cr: Add support for GN build file generation

tools/cr/cr/actions/gn.py

Index: tools/cr/cr/actions/gn.py
diff --git a/tools/cr/cr/actions/gn.py b/tools/cr/cr/actions/gn.py
new file mode 100644
index 0000000000000000000000000000000000000000..516537cecc152daac433fba67283f4b8fc97f849
--- /dev/null
+++ b/tools/cr/cr/actions/gn.py
@@ -0,0 +1,72 @@
+# 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.
+
+"""A module to add gn support to cr."""
+
+import cr
+import shlex
+import os
+
+GN_ARG_PREFIX = 'GN_ARG_'
+
+
+class GnPrepareOut(cr.PrepareOut):
+  """A prepare action that runs gn whenever you select an output directory."""
+
+  ACTIVE = cr.Config.From(
+      GN_ARG_is_component_build='true',
+  )
+
+  @property
+  def enabled(self):
+    # Disabled on Android for now.
+    return not cr.AndroidPlatform.GetInstance().is_active
+
+  def UpdateContext(self):
+    # Collapse GN_ARGS from all GN_ARG prefixes
+    gn_args = cr.context.Find('GN_ARGS') or ''
+    for key, value in cr.context.exported.items():
+      if key.startswith(GN_ARG_PREFIX):
+        gn_args += ' %s=%s' % (key[len(GN_ARG_PREFIX):], value)
+
+    gn_args += (' is_debug=%s' %
+        'true' if cr.context['CR_BUILDTYPE'] == 'Debug' else 'false')
+
+    cr.context['GN_ARGS'] = gn_args.strip()
+    if cr.context.verbose >= 1:
+      print cr.context.Substitute('GN_ARGS = {GN_ARGS}')
+
+  def Prepare(self):
+    if cr.context.verbose >= 1:
+      print cr.context.Substitute('Invoking gn with {GN_ARGS}')
+
+    args_file = os.path.join(cr.context['CR_SRC'],
+                             cr.context['CR_OUT_FULL'],
+                             'args.gn')
+    args = {}
+    for arg in shlex.split(cr.context['GN_ARGS']):
+      key, value = arg.split('=', 1)
+      args[key] = value
+
+    # Override any existing settings.
+    arg_lines = []
+    if os.path.exists(args_file):
+      with open(args_file) as f:
+        for line in f:
+          key = line.split('=', 1)[0].strip()
+          if key not in args:
+            arg_lines.append(line.strip())
+
+    # Append new settings.
+    for key, value in args.items():
+      arg_lines.append('%s = %s' % (key, value))
+
+    with open(args_file, 'w') as f:
+      f.write('\n'.join(arg_lines) + '\n')
+
+    cr.Host.Execute(
+        'gn',
+        'gen',
+        '{CR_OUT_FULL}',
+    )