Fix the gn wrapper to be aware of the --root argument.

gn.py

 #!/usr/bin/env python
 # 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.
 
 """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 gclient_utils
 import os
 import subprocess
 import sys
 
 
 def RunGN(sourceroot):
   # The binaries in platform-specific subdirectories in src/tools/gn/bin.
   gnpath = os.path.join(sourceroot,
                         'tools', 'gn', 'bin', gclient_utils.GetMacWinOrLinux(),
                         'gn' + gclient_utils.GetExeSuffix())
   return subprocess.call([gnpath] + sys.argv[1:])
 
 
 def main(args):
+  for arg in sys.argv:
+    # TODO(dpranke): It would be nice if '--root foo' worked in addition

[brettw, 2014/03/27 21:04:20]

I disagree that this should be supported and I'd just remove this todo. Normal
Unix command line parsing is either a single "-" with a single letter option and
no equal sign (optional space separating them) or a double "--" with an equal
sign. Chrome's command-line parsing follows this, so I don't see why GN should
be any different.

+    # to '--root=foo', but that would be inconsistent with how GN parses
+    # command line args. If we fix that in GN, we should fix this wrapper, too.
+    if arg.startswith('--root='):
+      sourceroot = arg.replace('--root=', '')
+      dotfile_path = os.path.join(sourceroot, '.gn')
+      if not os.path.exists(dotfile_path):
+        print >> sys.stderr, 'gn.py: "%s" not found, exiting.' % dotfile_path
+        sys.exit(1)
+      return RunGN(sourceroot)
+
   sourceroot = gclient_utils.FindFileUpwards('.gn')
   if not sourceroot:
-    print >> sys.stderr, '.gn file not found in any parent of the current path.'
+    print >> sys.stderr, ('gn.py: No .gn file found in any parent of '
+                          'the current path.')
+    print >> sys.stderr, ('\nYou need to either be inside a checkout, '
+                          'or use --root to specify the checkout root.')
     sys.exit(1)
   return RunGN(sourceroot)
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))