=== trac/core.py
==================================================================
--- trac/core.py	(revision 6827)
+++ trac/core.py	(local)
@@ -18,8 +18,8 @@
 
 from trac.util import TracError
 
-__all__ = ['Component', 'ExtensionPoint', 'implements', 'Interface',
-           'TracError']
+__all__ = ['Component', 'ExtensionPoint', 'SingletonExtensionPoint',
+           'implements', 'Interface', 'TracError']
 
 
 class Interface(object):
@@ -51,6 +51,24 @@
         return '<ExtensionPoint %s>' % self.interface.__name__
 
 
+class SingletonExtensionPoint(property):
+    def __init__(self, interface, cfg_section, cfg_property, default=None):
+        property.__init__(self, self.implementation)
+        self.xtnpt = ExtensionPoint(interface)
+        self.cfg_section = cfg_section
+        self.cfg_property = cfg_property
+        self.default = default
+
+    def implementation(self, component):
+        cfgvalue = component.config.get(self.cfg_section, self.cfg_property)
+        for impl in self.xtnpt.extensions(component):
+            if impl.__class__.__name__ == cfgvalue:
+                return impl
+        if self.default is not None:
+            return self.default
+        raise AttributeError, "Can't find implementation"
+
+
 class ComponentMeta(type):
     """Meta class for components.
     
=== trac/perm.py
==================================================================
--- trac/perm.py	(revision 6827)
+++ trac/perm.py	(local)
@@ -78,7 +78,7 @@
     implements(IPermissionRequestor)
 
     requestors = ExtensionPoint(IPermissionRequestor)
-    stores = ExtensionPoint(IPermissionStore)
+    store = SingletonExtensionPoint(IPermissionStore, 'trac', 'permission_store')
 
     # Public API
 
@@ -160,19 +160,7 @@
                     actions.append(action)
         return [('TRAC_ADMIN', actions)]
 
-    # Internal methods
 
-    def _get_store(self):
-        """Return the `IPermissionStore` implementation selected in the
-        configuration."""
-        selected_store = self.config.get('trac', 'permission_store')
-        for store in self.stores:
-            if store.__class__.__name__ == selected_store:
-                return store
-        raise TracError, 'Invalid permission store "%s"' % selected_store
-    store = property(fget=lambda self: self._get_store())
-
-
 class IPermissionGroupProvider(Interface):
     """
     Extension point interface for components that provide information about user
=== trac/web/main.py
==================================================================
--- trac/web/main.py	(revision 6827)
+++ trac/web/main.py	(local)
@@ -60,6 +60,8 @@
 
     authenticators = ExtensionPoint(IAuthenticator)
     handlers = ExtensionPoint(IRequestHandler)
+    default_handler = SingletonExtensionPoint(IRequestHandler,
+                                              'trac', 'default_handler')
 
     def authenticate(self, req):
         for authenticator in self.authenticators:
@@ -88,14 +90,13 @@
 
         # Select the component that should handle the request
         chosen_handler = None
-        default_handler = None
         if not req.path_info or req.path_info == '/':
-            default_handler = self.config.get('trac', 'default_handler')
-        for handler in self.handlers:
-            if handler.match_request(req) or \
-               handler.__class__.__name__ == default_handler:
-                chosen_handler = handler
-                break
+            chosen_handler = self.default_handler
+        else:
+            for handler in self.handlers:
+                if handler.match_request(req):
+                    chosen_handler = handler
+                    break
 
         chrome.populate_hdf(req, chosen_handler)