Several years ago I was developing a "robust serial console over USB" feature for Linux. THis feature allows you to have a console on, say /dev/ttyUSB0. The device doesn't have to be plugged in. When you plug in the USB-Serial dongle, you have a console. You can unplug your serial dongle right in the middle of a session, plug in one that uses a different chip, and your session is intact! Just "Ctrl-L" to refresh your Vim window where you were editing and there you are. The /dev/ttyUSB0 did actually disappear when unplugged and did re-appear on the re-insertion, but the TTY session was isolated from that, held in a suspended state.

Chopping apart the USB-Serial and TTY code in the kernel was relatively a piece of cake; but systemd threw up a wall, ironically! I got good help from the mailing list:

https://lists.freedesktop.org/archives/systemd-devel/2013-Ma...

At first I was stumped: everything should work, but what the heck is killing my login sessions when I unplug the device? I went over the code to make absolutely sure nothing would be getting a hangup signal from the TTY or the like. Through kernel printk messages, I traced it to systemd, slapping my forehead. Basically, even though the tty session was being kept intact, it was the fact that the USB device went away that systemd killed the session.

Once I solved the systemd issues, everything worked great.

systemd: it just likes to kill things, at least in its default configuration.