be performed on behalf
+  // of |referrer_|.
+  const AttachmentReferrer referrer_;
+
+  DISALLOW_COPY_AND_ASSIGN(AttachmentStore);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_API_ATTACHMENTS_ATTACHMENT_STORE_H_