NamedExtensionManager: call a callback when some names cannot be found

This will allow users to find out errors in their code that are currently
silently ignored.

Change-Id: I405511f9acc5319b1e4373be084f6a7c97556acf

stevedore/named.py

@@ -1,5 +1,9 @@
+import logging
+
 from .extension import ExtensionManager
 
+LOG = logging.getLogger(__name__)
+
 
 class NamedExtensionManager(ExtensionManager):
     """Loads only the named extensions.
@@ -34,6 +38,10 @@ class NamedExtensionManager(ExtensionManager):
         when this is called (when an entrypoint fails to load) are
         (manager, entrypoint, exception)
     :type on_load_failure_callback: function
+    :param on_missing_entrypoints_callback: Callback function that will be
+        called when one or more names cannot be found. The provided argument
+        will be a subset of the 'names' parameter.
+    :type on_missing_entrypoints_callback: function
     :param verify_requirements: Use setuptools to enforce the
         dependencies of the plugin(s) being loaded. Defaults to False.
     :type verify_requirements: bool
@@ -44,6 +52,7 @@ class NamedExtensionManager(ExtensionManager):
                  invoke_on_load=False, invoke_args=(), invoke_kwds={},
                  name_order=False, propagate_map_exceptions=False,
                  on_load_failure_callback=None,
+                 on_missing_entrypoints_callback=None,
                  verify_requirements=False):
         self._init_attributes(
             namespace, names, name_order=name_order,
@@ -53,6 +62,13 @@ class NamedExtensionManager(ExtensionManager):
                                         invoke_args,
                                         invoke_kwds,
                                         verify_requirements)
+        missing_entrypoints = set(names) - set([e.name for e in extensions])
+        if missing_entrypoints:
+            if on_missing_entrypoints_callback:
+                on_missing_entrypoints_callback(missing_entrypoints)
+            else:
+                LOG.warning('Could not load %s' %
+                            ', '.join(missing_entrypoints))
         self._init_plugins(extensions)
 
     @classmethod

stevedore/tests/test_callback.py

@@ -1,8 +1,10 @@
 """Tests for failure loading callback
 """
 from testtools.matchers import GreaterThan
+import mock
 
 from stevedore import extension
+from stevedore import named
 from stevedore.tests import utils
 
 
@@ -23,3 +25,19 @@ class TestCallback(utils.TestCase):
         for manager, entrypoint, error in errors:
             self.assertIs(manager, em)
             self.assertIsInstance(error, (IOError, ImportError))
+
+    @mock.patch('stevedore.named.NamedExtensionManager._load_plugins')
+    def test_missing_entrypoints_callback(self, load_fn):
+        errors = set()
+
+        def callback(names):
+            errors.update(names)
+
+        load_fn.return_value = [
+            extension.Extension('foo', None, None, None)
+        ]
+        named.NamedExtensionManager('stevedore.test.extension',
+                                    names=['foo', 'bar'],
+                                    invoke_on_load=True,
+                                    on_missing_entrypoints_callback=callback)
+        self.assertEqual(errors, {'bar'})