Fixes error on live-migration of volume-backed vm

Live-migrating a volume-backed vm (i.e. booted from volume)
is currently broken.

This patch fixes the case where a volume-backed vm is to be
live-migrated without shared storage on compute nodes or
ephemeral volumes attached to the instance. Specifically, it
stops create_images_and_backing() from blowing up when no disk
info is supplied.

Change-Id: Icec7a6e7225ebe029e24d3be303c9ab01818f30e
Fixes: bug 1235182
(cherry picked from commit 0cbb231cd1)

nova/tests/virt/libvirt/test_libvirt.py

@@ -2997,6 +2997,34 @@ class LibvirtConnTestCase(test.TestCase):
 
         db.instance_destroy(self.context, instance_ref['uuid'])
 
+    def test_create_images_and_backing(self):
+        conn = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
+        self.mox.StubOutWithMock(conn, '_fetch_instance_kernel_ramdisk')
+        self.mox.StubOutWithMock(libvirt_driver.libvirt_utils, 'create_image')
+
+        libvirt_driver.libvirt_utils.create_image(mox.IgnoreArg(),
+                                                  mox.IgnoreArg(),
+                                                  mox.IgnoreArg())
+        conn._fetch_instance_kernel_ramdisk(self.context, self.test_instance)
+        self.mox.ReplayAll()
+
+        self.stubs.Set(os.path, 'exists', lambda *args: False)
+        disk_info_json = jsonutils.dumps([{'path': 'foo', 'type': None,
+                                           'disk_size': 0,
+                                           'backing_file': None}])
+        conn._create_images_and_backing(self.context, self.test_instance,
+                                        "/fake/instance/dir", disk_info_json)
+
+    def test_create_images_and_backing_disk_info_none(self):
+        conn = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), False)
+        self.mox.StubOutWithMock(conn, '_fetch_instance_kernel_ramdisk')
+
+        conn._fetch_instance_kernel_ramdisk(self.context, self.test_instance)
+        self.mox.ReplayAll()
+
+        conn._create_images_and_backing(self.context, self.test_instance,
+                                        "/fake/instance/dir", None)
+
     def test_pre_live_migration_works_correctly_mocked(self):
         # Creating testdata
         vol = {'block_device_mapping': [

nova/virt/libvirt/driver.py

@@ -4169,7 +4169,10 @@ class LibvirtDriver(driver.ComputeDriver):
             json strings specified in get_instance_disk_info
 
         """
-        disk_info = jsonutils.loads(disk_info_json)
+        if not disk_info_json:
+            disk_info = []
+        else:
+            disk_info = jsonutils.loads(disk_info_json)
 
         for info in disk_info:
             base = os.path.basename(info['path'])