path: Add tracking of volatile paths.

recipe_modules/path/example.py

 # Copyright 2014 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.
 
 DEPS = [
   'path',
   'platform',
   'properties',
   'step',
 ]
@@ skipping 37 matching lines @@
   assert api.path.exists(file_path)
 
   realpath = api.path.realpath(file_path)
   assert api.path.exists(realpath)
 
   home_path = api.path.join(api.path.expanduser('~'), 'file')
   api.step('touch my home', ['touch', home_path])
   api.path.mock_add_paths(home_path)
   assert api.path.exists(home_path)
 
+  api.step('volatile tmp_dir', ['echo', api.path.is_volatile('tmp_dir')])
+
   # Convert strings to Paths
   paths_to_convert =  [
     api.path['start_dir'].join('some', 'thing'),
     api.path['start_dir'],
     api.path.resource("module_resource.py"),
     api.path.resource(),
     api.resource("recipe_resource.py"),
     api.resource(),
   ]
   for p in paths_to_convert:
@@ skipping 33 matching lines @@
     yield (api.test('%s_kitchen' % platform) +
            api.platform.name(platform) +
            api.properties(path_config='kitchen') +
            api.path.exists(api.path['tmp_base']))
 
     yield (api.test('%s_luci' % platform) +
            api.platform.name(platform) +
            api.properties(**{
               '$recipe_engine/path': {
                 'cache_dir': '/c',
-                'tmp_dir': '/t'
+                'tmp_dir': '/t',
+                'volatile': ['tmp_dir'],
               },
            }) +
            api.path.exists(api.path['tmp_base']))