fix(xcat-core): the Ubuntu POWER diskful install never leaves the installer

reg_linux_diskfull_installation_flat fails on ubuntu-22-ppc64le-devel and
ubuntu-24-ppc64le-devel in build #121 of xcat-core-devel-ubuntu-cd. The
Subiquity installer starts, errors in its early-commands, tars
/var/log/installer to port 8080 and reboots, nine times in 75 minutes. No
system is ever installed, so the address answers from the live installer and
the case ends on
"root@xcat25-cn: Permission denied (publickey,password)".

mkinstall in xCAT-server/lib/xcat/plugins/debian.pm selected
pre.ubuntu.subiquity and then replaced it with pre.ubuntu.ppc64 for every
ppc64 node, whichever installer was in use. pre.ubuntu.ppc64 writes a partman
recipe, and the early-commands append it to /autoinstall.yaml, which Subiquity
cannot parse.

pre.ubuntu.subiquity offered a UEFI branch and a BIOS branch, so a ppc64el node
took the BIOS branch and was given a bios_grub partition. POWER firmware loads
the boot loader from a PReP partition.

install_prescript now returns the script from the installer and the
architecture together, and the ppc64 script is reached only on the
debian-installer path. pre.ubuntu.subiquity gains a PReP branch, taken when
uname reports a POWER machine, which flags an 8M first partition prep and makes
that partition the grub device.

debian_install_prescript.t and the PReP case of ubuntu_subiquity_storage.t fail
without these changes.

Signed-off-by: Daniel Hilst <392820+dhilst@users.noreply.github.com>

xCAT-server/lib/xcat/plugins/debian.pm

@@ -383,6 +383,32 @@ sub _no_grub2_loader {
 
 #-------------------------------------------------------
 
+=head3  install_prescript
+
+    Descriptions: Return the pre-install script an Ubuntu or Debian install runs.
+    Arguments:
+        $platform   - the distribution family, for example ubuntu
+        $arch       - the architecture of the node
+        $subiquity  - true when the osimage template is a Subiquity autoinstall
+    Returns: the full path of the pre-install script
+
+=cut
+
+#-------------------------------------------------------
+sub install_prescript
+{
+    my ($platform, $arch, $subiquity) = @_;
+    my $base = "$::XCATROOT/share/xcat/install/scripts/pre.$platform";
+
+    # pre.ubuntu.ppc64 writes a partman recipe, which only the debian-installer
+    # reads. Subiquity gets its POWER partitioning from pre.ubuntu.subiquity.
+    return "$base.subiquity" if ($subiquity);
+    return "$base.ppc64" if (defined($arch) and $arch =~ /ppc64/i and $platform eq "ubuntu");
+    return $base;
+}
+
+#-------------------------------------------------------
+
 =head3  install_media_is_bootable
 
     Descriptions: Report whether copied media carries an install kernel and initrd.
@@ -1155,18 +1181,10 @@ sub mkinstall {
               );
         }
 
-        # maybe Debian will decide to use subiquity at some point?
-        my $prescript = "$::XCATROOT/share/xcat/install/scripts/pre.$platform";
-        if (using_subiquity($os,$tmplfile)) {
-            $prescript = $prescript . ".subiquity";
-        }
+        my $prescript =
+          install_prescript($platform, $arch, using_subiquity($os, $tmplfile));
         my $postscript = "$::XCATROOT/share/xcat/install/scripts/post.$platform";
 
-        # for powerkvm VM ubuntu LE#
-        if ($arch =~ /ppc64/i and $platform eq "ubuntu") {
-            $prescript = "$::XCATROOT/share/xcat/install/scripts/pre.$platform.ppc64";
-        }
-
 
         if (-r "$prescript") {
             $preerr = xCAT::Template->subvars($prescript,

xCAT-server/share/xcat/install/scripts/pre.ubuntu.subiquity

@@ -278,6 +278,60 @@ storage:
       path: /
       device: root-part-fs
 EOF
+elif [ "$(uname -m)" = "ppc64le" ] || [ "$(uname -m)" = "ppc64" ]; then
+        # POWER firmware reads neither an ESP nor a bios_grub partition. It loads the
+        # boot loader from a PReP partition, and curtin writes grub to the partition
+        # flagged prep, not to the disk.
+        cat <<EOF >/tmp/partitionfile
+storage:
+  version: 1
+  config:
+    - id: disk-detected
+      type: disk
+      ptable: gpt
+      path: $INSTALL_DISK
+      wipe: superblock-recursive
+      preserve: false
+    - id: prep-part
+      type: partition
+      device: disk-detected
+      size: 8M
+      flag: prep
+      number: 1
+      preserve: false
+      grub_device: true
+    - id: swap-part
+      type: partition
+      device: disk-detected
+      size: 2G
+      number: 2
+      preserve: false
+    - id: root-part
+      type: partition
+      device: disk-detected
+      size: -1
+      wipe: superblock
+      number: 3
+      preserve: false
+    - id: swap-part-fs
+      type: format
+      fstype: swap
+      volume: swap-part
+      preserve: false
+    - id: root-part-fs
+      type: format
+      fstype: ext4
+      volume: root-part
+      preserve: false
+    - id: swap-part-mount
+      type: mount
+      path: none
+      device: swap-part-fs
+    - id: root-part-mount
+      type: mount
+      path: /
+      device: root-part-fs
+EOF
 else
         cat <<EOF >/tmp/partitionfile
 storage: