Bugfix:

Defining recursive dependent names in IConfigurationPlugin 
dependencies, ends in recursive plugin lookup.

- Implemented check for recursive names
- Added unit test for recursive defined dependency names.

Thanks to Fred Drake

TODO:
Egg this!

src/z3c/configurator/CHANGES.txt

@@ -22,6 +22,9 @@ New features
 Bug fixes
 ---------
 
+- Defining recursive dependent names in IConfigurationPlugin dependencies,
+  ends in recursive plugin lookup.
+
 - SchemaConfigurationPluginBase now implements
   ISchemaConfigurationPluginBase.
 

src/z3c/configurator/README.txt

@@ -206,3 +206,44 @@ The same must happen for a schema base configurator.
   ...
   NotImplementedError
 
+
+No Recursion
+------------
+
+It's possible to define recursive dependencies without to run into recursion 
+errors. Let's define a new plugin free object:
+
+  >>> class IFoo(zope.interface.Interface):
+  ...     """Just a foo interface."""
+
+  >>> class Foo(object):
+  ...     """Implementation of foo."""
+  ...     zope.interface.implements(IFoo)
+
+Let's define another plugin named `first` which depends on a plugin named 
+`second`.
+
+  >>> class FirstPlugin(configurator.ConfigurationPluginBase):
+  ...     zope.component.adapts(IFoo)
+  ...     dependencies = ('second',)
+  ...
+  ...     def __call__(self, data):
+  ...         print 'FirstPlugin called'
+
+  >>> zope.component.provideAdapter(FirstPlugin, name='first')
+
+And define a plugin named `second` which depends on `first`:
+
+  >>> class SecondPlugin(configurator.ConfigurationPluginBase):
+  ...     zope.component.adapts(IFoo)
+  ...     dependencies = ('first',)
+  ...
+  ...     def __call__(self, data):
+  ...         print 'SecondPlugin called'
+
+  >>> zope.component.provideAdapter(SecondPlugin, name='second')
+
+  >>> foo = Foo()
+  >>> configurator.configure(foo, {})
+  FirstPlugin called
+  SecondPlugin called

src/z3c/configurator/configurator.py

@@ -54,14 +54,18 @@ def requiredPlugins(component, names=[]):
     # interfaces may change during execution
     plugins = getAdapterFactories(component,
                                   specific=False)
-    
+
     def _add(name, res):
+        if name in seen:
+            return
+        seen.add(name)
         deps = getattr(plugins[name], 'dependencies', ())
         for dep in deps:
             if not dep in res:
                 _add(dep, res)
         if name not in res:
             res.append(name)
+    seen = set()
     res = []
     for name in names:
         _add(name, res)