[gn] Detect location of Visual Studio in the registry.

build/vs_toolchain.py

 #!/usr/bin/env python
 # Copyright 2014 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 json
 import os
 import pipes
 import shutil
 import subprocess
@@ skipping 46 matching lines @@
     # values there.
     gyp_defines_dict = gyp.NameValueListToDict(gyp.ShlexEnv('GYP_DEFINES'))
     gyp_defines_dict['windows_sdk_path'] = win_sdk
     os.environ['GYP_DEFINES'] = ' '.join('%s=%s' % (k, pipes.quote(str(v)))
         for k, v in gyp_defines_dict.iteritems())
     os.environ['WINDOWSSDKDIR'] = win_sdk
     os.environ['WDK_DIR'] = wdk
     # Include the VS runtime in the PATH in case it's not machine-installed.
     runtime_path = ';'.join(vs_runtime_dll_dirs)
     os.environ['PATH'] = runtime_path + ';' + os.environ['PATH']
+  elif sys.platform == 'win32' and not depot_tools_win_toolchain:
+    if not 'GYP_MSVS_OVERRIDE_PATH' in os.environ:
+      os.environ['GYP_MSVS_OVERRIDE_PATH'] = DetectVisualStudioPath()
+
   return vs_runtime_dll_dirs
 
 
+def _RegistryGetValueUsingWinReg(key, value):
+  """Use the _winreg module to obtain the value of a registry key.
+
+  Args:
+    key: The registry key.
+    value: The particular registry value to read.
+  Return:
+    contents of the registry key's value, or None on failure.  Throws
+    ImportError if _winreg is unavailable.
+  """
+  import _winreg
+  try:
+    root, subkey = key.split('\\', 1)
+    assert root == 'HKLM'  # Only need HKLM for now.
+    with _winreg.OpenKey(_winreg.HKEY_LOCAL_MACHINE, subkey) as hkey:
+      return _winreg.QueryValueEx(hkey, value)[0]
+  except WindowsError:
+    return None
+
+
+def _RegistryGetValue(key, value):
+  try:
+    return _RegistryGetValueUsingWinReg(key, value)
+  except ImportError:
+    raise Exception('The python library _winreg not found.')
+
+
+def DetectVisualStudioPath():
+  """Return path to the GYP_MSVS_VERSION of Visual Studio.
+  """
+
+  # Note that this code is used from
+  # build/toolchain/win/setup_toolchain.py as well.
+
+  # Default to Visual Studio 2013 for now.
+  version_as_year = os.environ.get('GYP_MSVS_VERSION', '2013')
+  year_to_version = {
+      '2013': '12.0',
+      '2015': '14.0',
+  }
+  if version_as_year not in year_to_version:
+    raise Exception(('Visual Studio version %s (from GYP_MSVS_VERSION)'
+                     ' not supported. Supported versions are: %s') % (
+                       version_as_year, ', '.join(year_to_version.keys())))
+  version = year_to_version[version_as_year]
+  keys = [r'HKLM\Software\Microsoft\VisualStudio\%s' % version,
+          r'HKLM\Software\Wow6432Node\Microsoft\VisualStudio\%s' % version]
+  for key in keys:
+    path = _RegistryGetValue(key, 'InstallDir')
+    if not path:
+      continue
+    path = os.path.normpath(os.path.join(path, '..', '..'))
+    return path
+
+  raise Exception(('Visual Studio Version %s (from GYP_MSVS_VERSION)'
+                   ' not found.') % (version_as_year))
+
+
 def _VersionNumber():
   """Gets the standard version number ('120', '140', etc.) based on
   GYP_MSVS_VERSION."""
   if os.environ['GYP_MSVS_VERSION'] == '2013':
     return '120'
   elif os.environ['GYP_MSVS_VERSION'] == '2015':
     return '140'
   else:
     raise ValueError('Unexpected GYP_MSVS_VERSION')
 
@@ skipping 183 matching lines @@
       'copy_dlls': CopyDlls,
   }
   if len(sys.argv) < 2 or sys.argv[1] not in commands:
     print >>sys.stderr, 'Expected one of: %s' % ', '.join(commands)
     return 1
   return commands[sys.argv[1]](*sys.argv[2:])
 
 
 if __name__ == '__main__':
   sys.exit(main())