Add a get_root_bdm utility function

Getting the root block device out of a list of block devices is a common
idiom and we use it in several places in our code base. This patch
factors it out in a handy utilty method and adds tests for it.

Change-Id: I4a2cb3c689fed9fbc0bc51ddba095e1269cacb2d
blueprint: clean-up-legacy-block-device-mapping

nova/block_device.py

@@ -388,6 +388,13 @@ def new_format_is_ephemeral(bdm):
     return False
 
 
+def get_root_bdm(bdms):
+    try:
+        return (bdm for bdm in bdms if bdm.get('boot_index', -1) == 0).next()
+    except StopIteration:
+        return None
+
+
 def mappings_prepend_dev(mappings):
     """Prepend '/dev/' to 'device' entry of swap/ephemeral virtual type."""
     for m in mappings:

nova/compute/manager.py

@@ -1297,10 +1297,8 @@ class ComputeManager(manager.SchedulerDependentManager):
         It also ensures that there is a root_device_name and is set to the
         first block device in the boot sequence (boot_index=0).
         """
-        try:
-            root_bdm = (bdm for bdm in block_devices
-                        if bdm['boot_index'] == 0).next()
-        except StopIteration:
+        root_bdm = block_device.get_root_bdm(block_devices)
+        if not root_bdm:
             return
 
         # Get the root_device_name from the root BDM or the instance

nova/tests/test_block_device.py

@@ -129,6 +129,19 @@ class BlockDeviceTestCase(test.NoDBTestCase):
         _assert_volume_in_mapping('sdg', False)
         _assert_volume_in_mapping('sdh1', False)
 
+    def test_get_root_bdm(self):
+        root_bdm = {'device_name': 'vda', 'boot_index': 0}
+        bdms = [root_bdm,
+                {'device_name': 'vdb', 'boot_index': 1},
+                {'device_name': 'vdc', 'boot_index': -1},
+                {'device_name': 'vdd'}]
+        self.assertEqual(root_bdm, block_device.get_root_bdm(bdms))
+        self.assertEqual(root_bdm, block_device.get_root_bdm([bdms[0]]))
+        self.assertEqual(None, block_device.get_root_bdm(bdms[1:]))
+        self.assertEqual(None, block_device.get_root_bdm(bdms[2:]))
+        self.assertEqual(None, block_device.get_root_bdm(bdms[3:]))
+        self.assertEqual(None, block_device.get_root_bdm([]))
+
 
 class TestBlockDeviceDict(test.NoDBTestCase):
     def setUp(self):

nova/virt/libvirt/blockinfo.py

@@ -517,15 +517,11 @@ def get_disk_mapping(virt_type, instance,
 
         return mapping
 
-    try:
-        root_bdm = (bdm for bdm in
-                    driver.block_device_info_get_mapping(block_device_info)
-                    if bdm.get('boot_index') == 0).next()
-    except StopIteration:
-        # NOTE (ndipanov): This happens when we boot from image as
-        # there is no driver representation of local targeted images
-        # and they will not be in block_device_info list.
-        root_bdm = None
+    # NOTE (ndipanov): root_bdm can be None when we boot from image
+    # as there is no driver represenation of local targeted images
+    # and they will not be in block_device_info list.
+    root_bdm = block_device.get_root_bdm(
+        driver.block_device_info_get_mapping(block_device_info))
 
     root_device_name = block_device.strip_dev(
         driver.block_device_info_get_root(block_device_info))