Add module for loading specific classes

This adds nova/loadables.py which contains code that is to be shared by
host and cell scheduling filtering and weighing.

Most of this code originated from nova/scheduler/filters/__init__.py
(which was copied from nova/api extension loading).

This makes it more generic so that it can be shared.  Note that this
functionality is quite different than the generic plugin manager that
exists in openstack-common.  That code could not be used here without
some significant changes.  It also seems pretty overkill for the
functionality required by scheduling filtering and weighing.

Change-Id: I1b217dc2bc2d1dc2235c8251318d06b46597e8f4

nova/loadables.py

@@ -0,0 +1,116 @@
+# Copyright (c) 2011-2012 OpenStack, LLC.
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+"""
+Generic Loadable class support.
+
+Meant to be used by such things as scheduler filters and weights where we
+want to load modules from certain directories and find certain types of
+classes within those modules.  Note that this is quite different than
+generic plugins and the pluginmanager code that exists elsewhere.
+
+Usage:
+
+Create a directory with an __init__.py with code such as:
+
+class SomeLoadableClass(object):
+    pass
+
+
+class MyLoader(nova.loadables.BaseLoader)
+    def __init__(self):
+        super(MyLoader, self).__init__(SomeLoadableClass)
+
+If you create modules in the same directory and subclass SomeLoadableClass
+within them, MyLoader().get_all_classes() will return a list
+of such classes.
+"""
+
+import inspect
+import os
+import sys
+
+from nova import exception
+from nova.openstack.common import importutils
+
+
+class BaseLoader(object):
+    def __init__(self, loadable_cls_type):
+        mod = sys.modules[self.__class__.__module__]
+        self.path = mod.__path__[0]
+        self.package = mod.__package__
+        self.loadable_cls_type = loadable_cls_type
+
+    def _is_correct_class(self, obj):
+        """Return whether an object is a class of the correct type and
+        is not prefixed with an underscore.
+        """
+        return (inspect.isclass(obj) and
+                (not obj.__name__.startswith('_')) and
+                issubclass(obj, self.loadable_cls_type))
+
+    def _get_classes_from_module(self, module_name):
+        """Get the classes from a module that match the type we want."""
+        classes = []
+        module = importutils.import_module(module_name)
+        for obj_name in dir(module):
+            # Skip objects that are meant to be private.
+            if obj_name.startswith('_'):
+                continue
+            itm = getattr(module, obj_name)
+            if self._is_correct_class(itm):
+                classes.append(itm)
+        return classes
+
+    def get_all_classes(self):
+        """Get the classes of the type we want from all modules found
+        in the directory that defines this class.
+        """
+        classes = []
+        for dirpath, dirnames, filenames in os.walk(self.path):
+            relpath = os.path.relpath(dirpath, self.path)
+            if relpath == '.':
+                relpkg = ''
+            else:
+                relpkg = '.%s' % '.'.join(relpath.split(os.sep))
+            for fname in filenames:
+                root, ext = os.path.splitext(fname)
+                if ext != '.py' or root == '__init__':
+                    continue
+                module_name = "%s%s.%s" % (self.package, relpkg, root)
+                mod_classes = self._get_classes_from_module(module_name)
+                classes.extend(mod_classes)
+        return classes
+
+    def get_matching_classes(self, loadable_class_names):
+        """Get loadable classes from a list of names.  Each name can be
+        a full module path or the full path to a method that returns
+        classes to use.  The latter behavior is useful to specify a method
+        that returns a list of classes to use in a default case.
+        """
+        classes = []
+        for cls_name in loadable_class_names:
+            obj = importutils.import_class(cls_name)
+            if self._is_correct_class(obj):
+                classes.append(obj)
+            elif inspect.isfunction(obj):
+                # Get list of classes from a function
+                for cls in obj():
+                    classes.append(cls)
+            else:
+                error_str = 'Not a class of the correct type'
+                raise exception.ClassNotFound(class_name=cls_name,
+                                              exception=error_str)
+        return classes