path: Add tracking of volatile paths.

recipe_modules/path/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.
 
-from recipe_engine.config import config_item_context, ConfigGroup, Dict, Static
+from recipe_engine.config import config_item_context, ConfigGroup, Dict, \
+                                 Set, Static
 from recipe_engine.config_types import Path
 
 def BaseConfig(PLATFORM, START_DIR, TEMP_DIR, CACHE_DIR, **_kwargs):
   assert START_DIR[0].endswith(('\\', '/')), START_DIR
   assert TEMP_DIR[0].endswith(('\\', '/')), TEMP_DIR
   assert CACHE_DIR[0].endswith(('\\', '/')), CACHE_DIR
   return ConfigGroup(
     # base path name -> [tokenized absolute path]
     base_paths    = Dict(value_type=tuple),
 
     # dynamic path name -> Path object (referencing one of the base_paths)
     dynamic_paths = Dict(value_type=(Path, type(None))),
 
+    # Path keys from "base_paths" and "dynamic_paths" that are known to be
+    # volatile.
+    volatile_paths = Set(str),
+
     PLATFORM = Static(PLATFORM),
     START_DIR = Static(tuple(START_DIR)),
     TEMP_DIR = Static(tuple(TEMP_DIR)),
     CACHE_DIR = Static(tuple(CACHE_DIR)),
   )
 
 config_ctx = config_item_context(BaseConfig)
 
 @config_ctx(is_root=True)
 def BASE(c):
   c.base_paths['start_dir'] = c.START_DIR
   c.base_paths['tmp_base'] = c.TEMP_DIR
   c.base_paths['cache'] = c.CACHE_DIR
   c.dynamic_paths['checkout'] = None