Regenerate missing resized backing files

Fixes bug 1185588

The only way to reliably determine whether we are using a legacy
(pre-Grizzly) backing structure is to look at the backing file
reference for the instance/disk itself. Thus, if we determine the
backing file contains an underscore, we use the value following it
as our legacy_backing_size, which is then used to create the
necessary backing file.

Change-Id: I145f71068c972a604e5361ae79d6315286136786

1 files changed, 25 insertions, 0 deletions

diff --git a/nova/virt/libvirt/imagebackend.py b/nova/virt/libvirt/imagebackend.py
index 3d394fb79..844d9dfde 100755
--- a/nova/virt/libvirt/imagebackend.py
+++ b/nova/virt/libvirt/imagebackend.py
@@ -250,8 +250,33 @@ class Qcow2(Image):
             if size:
                 disk.extend(target, size)
 
+        # Download the unmodified base image unless we already have a copy.
         if not os.path.exists(base):
             prepare_template(target=base, *args, **kwargs)
+
+        legacy_backing_size = None
+        legacy_base = base
+
+        # Determine whether an existing qcow2 disk uses a legacy backing by
+        # actually looking at the image itself and parsing the output of the
+        # backing file it expects to be using.
+        if os.path.exists(self.path):
+            backing_path = libvirt_utils.get_disk_backing_file(self.path)
+            backing_file = os.path.basename(backing_path)
+            backing_parts = backing_file.rpartition('_')
+            if backing_file != backing_parts[-1] and \
+                    backing_parts[-1].isdigit():
+                legacy_backing_size = int(backing_parts[-1])
+                legacy_base += '_%d' % legacy_backing_size
+                legacy_backing_size *= 1024 * 1024 * 1024
+
+        # Create the legacy backing file if necessary.
+        if legacy_backing_size:
+            if not os.path.exists(legacy_base):
+                with utils.remove_path_on_error(legacy_base):
+                    libvirt_utils.copy_image(base, legacy_base)
+                    disk.extend(legacy_base, legacy_backing_size)
+
         # NOTE(cfb): Having a flavor that sets the root size to 0 and having
         #            nova effectively ignore that size and use the size of the
         #            image is considered a feature at this time, not a bug.