mysql socket disappears when restarting related jail with nullfs mount point

[Luis P. Mendes]

I'm running FreeBSD 11.3-RELEASE-p3 on a VPS.
The jail manager is qjail, version 5.4.

The server has two jails: a database `mariadb` and a web `webjail`.
Access to the database is done only through a unix socket to be null
mounted at the latter jail.

When started, 'service mysql-server start' on the `mariadb` jail
places the socket at /var/run/mysql/mysql.sock

When started, the `webjail` should mount that socket in its own
/var/run/mysql directory.

This is the line of the corresponding
/usr/local/etc/qjail.fstab/webjail file:
/usr/jails/mariadb/var/run/mysql  /usr/jails/webjail/var/run/mysql
nullfs  rw  0  0


What happens is that everytime that the `webjail` is started (the
first time or when restarted), the socket at `mariadb`'s
/var/run/mysql/mysql.sock
is deleted somehow.
Although this happens, the mysqld_safe and mysqld daemons keep
running.

As a note, when `webjail` is stopped, nothing happens to the socket at
the jail of the mariadb database server.

If the `mariadb` jail is started after `webjail`, the socket is
correctly mounted at the latter /var/run/mysql directory.


Using qjail's archive and restore commands, I copied these two jails
to another FreeBSD 11.3-RELEASE virtual machine, at my own computer,
and the strange behavior still happens.

There are no rc.d scripts made by me.

The contents of the config file for the `webjail` are:
/usr/local/etc/qjail.config/webjail

webjail { host.hostname       =  "webjail";
path                =  "/usr/jails/webjail";
mount.fstab         =  "/usr/local/etc/qjail.fstab/webjail";
exec.consolelog     =  "/var/log/qjail.webjail.console.log";
mount.devfs;
ip4.addr            =  127.0.0.85;
interface           =  "lo1";
devfs_ruleset       =  "4";
exec.start          =  "/bin/sh /etc/rc";
exec.stop           =  "/bin/sh /etc/rc.shutdown";


What's wrong with this?
Why starting a jail deletes a socket in another jail?




--


Luís Mendes