path: Add tracking of volatile paths.

recipe_modules/path/config.py

Index: recipe_modules/path/config.py
diff --git a/recipe_modules/path/config.py b/recipe_modules/path/config.py
index 98580f47ca011c362c74465f1af26741c33f5968..dcf83b02675d58beef72ce5dfcdc53eca79d1509 100644
--- a/recipe_modules/path/config.py
+++ b/recipe_modules/path/config.py
@@ -2,7 +2,8 @@
 # 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):
@@ -16,6 +17,10 @@ def BaseConfig(PLATFORM, START_DIR, TEMP_DIR, CACHE_DIR, **_kwargs):
     # 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)),