sync: Introduce CryptographerProvider interface

sync/engine/directory_cryptographer_provider.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_ENGINE_DIRECTORY_CRYPTOGRAPHER_PROVIDER_H_
+#define SYNC_ENGINE_DIRECTORY_CRYPTOGRAPHER_PROVIDER_H_
+
+#include "sync/engine/cryptographer_provider.h"
+
+#include "sync/syncable/syncable_read_transaction.h"
+
+namespace syncer {
+
+namespace syncable {
+class Directory;
+}
+
+// Provides access to the Directory's cryptographer through the
+// CryptographerProvider interface.
+class DirectoryCryptographerProvider : public CryptographerProvider {
+ public:
+  DirectoryCryptographerProvider(syncable::Directory* dir);
+  virtual ~DirectoryCryptographerProvider();
+
+  virtual bool InitScopedCryptographer(ScopedCryptographer* scoped) OVERRIDE;
+
+ private:
+  syncable::Directory* dir_;
+};
+
+// An implementation detail of the DirectoryCryptographerProvider.  Contains
+// the ReadTransaction used to safely access the Cryptographer.
+class ScopedDirectoryCryptographerImpl : public ScopedCryptographerImpl {
+ public:
+  ScopedDirectoryCryptographerImpl(syncable::Directory* dir);
+  virtual ~ScopedDirectoryCryptographerImpl();
+
+  virtual Cryptographer* Get() const OVERRIDE;
+
+ private:
+  syncable::Directory* dir_;
+  syncable::ReadTransaction trans_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedDirectoryCryptographerImpl);
+};
+
+}  // namespace syncer
+
+#endif  // SYNC_ENGINE_DIRECTORY_CRYPTOGRAPHER_PROVIDER_H_