Bug#945404: grub2-common: '/boot/grub/.background_cache.png' is not created on LUKS encrypted system

[Gábor Gombás]

Package: grub2-common
Version: 2.04-9
Followup-For: Bug #945404

The reason is lack of LUKS2 support. Specifically, in
grub-core/osdep/devmapper/getroot.c, grub_util_get_dm_abstraction() has:

  if (strncmp (uuid, "CRYPT-LUKS1-", 12) == 0)
    {
      grub_free (uuid);
      return GRUB_DEV_ABSTRACTION_LUKS;
    }

That will not match LUKS2-encrypted volumes, so grub-probe thinks the
filesystem will be readable.

Although grub upstream has some LUKS2 support now (see
https://savannah.gnu.org/bugs/?55093), that does not seem to cover this
part of the code yet.