Ubuntu 18.04 won't boot on full disk

[dronus]

The basic systemd configuration is not tolerating a full root fs.

Ubuntu 18.04 with a full root fs boots into a state where systemd messages are visible on screen, no graphical environment, no response to keyboard, no possibility to switch VTs. It still reacts to ACPI power switch, shutting down.

The startup process should be configured in a way that would allow back off from full fs situations.

This is critical as any user is allowed to fill the root fs as long as home is inside the root fs.

That can easily happen accidentally, eg. by download tools not removing interrupted downloads for later resuming them. Those would exit when the disk is full but leave the file in place. If shutdown is still working (which it is under some circumstances I've tested) this leads to a full disk at boot scenario, braking the whole system just by legitimate user operations.

Test procedure:
Open terminal.
Run
cat /dev/zero >zeroes

Wait until disk full.
Reboot by UI buttons.

What should happen:
System boots into a intermediate state where the problem is clearly mentioned and there is some back off. Ideally it should run the graphical desktop and notify the user to delete some files or present a tool to do so. If not possible, it could exit to a emergency root console showing some explanation what happened and what can be done about now.
System could also provide a small emergency reservation file, which it can automatically remove on boot if running out of disk space and notifiy the user about.

What happens:
System boots to an almost completely unresponsive state (keyboard locked).

Also it seems the Guest accounts can be used to fill up the disk either, so this could be used for denial of service, breaking a users machine without having login access.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Dan Streetman (ddstreet)]

please reopen if this is still an issue