The DocumentSource::usedDisk() virtual method must be implemented by any agg stage which can spill to disk, such as $group or $sort, to indicate whether spilling to disk actually occurred at runtime. This seems like an overly specific virtual method to exist on DocumentSource, the aggregation's abstract concept for any runtime query execution stage. Further, we have since extended DocumentSource to support a more generic execution stats reporting interface which closely resembles that used by both the classic execution engine and the slot-based execution engine. In particular, we now have DocumentSource::getCommonStats() and DocumentSource::getSpecificStats(). As a simplification of the code, and an improvement of the DocumentSource interface, we should consolidate usedDisk() with these more generic agg execution stats methods and delete usedDisk().