Bug#860833: 50mounted-tests hangs at dmsetup when using biosgrub partition on a left over fat32 partition

[Ivo De Decker <ivodd@debian.org>]

Hi Marga,

On Fri, Apr 21, 2017 at 09:44:42AM +0000, Marga Manterola wrote:
> I have a machine where I can reproduce this every time (since the partition
> never gets written to), so let me know if you want me to gather any additional
> information.

Thanks for investigating this in detail.

The changes introduced in os-prober 1.72 (just before the freeze) are meant to
provide a fix for #701814, which has been around for quite some time. The
dmsetup patch was tested on a regular installation. There probably wasn't that
much testing in d-i, which explains why your investigation shows it probably
don't work at all. The issue doesn't show up that often, because the code path
isn't used in most cases (see below).

The code path causing the issues in #701814 is used when grub-mount doesn't
work. At the time, grub-mount was only available on some architectures. See

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=701814#72

Now, grub-mount exists on all release architectures and is a dependency of
os-prober (via grub-common) and os-prober-udeb, so it will always be
available.

Looking at 50mounted-tests line 66:
https://sources.debian.net/src/os-prober/1.74/os-probes/common/50mounted-tests/#L66

The dmsetup code will only be used when grub-mount doesn't work on the device.
This means the dmsetup code path is only used when there is a partition that
can't be mounted by grub-mount (that was the case in #853277). Your tests show
something similar: there is a partition that cannot be mounted by grub-mount,
which means the dmsetup path is used. And that seems to be broken (at least in
d-i).

Given that grub-mount should be able to handle almost all common cases and
that the dmsetup path probably doesn't work at all in d-i, my proposal would
be to remove (or disable) the dmsetup code path and only rely on grub-mount.
This will still fix the original issue in #701814, and it will fix the issues
with the dmsetup code path (there is also #853927 and #853163). The 'cost' of
this change would be that devices which contain a partition that can't be
mounted by grub-mount, but can be mounted by the kernel, will no longer be
probed for other operating systems. I don't know if there are common examples
like that. Another option is to revert to the original code path without
dmsetup, but that would reintroduce #701814.

Cheers,

Ivo