mounter: use custom exception for absolute path error

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

subiquity/server/mounter.py

@@ -51,11 +51,15 @@ class OverlayCleanupError(Exception):
     """Exception to raise when an overlay could not be cleaned up."""
 
 
+class AbsolutePathError(Exception):
+    pass
+
+
 class _MountBase:
     def p(self, *args: str) -> str:
         for a in args:
             if a.startswith("/"):
-                raise Exception("no absolute paths here please")
+                raise AbsolutePathError("no absolute paths here please")
         return os.path.join(self.mountpoint, *args)
 
     def write(self, path, content):

subiquity/server/tests/test_mounter.py

@@ -18,15 +18,33 @@ import pathlib
 from unittest.mock import AsyncMock, Mock, call, patch
 
 from subiquity.server.mounter import (
+    AbsolutePathError,
     Mounter,
     Mountpoint,
     OverlayMountpoint,
+    _MountBase,
     lowerdir_for,
 )
 from subiquitycore.tests import SubiTestCase
 from subiquitycore.tests.mocks import make_app
 
 
+class Test_MountBase(SubiTestCase):
+    def setUp(self):
+        self.mountbase = _MountBase()
+        self.mountbase.mountpoint = "/target"
+
+    def test_p(self):
+        mnt = self.mountbase
+
+        self.assertEqual("/target/d1", mnt.p("d1"))
+        self.assertEqual("/target/d1/d2/d3/d4", mnt.p("d1", "d2/d3", "d4"))
+
+    def test_p__absolute(self):
+        with self.assertRaises(AbsolutePathError):
+            self.mountbase.p("a", "/b")
+
+
 class TestMounter(SubiTestCase):
     def setUp(self):
         self.model = Mock()