xenial: Watch out for boot failures due to /etc/mtab file

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for information.

Note the existing bug number

Cheers


-------- Forwarded Message --------
Subject: xenial: Watch out for boot failures due to /etc/mtab file
Date: Thu, 29 Oct 2015 14:43:22 +0100
From: Martin Pitt <***@ubuntu.com>
To: Ubuntu Development <ubuntu-***@lists.ubuntu.com>



Hello all,

/etc/mtab is supposed to be a symlink to /proc/self/mounts (or
/proc/mounts) for stuff to work. We've also shipped
"debian-fixup.service" since vivid to turn a file into a symlink on
boot.

However, if you have /etc/mtab as a file (e. g. right after
installation), current xenial's systemd will fail hard to boot with
something like

| systemd [1]: /etc/mtab is not a symlink or not pointing to /proc/self/mounts. This is not supported anymore. Please replace /etc/mtab with a symlink to /proc/self/mounts.
| systemd [1]: Freezing execution.

This is of course a really harsh failure mode, and it will be changed
again in a few weeks or so to merely warn about that. But as these
warnings have existed for a long time already, and nobody notices
them, having this hard failure mode will actually tell us which places
we need to fix. E. g. a week ago debian-installer was fixed [1], and I
just heared on IRC that a Xubuntu desktop install fails as well; bug
was just filed [2].

We really must stop writing /etc/mtab files. It breaks libmount's
monitoring and all kinds of old school software which still reads
/etc/mtab directly; ideally they should of course be fixed to look
into /proc, and /etc/mtab should just die, but there's still some way
to go (e. g. [3]).

So for now, if you see a boot failure like this, please report it and
ping me, so that we can fix our installers, images etc. to not produce
a broken /etc/mtab any more. After that, just remove /etc/mtab (e. g.
temporarily boot with upstart) to resume.

After we fixed our installers (ubiquity, d-i, cloud-init, etc.), we'll
go back to warning (IOW: ignore) mode.

Thanks,

Martin

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802187
[2] https://launchpad.net/bugs/1511376
[3] https://sourceware.org/bugzilla/show_bug.cgi?id=19108