Remedies LP Bug #928910 - Use libvirt lookupByName() to check existence

Make determining if an instance exists on a host
more efficient by adding an instance_exists() method to the
base virt driver that can be overridden by drivers that
have a more efficient mechanism of looking up an instance
by its ID / name. Modifies the _check_instance_already_created
method of the compute manager to use this new instance_exists() method.

Someone from Citrix should look into how to make the instance_exists()
method in the Xen and VMWare virt drivers more efficient than the
base "loop over all domains and see if the instance ID exists" method
now in the base driver class.

Change-Id: Ibf219788f9c104698057367da89300a060945778

nova/compute/manager.py

@@ -440,7 +440,7 @@ def _run_instance(self, context, instance_uuid,
 
     def _check_instance_not_already_created(self, context, instance):
         """Ensure an instance with the same name is not already present."""
-        if instance['name'] in self.driver.list_instances():
+        if self.driver.instance_exists(instance['name']):
             raise exception.Error(_("Instance has already been created"))
 
     def _check_image_size(self, context, instance):

nova/virt/driver.py

@@ -114,6 +114,21 @@ def get_info(self, instance_name):
         # TODO(Vek): Need to pass context in for access to auth_token
         raise NotImplementedError()
 
+    def instance_exists(self, instance_id):
+        """Checks existence of an instance on the host.
+
+        Returns True if an instance with the supplied ID exists on
+        the host, False otherwise.
+
+        :note This implementation works for all drivers, but it is
+              not particularly efficient. Maintainers of the virt drivers are
+              encouraged to override this method with something more
+              efficient.
+
+        :param instance_id: The ID / name of the instance to lookup
+        """
+        return instance_id in self.list_instances()
+
     def list_instances(self):
         """
         Return the names of all the instances known to the virtualization

nova/virt/libvirt/connection.py

@@ -281,6 +281,14 @@ def _connect(uri, read_only):
         else:
             return libvirt.openAuth(uri, auth, 0)
 
+    def instance_exists(self, instance_id):
+        """Efficient override of base instance_exists method."""
+        try:
+            _ignored = self._conn.lookupByName(instance_id)
+            return True
+        except libvirt.libvirtError:
+            return False
+
     def list_instances(self):
         return [self._conn.lookupByID(x).name()
                 for x in self._conn.listDomainsID()