Locking console with Ctrl-S hangs various processes (ttydcd)

[Mikhail T.]

Hello!

A fellow FreeBSD user was recently confounded by a problem: various processes on
his 10.0 system were hanging or otherwise misbehaving: su, certain daemons, syslogd.

They were all hung in the "ttydcd"-state. Searching
<https://www.google.com/search?q=FreeBSD+ttydcd> for that revealed only, that
other people have seen the problem, but nobody (including the entire
freebsd-stable@ on 2012
<http://lists.freebsd.org/pipermail/freebsd-stable/2012-June/068281.html>) was
able to offer a solution.

Apparently, the problem stems from the ttyv0 (which moonlights as /dev/console
for the vast majority of installs) being locked -- such as by pressing Ctrl-S.
One needs not be a root to do it...

After that, any attempts to write to /dev/console -- which even syslog(3) often
does -- would hang the calling process until the tty is unlocked (such as with
Ctrl-Q).

This may be the desired behavior for "normal" ttys, but, perhaps, /dev/console
should act differently?

    -mi