improve handling of the img_handlers config list

Allow a 'loop' entry to be optional.
Also improve the diagnostic with an empty
or non capable list.

Change-Id: Iec395497e43d1e84adbfc5f29a7bce82d4f91587

1 files changed, 4 insertions, 1 deletions

diff --git a/nova/virt/disk/api.py b/nova/virt/disk/api.py
index 47d56c954..9a457a318 100644
--- a/nova/virt/disk/api.py
+++ b/nova/virt/disk/api.py
@@ -120,9 +120,12 @@ class _DiskImage(object):
         # As a performance tweak, don't bother trying to
         # directly loopback mount a cow image.
         self.handlers = FLAGS.img_handlers[:]
-        if use_cow:
+        if use_cow and 'loop' in self.handlers:
             self.handlers.remove('loop')
 
+        if not self.handlers:
+            raise exception.Error(_('no capable image handler configured'))
+
     @property
     def errors(self):
         """Return the collated errors from all operations."""