[USS] implement CreateMetadataChangeList and GetClientTag

components/history/core/browser/history_backend.h

 // Copyright (c) 2012 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 COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_
 #define COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 15 matching lines @@
 #include "base/supports_user_data.h"
 #include "base/task/cancelable_task_tracker.h"
 #include "build/build_config.h"
 #include "components/favicon_base/favicon_usage_data.h"
 #include "components/history/core/browser/expire_history_backend.h"
 #include "components/history/core/browser/history_backend_notifier.h"
 #include "components/history/core/browser/history_types.h"
 #include "components/history/core/browser/keyword_id.h"
 #include "components/history/core/browser/thumbnail_database.h"
 #include "components/history/core/browser/visit_tracker.h"
+#include "components/sync/model/sync_metadata_store.h"
 #include "sql/init_status.h"
 
 class SkBitmap;
 class TestingProfile;
 
 namespace base {
 class SingleThreadTaskRunner;
 }
 
 namespace history {
@@ skipping 46 matching lines @@
 // Internal history implementation which does most of the work of the history
 // system. This runs on a background thread (to not block the browser when we
 // do expensive operations) and is NOT threadsafe, so it must only be called
 // from message handlers on the background thread. Invoking on another thread
 // requires threadsafe refcounting.
 //
 // Most functions here are just the implementations of the corresponding
 // functions in the history service. These functions are not documented
 // here, see the history service for behavior.
 class HistoryBackend : public base::RefCountedThreadSafe<HistoryBackend>,
-                       public HistoryBackendNotifier {
+                       public HistoryBackendNotifier,
+                       public syncer::SyncMetadataStore {

[pavely, 2017/05/01 17:24:16]

I don't really like exposing SyncMetadataStore to consumers of HistoryBackend.
What if instead we passed pointer to SyncMetadataStore into constructor of
TypedURLSyncBridge? We instantiate bridge in HistoryBackend::Init where we can
get the reference to HistoryDatabase.

  public:
   // Interface implemented by the owner of the HistoryBackend object. Normally,
   // the history service implements this to send stuff back to the main thread.
   // The unit tests can provide a different implementation if they don't have
   // a history service object.
   class Delegate {
    public:
     virtual ~Delegate() {}
 
     // Called when the database cannot be read correctly for some reason.
@@ skipping 364 matching lines @@
 
   ExpireHistoryBackend* expire_backend() { return &expirer_; }
 #endif
 
   // Returns true if the passed visit time is already expired (used by the sync
   // code to avoid syncing visits that would immediately be expired).
   virtual bool IsExpiredVisitTime(const base::Time& time);
 
   base::Time GetFirstRecordedTimeForTest() { return first_recorded_time_; }
 
+  // syncer::SyncMetadataStore implementation.
+  bool UpdateSyncMetadata(syncer::ModelType model_type,
+                          const std::string& storage_key,
+                          const sync_pb::EntityMetadata& metadata) override;
+  bool ClearSyncMetadata(syncer::ModelType model_type,
+                         const std::string& storage_key) override;
+  bool UpdateModelTypeState(
+      syncer::ModelType model_type,
+      const sync_pb::ModelTypeState& model_type_state) override;
+  bool ClearModelTypeState(syncer::ModelType model_type) override;
+
  protected:
   ~HistoryBackend() override;
 
  private:
   friend class base::RefCountedThreadSafe<HistoryBackend>;
   friend class HistoryBackendTest;
   friend class HistoryBackendDBBaseTest;  // So the unit tests can poke our
                                           // innards.
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, DeleteAll);
   FRIEND_TEST_ALL_PREFIXES(HistoryBackendTest, DeleteAllURLPreviouslyDeleted);
@@ skipping 422 matching lines @@
   // it unregisters itself as observer during destruction.
   std::unique_ptr<TypedUrlSyncableService> typed_url_syncable_service_;
   std::unique_ptr<TypedURLSyncBridge> typed_url_sync_bridge_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryBackend);
 };
 
 }  // namespace history
 
 #endif  // COMPONENTS_HISTORY_CORE_BROWSER_HISTORY_BACKEND_H_