Add sort_mount_point method

As this method can be introduced without any dependency,
provide it on an independent change to simplify reviews.
This is a partial refactor based on
I592c0b1329409307197460cfa8fd69798013f1f8

Change-Id: Idaf3d2b3b3e23d0b9d6bc071d67b961a829ae422
Co-Authored-By: Andreas Florath <andreas@florath.net>

diskimage_builder/block_device/utils.py

@@ -97,3 +97,23 @@ def exec_sudo(cmd):
         logger.error("Calling [%s] failed with [%s]" %
                      (" ".join(sudo_cmd), rval))
     return rval
+
+
+def sort_mount_points(mount_points):
+    logger.debug("sort_mount_points called [%s]" % mount_points)
+
+    def insert_sorted(mp, sorted_mount_points):
+        if len(sorted_mount_points) == 0:
+            sorted_mount_points.append(mp)
+            return
+        for idx in range(0, len(sorted_mount_points)):
+            if sorted_mount_points[idx].startswith(mp):
+                sorted_mount_points.insert(idx, mp)
+                return
+        sorted_mount_points.append(mp)
+
+    sorted_mount_points = []
+    for mp in mount_points:
+        insert_sorted(mp, sorted_mount_points)
+    logger.debug("sort_mount_points result [%s]" % sorted_mount_points)
+    return sorted_mount_points

diskimage_builder/tests/functional/test_blockdevice_utils.py

@@ -15,6 +15,7 @@
 
 from diskimage_builder.block_device.utils import parse_abs_size_spec
 from diskimage_builder.block_device.utils import parse_rel_size_spec
+from diskimage_builder.block_device.utils import sort_mount_points
 import testtools
 
 
@@ -47,3 +48,9 @@ class TestBlockDeviceUtils(testtools.TestCase):
         """Call parse_abs_size_spec with a completely broken unit spec"""
 
         self.assertRaises(RuntimeError, parse_abs_size_spec, "_+!HuHi+-=")
+
+    def test_sort_mount_points(self):
+        """Run sort_mount_points with a set of paths"""
+
+        smp = sort_mount_points(["/boot", "/", "/var/tmp", "/var"])
+        self.assertEqual(['/', '/boot', '/var', '/var/tmp'], smp)