From eee3ebdb2cf3e025294d387316b2601feb2a8ce6 Mon Sep 17 00:00:00 2001
From: Aaron Boman <aaron@aaronboman.com>
Date: Sat, 8 Oct 2016 15:58:38 -0500
Subject: [PATCH] Allow for actions to be empty.

This allows for an updating rule to not run an action itself,
rather it allows for that rule to delegate to other updating
actions.
---
 src/build/engine.py | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/src/build/engine.py b/src/build/engine.py
index 9d6b99c50..c409276b6 100644
--- a/src/build/engine.py
+++ b/src/build/engine.py
@@ -16,21 +16,23 @@ from b2.util import set_jam_action, is_iterable
 class BjamAction(object):
     """Class representing bjam action defined from Python."""
 
-    def __init__(self, action_name, function):
+    def __init__(self, action_name, function, has_command=False):
         assert isinstance(action_name, basestring)
         assert callable(function) or function is None
         self.action_name = action_name
         self.function = function
+        self.has_command = has_command
 
     def __call__(self, targets, sources, property_set_):
         assert is_iterable(targets)
         assert is_iterable(sources)
         assert isinstance(property_set_, property_set.PropertySet)
-        # Bjam actions defined from Python have only the command
-        # to execute, and no associated jam procedural code. So
-        # passing 'property_set' to it is not necessary.
-        bjam_interface.call("set-update-action", self.action_name,
-                            targets, sources, [])
+        if self.has_command:
+            # Bjam actions defined from Python have only the command
+            # to execute, and no associated jam procedural code. So
+            # passing 'property_set' to it is not necessary.
+            bjam_interface.call("set-update-action", self.action_name,
+                                targets, sources, [])
         if self.function:
             self.function(targets, sources, property_set_)
 
@@ -158,7 +160,7 @@ class Engine:
 
         self.do_set_update_action (action_name, targets, sources, properties)
 
-    def register_action (self, action_name, command, bound_list = [], flags = [],
+    def register_action (self, action_name, command='', bound_list = [], flags = [],
                          function = None):
         """Creates a new build engine action.
 
@@ -190,7 +192,8 @@ class Engine:
         if command:
             bjam_interface.define_action(action_name, command, bound_list, bjam_flags)
 
-        self.actions[action_name] = BjamAction(action_name, function)
+        self.actions[action_name] = BjamAction(
+            action_name, function, has_command=bool(command))
 
     def register_bjam_action (self, action_name, function=None):
         """Informs self that 'action_name' is declared in bjam.