[context] Split "env" values and prefixes.

recipe_modules/step/config.py

 # Copyright 2013 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import collections
 
 from recipe_engine.config import (config_item_context, ConfigGroup, ConfigList,
                                   Dict, Single, Set, List)
 from recipe_engine.config_types import Path
 from recipe_engine.util import Placeholder
 
 
 def BaseConfig(**_kwargs):
   def render_cmd(lst):
     return [(x if isinstance(x, Placeholder) else str(x)) for x in lst]
 
   return ConfigGroup(
     # For compatibility with buildbot, the step name must be ascii, which is why
     # this is a 'str' and not a 'basestring'.
     name = Single(str),
     base_name = Single(str, required=False),
     cmd = List(inner_type=(int,long,basestring,Path,Placeholder),
                jsonish_fn=render_cmd),
     timeout = Single(int, required=False),
 
     # optional
+    env_prefixes = Dict(value_type=List((basestring,int,Path,type(None))),
+        item_fn=lambda (k, v): (k, [str(e) for e in (v or ())] or None)),
     env = Dict(item_fn=lambda (k, v): (k, v if v is None else str(v)),
                value_type=(basestring,int,Path,type(None))),
     cwd = Single(Path, jsonish_fn=str, required=True),
 
     stdout = Single(Placeholder, required=False),
     stderr = Single(Placeholder, required=False),
     stdin = Single(Placeholder, required=False),
 
     allow_subannotations = Single(bool, required=False),
 
@@ skipping 14 matching lines @@
     step_nest_level = Single(int, required=False),
   )
 
 
 config_ctx = config_item_context(BaseConfig)
 
 @config_ctx()
 def test(c):  # pragma: no cover
   c.name = 'test'
   c.cmd = [Path('[CHECKOUT]', 'build', 'tools', 'cool_script.py')]