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

sync/api/sync_data.cc

Index: sync/api/sync_data.cc
diff --git a/sync/api/sync_data.cc b/sync/api/sync_data.cc
index fa48ea5edc18a89a1c20d15325e9e58bcee727b6..35a21f90f8be52997a8de25ff2346342f17a5662 100644
--- a/sync/api/sync_data.cc
+++ b/sync/api/sync_data.cc
@@ -36,6 +36,21 @@ syncer::AttachmentId ProtoToId(const sync_pb::AttachmentIdProto& proto) {
   return syncer::AttachmentId::CreateFromProto(proto);
 }
 
+// Return true iff |attachments| contains one or more elements with the same
+// AttachmentId.
+bool ContainsDuplicateAttachments(const syncer::AttachmentList& attachments) {
+  std::set<syncer::AttachmentId> id_set;
+  AttachmentList::const_iterator iter = attachments.begin();
+  AttachmentList::const_iterator end = attachments.end();
+  for (; iter != end; ++iter) {
+    if (id_set.find(iter->GetId()) != id_set.end()) {
+      return true;
+    }
+    id_set.insert(iter->GetId());
+  }
+  return false;
+}
+
 }  // namespace
 
 namespace syncer {
@@ -96,15 +111,13 @@ SyncData SyncData::CreateLocalData(const std::string& sync_tag,
       sync_tag, non_unique_title, specifics, attachments);
 }
 
-// TODO(maniscalco): What should happen if the same Attachment appears in the
-// list twice? Document the behavior and add a test case.
-//
 // Static.
 SyncData SyncData::CreateLocalDataWithAttachments(
     const std::string& sync_tag,
     const std::string& non_unique_title,
     const sync_pb::EntitySpecifics& specifics,
     const AttachmentList& attachments) {
+  DCHECK(!ContainsDuplicateAttachments(attachments));
   sync_pb::SyncEntity entity;
   entity.set_client_defined_unique_tag(sync_tag);
   entity.set_non_unique_name(non_unique_title);
@@ -113,9 +126,6 @@ SyncData SyncData::CreateLocalDataWithAttachments(
                  attachments.end(),
                  RepeatedFieldBackInserter(entity.mutable_attachment_id()),
                  AttachmentToProto);
-  // TODO(maniscalco): Actually pass the attachments to the ctor and make them
-  // available to the AttachmentService once this SyncData gets passed into
-  // GenericChangeProcesso::ProcessSyncChanges (bug 354530).
   AttachmentList copy_of_attachments(attachments);
   return SyncData(kInvalidId,
                   &entity,