* When receives unauthorized requests, RGW(rados auth only) runs four
* engines as follows:
* 1. anonymous engine in s3_main_strategy_plain
* 2. local engine in s3_main_strategy_plain
* 3. anonymous engine in s3_main_strategy_boto2
* 4. local engine in s3_main_strategy_boto2
* The third engine which is exactly the same with the first one is
* redundant.