Index: build/engine.py
===================================================================
--- build/engine.py	(revision 75605)
+++ build/engine.py	(working copy)
@@ -15,9 +15,10 @@
 class BjamAction:
     """Class representing bjam action defined from Python."""
     
-    def __init__(self, action_name, function):
+    def __init__(self, action_name, function, chain_function):
         self.action_name = action_name
         self.function = function
+        self.chain_function = chain_function
             
     def __call__(self, targets, sources, property_set):
         if self.function:
@@ -29,6 +30,11 @@
         bjam_interface.call("set-update-action", self.action_name,
                             targets, sources, [])
 
+        # Chain function allows invoking additional actions after
+        # the main update action.
+        if self.chain_function:
+            self.chain_function(targets, sources, property_set)
+
 class BjamNativeAction:
     """Class representing bjam action defined by Jam code.
 
@@ -109,7 +115,7 @@
         self.do_set_update_action (action_name, targets, sources, properties)
 
     def register_action (self, action_name, command, bound_list = [], flags = [],
-                         function = None):
+                         function = None, chain_function = None):
         """Creates a new build engine action.
 
         Creates on bjam side an action named 'action_name', with
@@ -132,10 +138,14 @@
         bjam_flags = reduce(operator.or_,
                             (action_modifiers[flag] for flag in flags), 0)
 
-        bjam_interface.define_action(action_name, command, bound_list, bjam_flags)
+        # We allow command to be empty so that we can define 'action' as pure
+        # python function that would do some conditional logic and then relay
+        # to other actions.
+        assert command or function
+        if command:
+            bjam_interface.define_action(action_name, command, bound_list, bjam_flags)
+        self.actions[action_name] = BjamAction(action_name, function, chain_function)
 
-        self.actions[action_name] = BjamAction(action_name, function)
-
     def register_bjam_action (self, action_name, function=None):
         """Informs self that 'action_name' is declared in bjam.