[bug #38284] GRUB should provide option to swap partition order

[Ulf Zibis]

Follow-up Comment #1, bug #38284 (project grub):

Please forget, what I have written for grub.cfg. I wasn't aware, that
partitions normally are determined by their UUID, so the assignment should
work even if partitions are swapped.

So I suggest to extend the os-prober script similar like the following:
(should be controlled by a new etc/default/grub entry)

if [ x"${swapped_part_1}" = xy && x"${swapped_part_2}" = xy ]; then
  parttool $swapped_part_1 swap-order $swapped_part_2
fi
set swapped_part_1=
set swapped_part_2=
save_env swapped_part_1
save_env swapped_part_2

menuentry 'Windows XP on sda1' {
  ...
}

menuentry 'Windows XP OLD on sda3' {
  ...
  set swapped_part_1='hd0,msdos1'
  set swapped_part_2=${root}
  parttool $swapped_part_1 swap-order $swapped_part_2
  save_env swapped_part_1
  save_env swapped_part_2
  chainloader +1
}


==========================================================================
A first draft, of what should be added to grub-core/parttool/msdospart.c :
(I do not have any environment to build GRUB, so hopefully others have)
==========================================================================
<code>
static struct grub_parttool_argdesc grub_pcpart_swapargs[] =
{
  {"swap-order", N_("Swap the order in partition table"),
GRUB_PARTTOOL_ARG_VAL},
  {0, 0, 0}
};

static grub_err_t grub_pcpart_swap (const grub_device_t dev1,
                                    const struct grub_parttool_args *args)
{
  grub_device_t dev2 = parse (args[0]); // TODO: replace parse() by valid
code
  grub_partition_t part1, part2;
  int index1, index2;
  struct grub_msdos_partition_mbr mbr;
  struct grub_msdos_partition_entry entry;

  part1 = dev1->disk->partition;
  part2 = dev2->disk->partition;
  index1 = part1->index;
  index2 = part2->index;

  if (dev1->disk != dev2->disk)
    return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("partitions must be on same
disk"));
  if (index1 >= 4 || index2 >= 4)
    return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("logical partition not
allowed"));

  dev1->disk->partition = part1->parent;

  /* Read the MBR.  */
  if (grub_disk_read (dev1->disk, 0, 0, sizeof (mbr), &mbr))
  {
    dev1->disk->partition = part1;
    return grub_errno;
  }

  entry = mbr.entries[index1];
  mbr.entries[index1] = mbr.entries[index2];
  mbr.entries[index2] = entry;
  grub_printf_ (N_("Partitions %d, %d are swapped now. n"), index1, index2);

   /* Write the MBR.  */
  grub_disk_write (dev1->disk, 0, 0, sizeof (mbr), &mbr);

  dev1->disk->partition = part1;

  return GRUB_ERR_NONE;
}
</code>


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