[USS] Add SyncMetadataStore interface

components/sync/model/metadata_table.h

Index: components/sync/model/metadata_table.h
diff --git a/components/sync/model/metadata_table.h b/components/sync/model/metadata_table.h
new file mode 100644
index 0000000000000000000000000000000000000000..bc25755aa57c72346bb21d04c9d5a4decc8d44ce
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+++ b/components/sync/model/metadata_table.h
@@ -0,0 +1,49 @@
+// Copyright 2017 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_SYNC_MODEL_METADATA_TABLE_H_
+#define COMPONENTS_SYNC_MODEL_METADATA_TABLE_H_
+
+#include <string>
+
+#include "components/sync/base/model_type.h"
+
+namespace sync_pb {
+class EntityMetadata;
+class ModelTypeState;
+}  // namespace sync_pb
+
+namespace syncer {
+
+// Interface used by the MetadataChangeList and database to communicate about

[pavely, 2017/04/06 22:56:42]

This interface will be used by developers outside of sync team. Class comment
should explain when would someone need this class and how to use/implement it.
For example:

MetadataTable defines interface implemented by model types for persisting sync
metadata and datatype state. It allows model type to use common implementation
of MetadataChangeList (OnDiskMetadataChangeList) instead of implementing their
own.

Model type in implementation of ModelTypeSyncBridge::CreateMetadataChangeList
should create instance of OnDiskMetadataChangeList passing pointer to
MetadataTable to its constructor.

Implementations of MetadataTable methods should write updated metadata in model
type specific sync metadata storage.

[Gang Wu, 2017/04/21 05:28:03]

Done.

+// metadata. The purpose of the interface is to record metadata changes to
+// database for each datatype. The implementation of the interface is supposed
+// to add/update/delete metadata for each datatype in database.
+class MetadataTable {

[pavely, 2017/04/06 22:56:42]

Word "Table" in MetadataTable was probably inspired by AutofillTable hierarchy
of classes. When this interface is used with other types (Typed URL) Table might
look out of place. This class defines interface for storing sync metadata in any
storage (doesn't have to be table or sql). I think the name should reflect that
it concerns sync metadata (there could be other metadata in model type's
implementation) and shouldn't be restricted to tables. Here are some potential
names:
SyncMetadataStore
SyncMetadataWriter
SyncMetadataProvider

[Gang Wu, 2017/04/21 05:28:03]

Done.

+ public:
+  MetadataTable() {}
+  virtual ~MetadataTable() {}
+
+  // Update the metadata row for |model_type|, keyed by |storage_key|, to
+  // contain the contents of |metadata|.
+  virtual bool UpdateSyncMetadata(syncer::ModelType model_type,

[pavely, 2017/04/06 22:56:42]

Could you add description of return value for all these functions.

[Gang Wu, 2017/04/21 05:28:03]

Done.

+                                  const std::string& storage_key,
+                                  const sync_pb::EntityMetadata& metadata) = 0;
+
+  // Remove the metadata row of type |model_type| keyed by |storage_key|.
+  virtual bool ClearSyncMetadata(syncer::ModelType model_type,
+                                 const std::string& storage_key) = 0;
+
+  // Update the stored sync state for the |model_type|.
+  virtual bool UpdateModelTypeState(
+      syncer::ModelType model_type,
+      const sync_pb::ModelTypeState& model_type_state) = 0;
+
+  // Clear the stored sync state for |model_type|.
+  virtual bool ClearModelTypeState(syncer::ModelType model_type) = 0;
+};
+
+}  // namespace syncer
+
+#endif  // COMPONENTS_SYNC_MODEL_METADATA_TABLE_H_