Add Paths as first-class types in configs.

scripts/slave/recipe_modules/git/api.py

 # Copyright 2013 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.
 
 from slave import recipe_api
 
 class GitApi(recipe_api.RecipeApi):
   def __call__(self, *args, **kwargs):
     """Return a git command step."""
     name = 'git '+args[0]
     # Distinguish 'git config' commands by the variable they are setting.
     if args[0] == 'config' and not args[1].startswith('-'):
       name += ' ' + args[1]
     if 'cwd' not in kwargs:
-      kwargs.setdefault('cwd', self.m.path.checkout())
+      kwargs.setdefault('cwd', self.m.path.checkout)
     git_cmd = 'git'
     if self.m.platform.is_win:
       git_cmd = self.m.path.depot_tools('git.bat')
     return self.m.step(name, [git_cmd] + list(args), **kwargs)
 
   def checkout(self, url, ref='master', dir_path=None, recursive=False,
                keep_paths=None):
     """Returns an iterable of steps to perform a full git checkout.
     Args:
       url (string): url of remote repo to use as upstream
       ref (string): ref to check out after fetching
-      dir_path (string): optional directory to clone into
+      dir_path (Path): optional directory to clone into
       recursive (bool): whether to recursively fetch submodules or not
       keep_paths (iterable of strings): paths to ignore during git-clean;
           paths are gitignore-style patterns relative to checkout_path.
     """
     if not dir_path:
       dir_path = url.rsplit('/', 1)[-1]
       if dir_path.endswith('.git'):  # ex: https://host/foobar.git
         dir_path = dir_path[:-len('.git')]
 
       # ex: ssh://host:repo/foobar/.git
       dir_path = dir_path or dir_path.rsplit('/', 1)[-1]
 
       dir_path = self.m.path.slave_build(dir_path)
-    assert self.m.path.pardir not in dir_path
-    self.m.path.add_checkout(dir_path)
+    self.m.path.set_dynamic_path('checkout', dir_path, overwrite=False)
 
     full_ref = 'refs/heads/%s' % ref if '/' not in ref else ref
 
     recursive_args = ['--recurse-submodules'] if recursive else []
     clean_args = list(self.m.itertools.chain(
         *[('-e', path) for path in keep_paths or []]))
 
     git_setup_args = ['--path', dir_path, '--url', url]
     if self.m.platform.is_win:
       git_setup_args += ['--git_cmd_path', self.m.path.depot_tools('git.bat')]
 
     return [
       self.m.python('git setup',
                     self.m.path.build('scripts', 'slave', 'git_setup.py'),
                     git_setup_args),
       # git_setup.py always sets the repo at the given url as remote 'origin'.
       self('fetch', 'origin', *recursive_args),
       self('update-ref', full_ref, 'origin/' + ref),
       self('clean', '-f', '-d', '-x', *clean_args),
       self('checkout', '-f', ref),
       self('submodule', 'update', '--init', '--recursive',
            cwd=dir_path),
     ]