Always use bdm in instance_block_mapping on Xen

It doesn't seem that Xen will set the 'root_device_name' property on
instances.  This is causing instance_block_mapping to return early
without evaluating the bdm before returning the list of device_names
in use.  I'm not sure why instance_block_mapping does this
(optimization?) but on Xen at least it seems that it should not.

Added a check for Xen compute_driver flag in instance_block_mapping to
"guess" the root_device_name (similarlly to what is done in
compute.utils for swap and ephemeral).

fixes bug #1061944

Change-Id: If5b1a2b7377232c78f0629a3624552ecf6ceb0ee
(cherry picked from commit 07845ad)

nova/block_device.py

@@ -17,6 +17,9 @@
 
 import re
 
+from nova import flags
+
+FLAGS = flags.FLAGS
 
 DEFAULT_ROOT_DEV_NAME = '/dev/sda1'
 _DEFAULT_MAPPINGS = {'ami': 'sda1',
@@ -89,8 +92,12 @@ def strip_prefix(device_name):
 
 def instance_block_mapping(instance, bdms):
     root_device_name = instance['root_device_name']
+    # NOTE(clayg): remove this when xenapi is setting default_root_device
     if root_device_name is None:
-        return _DEFAULT_MAPPINGS
+        if FLAGS.compute_driver.endswith('xenapi.XenAPIDriver'):
+            root_device_name = '/dev/xvda'
+        else:
+            return _DEFAULT_MAPPINGS
 
     mappings = {}
     mappings['ami'] = strip_dev(root_device_name)

nova/tests/compute/test_compute_utils.py

@@ -44,12 +44,21 @@ class ComputeValidateDeviceTestCase(test.TestCase):
     def setUp(self):
         super(ComputeValidateDeviceTestCase, self).setUp()
         self.context = context.RequestContext('fake', 'fake')
-        self.instance = {
-                'uuid': 'fake',
-                'root_device_name': '/dev/vda',
-                'default_ephemeral_device': '/dev/vdb',
-                'instance_type_id': 'fake',
-        }
+        # check if test name includes "xen"
+        if 'xen' in self.id():
+            self.flags(compute_driver='xenapi.XenAPIDriver')
+            self.instance = {
+                    'uuid': 'fake',
+                    'root_device_name': None,
+                    'instance_type_id': 'fake',
+            }
+        else:
+            self.instance = {
+                    'uuid': 'fake',
+                    'root_device_name': '/dev/vda',
+                    'default_ephemeral_device': '/dev/vdb',
+                    'instance_type_id': 'fake',
+            }
         self.data = []
 
         def fake_get(instance_type_id, ctxt=None):
@@ -150,8 +159,6 @@ def test_swap_no_ephemeral(self):
         self.assertEqual(device, '/dev/vdc')
 
     def test_ephemeral_xenapi(self):
-        self.flags(compute_driver='xenapi.XenAPIDriver')
-        del self.instance['default_ephemeral_device']
         self.instance_type = {
             'ephemeral_gb': 10,
             'swap': 0,
@@ -162,8 +169,6 @@ def test_ephemeral_xenapi(self):
         self.assertEqual(device, '/dev/xvdc')
 
     def test_swap_xenapi(self):
-        self.flags(compute_driver='xenapi.XenAPIDriver')
-        del self.instance['default_ephemeral_device']
         self.instance_type = {
             'ephemeral_gb': 0,
             'swap': 10,
@@ -174,8 +179,6 @@ def test_swap_xenapi(self):
         self.assertEqual(device, '/dev/xvdb')
 
     def test_swap_and_ephemeral_xenapi(self):
-        self.flags(compute_driver='xenapi.XenAPIDriver')
-        del self.instance['default_ephemeral_device']
         self.instance_type = {
             'ephemeral_gb': 10,
             'swap': 10,
@@ -185,6 +188,19 @@ def test_swap_and_ephemeral_xenapi(self):
         device = self._validate_device()
         self.assertEqual(device, '/dev/xvdd')
 
+    def test_swap_and_one_attachment_xenapi(self):
+        self.instance_type = {
+            'ephemeral_gb': 0,
+            'swap': 10,
+        }
+        self.stubs.Set(instance_types, 'get_instance_type',
+                       lambda instance_type_id, ctxt=None: self.instance_type)
+        device = self._validate_device()
+        self.assertEqual(device, '/dev/xvdb')
+        self.data.append(self._fake_bdm(device))
+        device = self._validate_device()
+        self.assertEqual(device, '/dev/xvdd')
+
 
 class UsageInfoTestCase(test.TestCase):