There are several calls to BSONObj::getField that are made by functions called by RunCommandImpl::_epilogue. These are chiefly CommandHelpers::extractOrAppendOK and mongo::getStatusFromCommandResult. These command parsers can be consolidated with the help of an IDL generated parser to reduce the amount of parsing of the BSON reply.

Also, CommandHelpers::checkForInternalError can be made debug only to avoid yet another pass over the reply.

Example of the reply fields that can be consolidated in an IDL definition:

  CommonReplyArgs:
      description: TODO
      strict: false
      unsafe_dangerous_disable_extra_field_duplicate_checks: true
      fields:
        ok:
          type: safeBool
          optional: true
        code:
          type: safeInt
          optional: true
        errmsg:
          type: string
          optional: true
        $err:
          cpp_name: dollarErr
          type: string
          optional: true
        writeConcernError:
          type: object
          optional: true

Note: There may be opportunity to align with the reply fields defined in src/mongo/idl/generic_argument.idl. On the other hand, a more narrow parser for this error handling will be better then a wider parser handling all generic reply fields.

POC: https://github.com/markbenvenuto/mongo/tree/tiger_perf_bson