[Sync] Handle invalid specifics field numbers gracefully

sync/internal_api/public/base/model_type.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.
 
 // Enumerate the various item subtypes that are supported by sync.
 // Each sync object is expected to have an immutable object type.
 // An object's type is inferred from the type of data it holds.
 
 #ifndef SYNC_INTERNAL_API_PUBLIC_BASE_MODEL_TYPE_H_
 #define SYNC_INTERNAL_API_PUBLIC_BASE_MODEL_TYPE_H_
@@ skipping 142 matching lines @@
 // - They support custom update application and conflict resolution logic.
 // - All change processing occurs on the sync thread (GROUP_PASSIVE).
 ModelTypeSet ControlTypes();
 
 // Returns true if this is a control type.
 //
 // See comment above for more information on what makes these types special.
 bool IsControlType(ModelType model_type);
 
 // Determine a model type from the field number of its associated
-// EntitySpecifics field.
+// EntitySpecifics field.  Returns UNSPECIFIED if the field number is
+// not recognized.
+//
+// If you're putting the result in a ModelTypeSet, you should use the
+// following pattern:
+//
+//   ModelTypeSet model_types;
+//   // Say we're looping through a list of items, each of which has a
+//   // field number.
+//   for (...) {
+//     int field_number = ...;
+//     ModelType model_type =
+//         GetModelTypeFromSpecificsFieldNumber(field_number);
+//     if (!IsRealDataType(model_type)) {
+//       NOTREACHED() << "Unknown field number " << field_number;
+//       continue;
+//     }
+//     model_types.Put(model_type);
+//   }
 ModelType GetModelTypeFromSpecificsFieldNumber(int field_number);
 
 // Return the field number of the EntitySpecifics field associated with
 // a model type.
 //
 // Used by tests outside of sync.
 SYNC_EXPORT int GetSpecificsFieldNumberFromModelType(
     ModelType model_type);
 
 FullModelTypeSet ToFullModelTypeSet(ModelTypeSet in);
@@ skipping 38 matching lines @@
 // type and |model_type| was filled in.
 bool NotificationTypeToRealModelType(const std::string& notification_type,
                                      ModelType* model_type);
 
 // Returns true if |model_type| is a real datatype
 SYNC_EXPORT bool IsRealDataType(ModelType model_type);
 
 }  // namespace syncer
 
 #endif  // SYNC_INTERNAL_API_PUBLIC_BASE_MODEL_TYPE_H_