Add a wrapper script for GN to depot_tools

gn.py

Index: gn.py
===================================================================
--- gn.py	(revision 0)
+++ gn.py	(revision 0)
@@ -0,0 +1,66 @@
+#!/usr/bin/env python
+# Copyright (c) 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.
+
+"""This script is a wrapper around the GN binary that is pulled from Google
+Cloud Storage when you sync Chrome. The binaries go into platform-specific
+subdirectories in the source tree.
+
+This script makes there be one place for forwarding to the correct platform's
+binary. It will also automatically try to find the gn binary when run inside
+the chrome source tree, so users can just type "gn" on the command line
+(normally depot_tools is on the path)."""
+
+import os
+import subprocess
+import sys
+
+
+class PlatformUnknownError(IOError):
+  pass
+
+
+def HasDotfile(path):
+  """Returns True if the given path has a .gn file in it."""
+  return os.path.exists(path + '/.gn')
+
+
+def FindSourceRootOnPath():
+  """Searches upward from the current directory for the root of the source
+  tree and returns the found path. Returns None if no source root could
+  be found."""
+  cur = os.getcwd()
+  while True:
+    if HasDotfile(cur):
+      return cur
+    up_one = os.path.dirname(cur)
+    if up_one == cur:
+      return None  # Reached the top of the directory tree
+    cur = up_one
+
+
+def RunGN(sourceroot):
+  # The binaries in platform-specific subdirectories in src/tools/gn/bin.
+  gnpath = sourceroot + '/tools/gn/bin/'
+  if sys.platform == 'win32':
+    gnpath += 'win/gn.exe'
+  elif sys.platform.startswith('linux'):
+    gnpath += 'linux/gn'
+  elif sys.platform == 'darwin':
+    gnpath += 'mac/gn'
+  else:
+    raise PlatformUnknownError('Unknown platform for GN: ' + sys.platform)
+
+  return subprocess.call([gnpath] + sys.argv[1:])
+
+
+def main(args):
+  sourceroot = FindSourceRootOnPath()
+  if not sourceroot:
+    print >> sys.stderr, '.gn file not found in any parent of the current path.'
+    sys.exit(1)
+  return RunGN(sourceroot)
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv))