Allow specificying that only target_os should be used in a gclient file

gclient.py

 #!/usr/bin/env python
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Meta checkout manager supporting both Subversion and GIT.
 
 Files
   .gclient      : Current client configuration, written by 'config' command.
                   Format is a Python script defining 'solutions', a list whose
@@ skipping 37 matching lines @@
         "action":  ["python", "image_indexer.py", "--all"]},
     ]
 
 Specifying a target OS
   An optional key named "target_os" may be added to a gclient file to specify
   one or more additional operating systems that should be considered when
   processing the deps_os dict of a DEPS file.
 
   Example:
     target_os = [ "android" ]
+
+  If the "target_os_only" key is also present and true, then *only* the
+  operating systems listed in "target_os" will be used.
+
+  Example:
+    target_os = [ "ios" ]
+    target_os_only = True
 """
 
 __version__ = "0.6.4"
 
 import copy
 import logging
 import optparse
 import os
 import platform
 import posixpath
@@ skipping 811 matching lines @@
     assert not self.dependencies
     config_dict = {}
     self.config_content = content
     try:
       exec(content, config_dict)
     except SyntaxError, e:
       gclient_utils.SyntaxErrorToError('.gclient', e)
 
     # Append any target OS that is not already being enforced to the tuple.
     target_os = config_dict.get('target_os', [])
-    self._enforced_os = tuple(set(self._enforced_os).union(target_os))
+    if config_dict.get('target_os_only', False):
+      self._enforced_os = tuple(set(target_os))
+    else:
+      self._enforced_os = tuple(set(self._enforced_os).union(target_os))
+
+    if not target_os and config_dict.get('target_os_only', False):
+      raise gclient_utils.Error('Can\'t use target_os_only if target_os is '
+                                'not specified')
 
     deps_to_add = []
     for s in config_dict.get('solutions', []):
       try:
         deps_to_add.append(Dependency(
             self, s['name'], s['url'],
             s.get('safesync_url', None),
             s.get('managed', True),
             s.get('custom_deps', {}),
             s.get('custom_vars', {}),
@@ skipping 759 matching lines @@
   except (gclient_utils.Error, subprocess2.CalledProcessError), e:
     print >> sys.stderr, 'Error: %s' % str(e)
     return 1
 
 
 if '__main__' == __name__:
   fix_encoding.fix_encoding()
   sys.exit(Main(sys.argv[1:]))
 
 # vim: ts=2:sw=2:tw=80:et: