Fix exception catch when volume mount fails

When we call nova.privsep.fs.mount(), we are catching
ProcessExecutionError() (an instance of a class) and that is not allowed
in python3.

This changes the except statement to catch ProcessExecutionError
(class) instead.

Closes-Bug: #1984736

Change-Id: I24a269f9809063f864a4f0443d6724dfa8703d9d
(cherry picked from commit f996674)

Author: melwitt

nova/tests/unit/virt/libvirt/volume/test_mount.py

@@ -580,6 +580,32 @@ def test_umount_log_failure(self, mock_log):
 
         mock_log.assert_called()
 
+    @mock.patch.object(mount.LOG, 'exception')
+    def test_mount_failure(self, mock_log_exc):
+        m = self._get_clean_hostmountstate()
+        err = processutils.ProcessExecutionError
+        self.mock_mount.side_effect = err
+
+        # Mount vol_a
+        self.assertRaises(err, self._sentinel_mount, m, mock.sentinel.vol_a)
+
+        # Verify the mountpoint got removed after the failure
+        self.assertEqual({}, m.mountpoints)
+
+        # Now try a scenario where the mount failed because the volume was
+        # already mounted
+        self.mock_ismount.side_effect = [False, True]
+
+        # Mount vol_a
+        self._sentinel_mount(m, mock.sentinel.vol_a)
+
+        # Verify the mountpoint is present despite the error
+        self.assertEqual(1, len(m.mountpoints))
+        self.assertIn(mock.sentinel.mountpoint, m.mountpoints)
+
+        # Verify we logged an exception
+        mock_log_exc.assert_called()
+
 
 class MountManagerTestCase(test.NoDBTestCase):
     class FakeHostMountState(object):

nova/virt/libvirt/volume/mount.py

@@ -306,7 +306,7 @@ def mount(self, fstype, export, vol_name, mountpoint, instance, options):
 
                 try:
                     nova.privsep.fs.mount(fstype, export, mountpoint, options)
-                except processutils.ProcessExecutionError():
+                except processutils.ProcessExecutionError:
                     # Check to see if mountpoint is mounted despite the error
                     # eg it was already mounted
                     if os.path.ismount(mountpoint):