Extract the shell runner into devtools.

mojo/devtools/common/devtoolslib/default_paths.py

+# Copyright 2015 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.
+
+"""Supports inferring locations of files in default checkout layouts.
+
+These functions allow devtools scripts to work out-of-the-box with regular Mojo
+checkouts.
+"""
+
+import os.path
+
+
+def _lowest_ancestor_containing_relpath(relpath):
+  """Returns the lowest ancestor of this file that contains |relpath|."""
+  cur_dir_path = os.path.abspath(os.path.dirname(__file__))
+  while True:
+    if os.path.exists(os.path.join(cur_dir_path, relpath)):
+      return cur_dir_path
+
+    next_dir_path = os.path.dirname(cur_dir_path)
+    if next_dir_path != cur_dir_path:
+      cur_dir_path = next_dir_path
+    else:
+      return None
+
+
+def infer_default_paths(is_android, is_debug, target_cpu):
+  """Infers the locations of select build output artifacts in a regular Mojo
+  checkout.
+
+  Returns:
+    Tuple of path dictionary, error message. Only one of the two will be
+    not-None.
+  """
+  build_dir = (('android_' if is_android else '') +
+               (target_cpu + '_' if target_cpu else '') +
+               ('Debug' if is_debug else 'Release'))
+  out_build_dir = os.path.join('out', build_dir)
+
+  root_path = _lowest_ancestor_containing_relpath(out_build_dir)
+  if not root_path:
+    return None, ('Failed to find build directory: ' + out_build_dir)
+
+  paths = {}
+  paths['root'] = root_path
+  build_dir_path = os.path.join(root_path, out_build_dir)
+  paths['build'] = build_dir_path
+  if is_android:
+    paths['shell'] = os.path.join(build_dir_path, 'apks', 'MojoShell.apk')
+    paths['adb'] = os.path.join(root_path, 'third_party', 'android_tools',
+                                'sdk', 'platform-tools', 'adb')
+  else:
+    paths['shell'] = os.path.join(build_dir_path, 'mojo_shell')
+
+  paths['sky_packages'] = os.path.join(build_dir_path, 'gen', 'dart-pkg',
+                                       'packages')
+  return paths, ''

[qsr, 2015/07/16 14:19:20]

According to your doc, this should be paths, None

[ppi, 2015/07/16 14:23:45]

Done.