The problem here seem to be that when mysql
is redirecting stderr to a file, stderr becomes 
buffered, whereas it is unbuffered by definition.

The solution is to unbuffer it by setting buffer
to null.

sql/log.cc:
  use setbuf(stderr, NULL) to set the buffer to null.
sql/mysqld.cc:
  use setbuf(stderr, NULL) to set the buffer to null.