a quick and simple fix, yes?

# in post_stuff()

# after "# proxy check"
$dbh->do("LOCK TABLES ".SQL_TABLE." WRITE");

# before "# set the name, email and password cookies"
$dbh->do("UNLOCK TABLES");

N.B. the implications of this code: all posts are serialized, which means some requests will take a very long time to execute, tying up a mysql connection slot all the while.