InnoDB engine changes internal auto_increment counter only after
ha_innodb::write_row, so two threads can't simultaneously
operate between ha_innodb::update_autoincrement and
ha_innodb::write_row.
So concurrent execution of ha_partition::write_row prevented


sql/ha_partition.cc:
  use table_share->mutex to avoid concurrent write_row for AUTO_INCREMENTed table