imagebackend: default by_name image_type to config correctly

Callers of by_name shouldn't need to pass in the default from CONF.

This has implications for the libvirt_imagebackend fixture. The
_mock_backend() method in the libvirt_imagebackend fixture is what
mocks out Backend.backend(). Before this patch, the defaulting of
image_type to CONF.libvirt.images_type was done *inside*
Backend.backend() whenever image_type=None was passed in. As of this
patch, this is no longer case, and the default value from CONF gets
passed in instead.

Unit tests were testing the previous behaviour and essentially just
asserting that None was passed in. This was never really correct to
begin with, so a couple of assertIsNone() assertions need to be
updated.

Change-Id: Ib95aa440ffff0989c90f09f5b5d4e26b30db9683

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

@@ -24631,7 +24631,7 @@ class LibvirtDriverTestCase(test.NoDBTestCase, TraitsComparisonMixin):
             self.assertEqual('raw', disk.image_type)
 
         # Assert that the root rescue disk was created as the default type
-        self.assertIsNone(disks['disk.rescue'].image_type)
+        self.assertEqual('default', disks['disk.rescue'].image_type)
 
         # We expect the generated domain to contain disk.rescue and
         # disk, in that order
@@ -24703,7 +24703,7 @@ class LibvirtDriverTestCase(test.NoDBTestCase, TraitsComparisonMixin):
             self.assertEqual('raw', disk.image_type)
 
         # Assert that the root rescue disk was created as the default type
-        self.assertIsNone(disks['disk.rescue'].image_type)
+        self.assertEqual('default', disks['disk.rescue'].image_type)
 
         # We expect the generated domain to contain disk.rescue, disk, and
         # disk.config.rescue in that order

nova/virt/libvirt/driver.py

@@ -4804,8 +4804,7 @@ class LibvirtDriver(driver.ComputeDriver):
             if size == 0 or suffix == '.rescue':
                 size = None
 
-            backend = self.image_backend.by_name(instance, 'disk' + suffix,
-                                                 CONF.libvirt.images_type)
+            backend = self.image_backend.by_name(instance, 'disk' + suffix)
             created_disks = not backend.exists()
 
             if instance.task_state == task_states.RESIZE_FINISH:
@@ -5357,6 +5356,11 @@ class LibvirtDriver(driver.ComputeDriver):
         self, instance, name, disk_mapping, flavor, image_type=None,
         boot_order=None,
     ):
+        # NOTE(artom) To pass unit tests, wherein the code here is loaded
+        # *before* any config with self.flags() is done, we need to have the
+        # default inline in the method, and not in the kwarg declaration.
+        if image_type is None:
+            image_type = CONF.libvirt.images_type
         disk_unit = None
         disk = self.image_backend.by_name(instance, name, image_type)
         if (name == 'disk.config' and image_type == 'rbd' and
@@ -5381,7 +5385,9 @@ class LibvirtDriver(driver.ComputeDriver):
             disk_unit=disk_unit, boot_order=boot_order)
         return conf
 
-    def _get_guest_fs_config(self, instance, name, image_type=None):
+    def _get_guest_fs_config(
+        self, instance, name, image_type=CONF.libvirt.images_type
+    ):
         disk = self.image_backend.by_name(instance, name, image_type)
         return disk.libvirt_fs_info("/", "ploop")
 
@@ -10721,8 +10727,7 @@ class LibvirtDriver(driver.ComputeDriver):
                 # Creating backing file follows same way as spawning instances.
                 cache_name = os.path.basename(info['backing_file'])
 
-                disk = self.image_backend.by_name(instance, instance_disk,
-                                                  CONF.libvirt.images_type)
+                disk = self.image_backend.by_name(instance, instance_disk)
                 if cache_name.startswith('ephemeral'):
                     # The argument 'size' is used by image.cache to
                     # validate disk size retrieved from cache against

nova/virt/libvirt/imagebackend.py

@@ -1311,6 +1311,10 @@ class Backend(object):
         :return: An Image object for the disk with given name and instance.
         :rtype: Image
         """
+        # NOTE(artom) To pass functional tests, wherein the code here is loaded
+        # *before* any config with self.flags() is done, we need to have the
+        # default inline in the method, and not in the kwarg declaration.
+        image_type = image_type or CONF.libvirt.images_type
         backend = self.backend(image_type)
         return backend(instance=instance, disk_name=name)