Thread 1 "mysqld" hit Breakpoint 1, innobase_mysql_tmpfile () at /home/dan/repos/mariadb-server-5.5/storage/xtradb/handler/ha_innodb.cc:1639
1639            os_event_wait(srv_allow_writes_event);

(gdb) p fd
$2 = 7
(gdb) n
1682                    fd2 = fcntl(fd, F_DUPFD_CLOEXEC, 0);
(gdb)
1687                    if (fd2 < 0) {
(gdb) p fd2
$3 = 8

cat /proc/20448/fdinfo/{7,8}
  2051972      0 -r--------   1  dan      dan             0 Mar  2 12:04 fdinfo/7
pos:    0
flags:  0100002
mnt_id: 82
  2051973      0 -r--------   1  dan      dan             0 Mar  2 12:04 fdinfo/8
pos:    0
flags:  02100002
mnt_id: 82

So fd 8 has 02000000 hence CLOEXEC