utils: add functions to obtain disk and partition UUIDs

Add get_partition_id and get_disk_id to obtain the UUID of a disk
or partition as a string. These values are obtained invoking the
program blkid as a subprocess.

This commit is preparatory work for the new native postinstall code.

src/utils/disk.py

@@ -8,6 +8,8 @@
 
 import os
 import logging
+import shlex
+import subprocess
 from src.log import OgError
 
 import fdisk
@@ -69,3 +71,26 @@ def get_efi_partition(disknum, enforce_gpt):
         if pa.type.name == 'EFI System':
             return cxt.partition_to_string(pa, fdisk.FDISK_FIELD_DEVICE), f'/dev/{disk}', pa.partno + 1
     raise OgError(f'Cannot find "EFI System" partition at /dev/{disk}')
+
+
+def get_partition_id(disk_index, part_index):
+    device = get_partition_device(disk_index, part_index)
+    cmd = f'blkid -s PARTUUID -o value {device}'
+    proc = subprocess.run(shlex.split(cmd),
+                          stdout=subprocess.PIPE,
+                          encoding='utf-8')
+    if proc.returncode != 0:
+            raise OgError(f'failed to query partition UUID for {device}')
+    return proc.stdout.strip()
+
+
+def get_disk_id(disk_index):
+    disk = get_disks()[disk_index - 1]
+    disk_path = f'/dev/{disk}'
+    cmd = f'blkid -s PTUUID -o value {disk_path}'
+    proc = subprocess.run(shlex.split(cmd),
+                          stdout=subprocess.PIPE,
+                          encoding='utf-8')
+    if proc.returncode != 0:
+            raise OgError(f'failed to query disk UUID for {disk_path}')
+    return proc.stdout.strip()