Add a wrapper script for GN to depot_tools

gn.py

+#!/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

[M-A Ruel, 2013/11/08 00:48:31]

Make that a docstring

"""Wrapper around the GN binary that ...

+# Cloud Storage when you sync Chrome. The binaries go into platform-specific
+# subdirectories in the source tree.
+#
+# This script makes there be one place that we can forward to the correct

[M-A Ruel, 2013/11/08 00:48:31]

unusual English structure to me. I should read more.

+# 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
+

[M-A Ruel, 2013/11/08 00:48:31]

2 lines between file level symbols

+class SourceTreeNotFoundError(IOError):
+  pass
+
+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 nil if no source root could

[M-A Ruel, 2013/11/08 00:48:31]

s/nil/None/

+  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)

[M-A Ruel, 2013/11/08 00:48:31]

I'd recommend to just print >> sys.stderr and return 1.

Using exceptions is useful if you plan to use this file as a library. Otherwise
it just prints an unneeded stack trace to the user.

[brettw, 2013/11/08 20:49:16]

I kept this exception. I'm going to use this as a library for gyp_chromium, I
think. I replaced the other exception since that's only for the command-line
facing interface.

+
+  return subprocess.call([gnpath] + sys.argv[1:])
+
+def main(args):
+  # We don't use optparse since that does things like handle --help that
+  # we explicitly want to pass to the GN binary.
+  #
+  # The only command line argument we support is sourceroot which allows us to
+  # find the GN binary. This also gets passed to the GN binary which it will
+  # interpret as the same thing.
+  ROOT_ARG = '--root='
+  sourceroot = None

[M-A Ruel, 2013/11/08 00:48:31]

I think it is not a bad idea to not support this at all. Like with git, the idea
is that you make sure the cwd is the proper directory you want to use, that's
it.

I feel this would make the code less brittle long-term and set expectation
clearly, e.g. "you need to cd to the source tree first" but I'm not strongly
opposed to this.

[brettw, 2013/11/08 20:49:16]

Okay, I removed this.

+  for arg in sys.argv[1:]:
+    if arg.startswith(ROOT_ARG):
+      sourceroot = arg[len(ROOT_ARG):]
+      break
+
+  if not sourceroot:
+    sourceroot = FindSourceRootOnPath()
+    if not sourceroot:
+      raise SourceTreeNotFoundError(

[M-A Ruel, 2013/11/08 00:48:31]

I'd recommend to just print >> sys.stderr and return 1.

+          '.gn file not found in any directory in the current path.' +
+          'If you want to run it outside the source tree, use --root')
+  return RunGN(sourceroot)
+
+if __name__ == '__main__':
+  sys.exit(main(sys.argv))