Update the toolchain if there's a mismatch between the env and win_toolchain.json

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 15 matching lines @@
   returns the location of the VS runtime DLLs so they can be copied into
   the output directory after gyp generation.
   """
   vs_runtime_dll_dirs = None
   depot_tools_win_toolchain = \
       bool(int(os.environ.get('DEPOT_TOOLS_WIN_TOOLCHAIN', '1')))
   # When running on a non-Windows host, only do this if the SDK has explicitly
   # been downloaded before (in which case json_data_file will exist).
   if ((sys.platform in ('win32', 'cygwin') or os.path.exists(json_data_file))
       and depot_tools_win_toolchain):
     if not os.path.exists(json_data_file):

[scottmg, 2016/01/15 01:05:19]

I don't think overriding the version that's coming from the .json file is right.
It's just a cache of the data that the toolchain zip stored, so you could end up
say that you're building with 2015 when you're actually building with 2013.

Woudl it work to instead check if the json matches the environment up here, and
if it doesn't then call Update()? I vaguely remember that maybe that's what this
if not exists is here for.

       Update()
     with open(json_data_file, 'r') as tempf:
       toolchain_data = json.load(tempf)
 
     toolchain = toolchain_data['path']
     version = toolchain_data['version']
     win_sdk = toolchain_data.get('win_sdk')
     if not win_sdk:
       win_sdk = toolchain_data['win8sdk']
     wdk = toolchain_data['wdk']
     # TODO(scottmg): The order unfortunately matters in these. They should be
     # split into separate keys for x86 and x64. (See CopyVsRuntimeDlls call
     # below). http://crbug.com/345992
     vs_runtime_dll_dirs = toolchain_data['runtime_dirs']
 
     os.environ['GYP_MSVS_OVERRIDE_PATH'] = toolchain
-    os.environ['GYP_MSVS_VERSION'] = version
+    if os.getenv('GYP_MSVS_VERSION'):
+      print ('Using the GYP_MSVS_VERSION value from the environment (%d).' %
+             os.getenv('GYP_MSVS_VERSION'))
+    else:
+      os.environ['GYP_MSVS_VERSION'] = version
     # We need to make sure windows_sdk_path is set to the automated
     # toolchain values in GYP_DEFINES, but don't want to override any
     # otheroptions.express
     # 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
@@ skipping 266 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())