From ecedbe717e05151f5811caf832a406217cb40c78 Mon Sep 17 00:00:00 2001
From: John Sirois <john.sirois@gmail.com>
Date: Tue, 3 Jun 2014 10:32:27 -0700
Subject: [PATCH] Add a hook to allow pants customizations to inject backends.

This may be provisional until the backend reorg finalizes, but
for now it assumes the additional backend packages injected
have been arranged to be on the PYTHONPATH by other means.

Testing Done:
$ ./build-support/bin/ci.sh

Bugs closed: 447

Reviewed at https://rbcommons.com/s/twitter/r/447/
---
 src/python/pants/base/BUILD                 |  3 +++
 src/python/pants/base/dev_backend_loader.py | 18 ++++++++++++++++--
 src/python/pants/bin/pants_exe.py           |  3 ++-
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/python/pants/base/BUILD b/src/python/pants/base/BUILD
index c3bc68ba767..06e3deba072 100644
--- a/src/python/pants/base/BUILD
+++ b/src/python/pants/base/BUILD
@@ -102,6 +102,9 @@ python_library(
 python_library(
   name = 'dev_backend_loader',
   sources = ['dev_backend_loader.py'],
+  dependencies = [
+    pants('3rdparty/python/twitter/commons:twitter.common.collections'),
+  ]
 )
 
 python_library(
diff --git a/src/python/pants/base/dev_backend_loader.py b/src/python/pants/base/dev_backend_loader.py
index e27d40c8d0f..95ef496e105 100644
--- a/src/python/pants/base/dev_backend_loader.py
+++ b/src/python/pants/base/dev_backend_loader.py
@@ -1,6 +1,20 @@
+# Copyright 2014 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
 
+from __future__ import (nested_scopes, generators, division, absolute_import, with_statement,
+                        print_function, unicode_literals)
 
-def load_backends_from_source(build_file_parser):
+from twitter.common.collections import OrderedSet
+
+
+def load_backends_from_source(build_file_parser, additional_backends=None):
+  """Installs pants backend packages to provide targets and helper functions to BUILD files and
+  goals to the cli.
+
+  :param build_file_parser: The parser to populate with target aliases and helper functions from
+    the backends.
+  :param additional_backends: An optional list of additional packages to load backends from.
+  """
   print("Loading pants backends from source")
   backend_packages = [
     'pants.backend.core',
@@ -9,7 +23,7 @@ def load_backends_from_source(build_file_parser):
     'pants.backend.codegen',
     'pants.backend.maven_layout',
   ]
-  for backend_package in backend_packages:
+  for backend_package in OrderedSet(backend_packages + (additional_backends or [])):
     module = __import__(backend_package + '.register',
                         {}, # globals
                         {}, # locals
diff --git a/src/python/pants/bin/pants_exe.py b/src/python/pants/bin/pants_exe.py
index d3fba32bc06..71a3982a484 100644
--- a/src/python/pants/bin/pants_exe.py
+++ b/src/python/pants/bin/pants_exe.py
@@ -131,7 +131,8 @@ def _run():
   build_file_parser = BuildFileParser(root_dir=root_dir, run_tracker=run_tracker)
   build_graph = BuildGraph(run_tracker=run_tracker)
 
-  load_backends_from_source(build_file_parser)
+  additional_backends = config.getlist('backends', 'packages')
+  load_backends_from_source(build_file_parser, additional_backends=additional_backends)
 
   command_class, command_args = _parse_command(root_dir, argv)
   command = command_class(run_tracker,