Keep track of which attachments are referenced by which sync entries.

sync/api/sync_data.h

 // Copyright 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 SYNC_API_SYNC_DATA_H_
 #define SYNC_API_SYNC_DATA_H_
 
 #include <iosfwd>
 #include <string>
 #include <vector>
@@ skipping 25 matching lines @@
 // in STL containers are supported and encouraged if helpful.
 class SYNC_EXPORT SyncData {
  public:
   // Creates an empty and invalid SyncData.
   SyncData();
   ~SyncData();
 
   // Default copy and assign welcome.
 
   // Helper methods for creating SyncData objects for local data.
-  // The sync tag must be a string unique to this datatype and is used as a node
+  //
+  // |sync_tag| Must be a string unique to this datatype and is used as a node
   // identifier server-side.
+  //
   // For deletes: |datatype| must specify the datatype who node is being
   // deleted.
-  // For adds/updates: the specifics must be valid and the non-unique title (can
-  // be the same as sync tag) must be specfied.
-  // Note: the non_unique_title is primarily for debug purposes, and will be
-  // overwritten if the datatype is encrypted.
+  //
+  // For adds/updates: |specifics| must be valid and |non_unique_title| (can be
+  // the same as |sync_tag|) must be specfied.  Note: |non_unique_title| is
+  // primarily for debug purposes, and will be overwritten if the datatype is
+  // encrypted.
+  //
+  // For data with attachments: |attachments| must not contain duplicates.
   static SyncData CreateLocalDelete(
       const std::string& sync_tag,
       ModelType datatype);
   static SyncData CreateLocalData(
       const std::string& sync_tag,
       const std::string& non_unique_title,
       const sync_pb::EntitySpecifics& specifics);
   static SyncData CreateLocalDataWithAttachments(
       const std::string& sync_tag,
       const std::string& non_unique_title,
@@ skipping 139 matching lines @@
 };
 
 // gmock printer helper.
 void PrintTo(const SyncData& sync_data, std::ostream* os);
 
 typedef std::vector<SyncData> SyncDataList;
 
 }  // namespace syncer
 
 #endif  // SYNC_API_SYNC_DATA_H_