SERVER-24152 added the $bucketAuto stage, but did not add the ability to spill to disk. It will currently error if the memory usage exceeds 100MB.

If the user has specified allowDiskUse: true to the aggregate command, the $bucketAuto stage should be able to take advantage of the sorter to spill to disk.

All that should be required is modifying this code to appropriately set SortOptions.extSortAllowed appropriately based off of pExpCtx->extSortAllowed and pExpCtx->inRouter.