disk/vfs: ensure guestfs capabilities

Ensures that guestfs is available and well configured. Also
when checking for capabilities this commit introduces an in-memory
flag to avoid repeating the process if succeeded.
If we are able to load guestfs but something wrong happens a fatal
error is raised.

DocImpact: We should add a not for ubuntu like systems that use
libguestfs to execute the command 'update-guestfs-appliance' for
configuring it.

Closes-Bug: #1275267
Closes-Bug: #1157922
Change-Id: I97b3a23829ea1f3aadfe08ca6448b35010d2f312

nova/tests/virt/disk/vfs/fakeguestfs.py

@@ -54,6 +54,9 @@ class GuestFS(object):
 
         self.drives.append((file, kwargs['format']))
 
+    def add_drive(self, file, format=None, *args, **kwargs):
+        self.add_drive_opts(file, format=None, *args, **kwargs)
+
     def inspect_os(self):
         return ["/dev/guestvgf/lv_root"]
 

nova/virt/disk/vfs/api.py

@@ -12,6 +12,7 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+from nova import exception
 from nova.i18n import _LW
 from nova.openstack.common import importutils
 from nova.openstack.common import log as logging
@@ -21,31 +22,42 @@ LOG = logging.getLogger(__name__)
 
 class VFS(object):
 
+    # Class level flag to indicate whether we can consider
+    # that guestfs is ready to be used.
+    guestfs_ready = False
+
     @staticmethod
     def instance_for_image(imgfile, imgfmt, partition):
         LOG.debug("Instance for image imgfile=%(imgfile)s "
                   "imgfmt=%(imgfmt)s partition=%(partition)s",
                   {'imgfile': imgfile, 'imgfmt': imgfmt,
                    'partition': partition})
-        hasGuestfs = False
-        try:
-            LOG.debug("Trying to import guestfs")
-            importutils.import_module("guestfs")
-            hasGuestfs = True
-        except Exception:
-            pass
 
-        if hasGuestfs:
+        vfs = None
+        try:
             LOG.debug("Using primary VFSGuestFS")
-            return importutils.import_object(
+            vfs = importutils.import_object(
                 "nova.virt.disk.vfs.guestfs.VFSGuestFS",
                 imgfile, imgfmt, partition)
-        else:
-            LOG.warn(_LW("Unable to import guestfs, "
-                         "falling back to VFSLocalFS"))
-            return importutils.import_object(
-                "nova.virt.disk.vfs.localfs.VFSLocalFS",
-                imgfile, imgfmt, partition)
+            if not VFS.guestfs_ready:
+                # Inspect for capabilities and keep
+                # track of the result only if succeeded.
+                vfs.inspect_capabilities()
+                VFS.guestfs_ready = True
+            return vfs
+        except exception.NovaException:
+            if vfs is not None:
+                # We are able to load libguestfs but
+                # something wrong happens when trying to
+                # check for capabilities.
+                raise
+            else:
+                LOG.warn(_LW("Unable to import guestfs"
+                             "falling back to VFSLocalFS"))
+
+        return importutils.import_object(
+            "nova.virt.disk.vfs.localfs.VFSLocalFS",
+            imgfile, imgfmt, partition)
 
     """
     The VFS class defines an interface for manipulating files within

nova/virt/disk/vfs/guestfs.py

@@ -52,10 +52,26 @@ class VFSGuestFS(vfs.VFS):
 
         global guestfs
         if guestfs is None:
-            guestfs = importutils.import_module('guestfs')
+            try:
+                guestfs = importutils.import_module('guestfs')
+            except Exception as e:
+                raise exception.NovaException(
+                    _("libguestfs is not installed (%s)") % e)
 
         self.handle = None
 
+    def inspect_capabilities(self):
+        """Determines whether guestfs is well configured."""
+        try:
+            g = guestfs.GuestFS()
+            g.add_drive("/dev/null")  # sic
+            g.launch()
+        except Exception as e:
+            raise exception.NovaException(
+                _("libguestfs installed but not usable (%s)") % e)
+
+        return self
+
     def setup_os(self):
         if self.partition == -1:
             self.setup_os_inspect()