Improve Windows GN build setup.

tools/gn/secondary/build/toolchain/win/setup_toolchain.py

Index: tools/gn/secondary/build/toolchain/win/setup_toolchain.py
diff --git a/tools/gn/secondary/build/toolchain/win/setup_toolchain.py b/tools/gn/secondary/build/toolchain/win/setup_toolchain.py
new file mode 100644
index 0000000000000000000000000000000000000000..b1d32ada638704034f2e5e39f24648b2f43b5c8f
--- /dev/null
+++ b/tools/gn/secondary/build/toolchain/win/setup_toolchain.py
@@ -0,0 +1,72 @@
+# 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.
+
+import os
+import re
+import sys
+
+def ExtractImportantEnvironment():
+  """Extracts environment variables required for the toolchain from the
+  current environment."""
+  envvars_to_save = (
+      'goma_.*', # TODO(scottmg): This is ugly, but needed for goma.
+      'Path',
+      'PATHEXT',
+      'SystemRoot',
+      'TEMP',
+      'TMP',
+      )
+  result = {}
+  for envvar in envvars_to_save:
+    if envvar in os.environ:
+      if envvar == 'Path':
+        # Our own rules (for running gyp-win-tool) and other actions in
+        # Chromium rely on python being in the path. Add the path to this
+        # python here so that if it's not in the path when ninja is run
+        # later, python will still be found.
+        result[envvar.upper()] = os.path.dirname(sys.executable) + \
+            os.pathsep + os.environ[envvar]
+      else:
+        result[envvar.upper()] = os.environ[envvar]
+  for required in ('SYSTEMROOT', 'TEMP', 'TMP'):
+    if required not in result:
+      raise Exception('Environment variable "%s" '
+                      'required to be set to valid path' % required)
+  return result
+
+def FormatAsEnvironmentBlock(envvar_dict):
+  """Format as an 'environment block' directly suitable for CreateProcess.
+  Briefly this is a list of key=value\0, terminated by an additional \0. See
+  CreateProcess documentation for more details."""
+  block = ''
+  nul = '\0'
+  for key, value in envvar_dict.iteritems():
+    block += key + '=' + value + nul
+  block += nul
+  return block
+
+def CopyTool(source_path):
+  """Copies the given tool to the current directory, including a warning not
+  to edit it."""
+  with open(source_path) as source_file:
+    tool_source = source_file.readlines()
+
+  # Add header and write it out to the current directory (which should be the
+  # root build dir).
+  with open("gyp-win-tool", 'w') as tool_file:
+    tool_file.write(''.join([tool_source[0],
+                             '# Generated by setup_toolchain.py do not edit.\n']
+                            + tool_source[1:]))
+
+# Find the tool source, it's the first argument, and copy it.
+if len(sys.argv) != 2:
+  print "Need one argument (win_tool source path)."
+  sys.exit(1)
+CopyTool(sys.argv[1])
+
+# Write the environment file to the current directory.
+environ = FormatAsEnvironmentBlock(ExtractImportantEnvironment())
+with open('environment.x86', 'wb') as env_file:
+  env_file.write(environ)
+