Make _rebase_with_qemu_img() generic

Move volume_delete related logic away from this method, in order to make
it generic and usable elsewhere.

Change-Id: I17357d85f845d4160cb7c7784772530a1e92af76
Related-Bug: #1732428
(cherry picked from commit ce22034)

nova/tests/unit/virt/libvirt/test_driver.py

@@ -25142,6 +25142,25 @@ def test_cache_image_existing(self, mock_fetch, mock_utime, mock_exists,
         mock_et.assert_not_called()
         mock_isdir.assert_not_called()
 
+    @mock.patch('nova.virt.images.qemu_img_info',
+                return_value=mock.Mock(file_format="fake_fmt"))
+    @mock.patch('oslo_concurrency.processutils.execute')
+    def test_rebase_with_qemu_img(self, mock_execute, mock_qemu_img_info):
+        """rebasing disk image to another backing file"""
+        self.drvr._rebase_with_qemu_img("disk", "backing_file")
+        mock_qemu_img_info.assert_called_once_with("backing_file")
+        mock_execute.assert_called_once_with('qemu-img', 'rebase',
+                                             '-b', 'backing_file', '-F',
+                                             'fake_fmt', 'disk')
+
+        # Flatten disk image when no backing file is given.
+        mock_qemu_img_info.reset_mock()
+        mock_execute.reset_mock()
+        self.drvr._rebase_with_qemu_img("disk", None)
+        self.assertEqual(0, mock_qemu_img_info.call_count)
+        mock_execute.assert_called_once_with('qemu-img', 'rebase',
+                                             '-b', '', 'disk')
+
 
 class LibvirtVolumeUsageTestCase(test.NoDBTestCase):
     """Test for LibvirtDriver.get_all_volume_usage."""

nova/virt/libvirt/driver.py

@@ -2934,32 +2934,15 @@ def _wait_for_snapshot():
         timer.start(interval=0.5).wait()
 
     @staticmethod
-    def _rebase_with_qemu_img(guest, device, active_disk_object,
-                              rebase_base):
-        """Rebase a device tied to a guest using qemu-img.
-
-        :param guest:the Guest which owns the device being rebased
-        :type guest: nova.virt.libvirt.guest.Guest
-        :param device: the guest block device to rebase
-        :type device: nova.virt.libvirt.guest.BlockDevice
-        :param active_disk_object: the guest block device to rebase
-        :type active_disk_object: nova.virt.libvirt.config.\
-                                    LibvirtConfigGuestDisk
+    def _rebase_with_qemu_img(source_path, rebase_base):
+        """Rebase a disk using qemu-img.
+
+        :param source_path: the disk source path to rebase
+        :type source_path: string
         :param rebase_base: the new parent in the backing chain
         :type rebase_base: None or string
         """
 
-        # It's unsure how well qemu-img handles network disks for
-        # every protocol. So let's be safe.
-        active_protocol = active_disk_object.source_protocol
-        if active_protocol is not None:
-            msg = _("Something went wrong when deleting a volume snapshot: "
-                    "rebasing a %(protocol)s network disk using qemu-img "
-                    "has not been fully tested") % {'protocol':
-                    active_protocol}
-            LOG.error(msg)
-            raise exception.InternalError(msg)
-
         if rebase_base is None:
             # If backing_file is specified as "" (the empty string), then
             # the image is rebased onto no backing file (i.e. it will exist
@@ -2974,7 +2957,7 @@ def _rebase_with_qemu_img(guest, device, active_disk_object,
             b_file_fmt = images.qemu_img_info(backing_file).file_format
             qemu_img_extra_arg = ['-F', b_file_fmt]
 
-        qemu_img_extra_arg.append(active_disk_object.source_path)
+        qemu_img_extra_arg.append(source_path)
         # execute operation with disk concurrency semaphore
         with compute_utils.disk_ops_semaphore:
             processutils.execute("qemu-img", "rebase", "-b", backing_file,
@@ -3123,7 +3106,18 @@ def _get_snap_dev(filename, backing_store):
             else:
                 LOG.debug('Guest is not running so doing a block rebase '
                           'using "qemu-img rebase"', instance=instance)
-                self._rebase_with_qemu_img(guest, dev, active_disk_object,
+
+                # It's unsure how well qemu-img handles network disks for
+                # every protocol. So let's be safe.
+                active_protocol = active_disk_object.source_protocol
+                if active_protocol is not None:
+                    msg = _("Something went wrong when deleting a volume "
+                            "snapshot: rebasing a %(protocol)s network disk "
+                            "using qemu-img has not been fully tested"
+                           ) % {'protocol': active_protocol}
+                    LOG.error(msg)
+                    raise exception.InternalError(msg)
+                self._rebase_with_qemu_img(active_disk_object.source_path,
                                            rebase_base)
 
         else: