Bug#940028: debian-installer multi-console race with preseeding

[Ian Jackson <ian.jackson@citrix.com>]

I just experienced this bug.  Thanks for some very useful hints and
pointers from Colin Watson.

This is particularly awkward to debug because one of the parallel
invocations of d-i is usually invisible.  And the precise results are
the results of races and can be different from one run to another.

In my tests I frequently saw:
  main-menu[330]: /var/lib/dpkg/status: No such file or directory

Now that we have looked at the code I think that message is quite
unlikely unless something is seriously wrong, such as a corrupted
initramfs or this parallel execution bug.  I mention this as a useful
search term, hoping that affected users may find this bug report.

You can see whether your configuration is going to be affected by
looking at /proc/consoles in the installer environment: if it contains
only one line, you are OK.  If it has several you will be hit by this
bug (which exists in buster and as of now has not yet been fixed, so
is in current bullseye too).

A workaround is to specify *exactly one* appropriate console=
on the kernel command line.  This causes the kernel to report only
that console in /proc/consoles and the bug is avoided.

FTR, the offending commit is
  https://salsa.debian.org/installer-team/rootskel/commit/b6048aafed7d73ba42da04d6f7a798710f271384
CCing its authors.

Thanks,
Ian.