Handling unexpected python error "NoneType object is not iterable"

The glance documentation states that None (empty image) is a valid
return value. We have to handle this outcome. Adding error handling
helps debugging this unhandled issue

Change-Id: If7c22ac4516f8c2a6ccd8bf6b6ed98409312b138
Closes-Bug: #1798147
(cherry picked from commit d3afc39467)

cinder/exception.py

@@ -727,6 +727,10 @@ class GlanceMetadataNotFound(NotFound):
     message = _("Glance metadata for volume/snapshot %(id)s cannot be found.")
 
 
+class ImageDownloadFailed(CinderException):
+    _msg_fmt = _("Failed to download image %(image_href)s, reason: %(reason)s")
+
+
 class ExportFailure(Invalid):
     message = _("Failed to export for volume: %(reason)s")
 

cinder/image/glance.py

@@ -357,6 +357,10 @@ class GlanceImageService(object):
         except Exception:
             _reraise_translated_image_exception(image_id)
 
+        if image_chunks is None:
+            raise exception.ImageDownloadFailed(
+                image_href=context, reason=_('image contains no data.'))
+
         if not data:
             return image_chunks
         else: