In debug builds of MySQL, there is an configuration variable
that allows an InnoDB log checkpoint to be initiated:

SET GLOBAL innodb_log_checkpoint_now=ON;

Setting this variable while a table-rebuilding ALTER TABLE is executing
may result in an infinite loop.

checkpoint_now_set(): Account for log_sys->append_on_checkpoint->size().
Note that this function contains race conditions, because it is accessing
fields of log_sys without holding log_sys->mutex. We think that this is
acceptable, because this variable only exists for debugging purposes, in
debug builds of MySQL.

RB: 9947
Reviewed-by: Sunny Bains <sunny.bains@oracle.com>