Make Visual Studio version selection more smart

This CL enhances the Visual Studio version selection by considering a
prioritized support list, env variable and the ones installed in system.

BUG=956482

Change-Id: I7ab5286057ad6b7bca33b01f83e2cca03b606112
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1587836
Commit-Queue: Yang Gu <yang.gu@intel.com>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656684}

build/vs_toolchain.py

@@ -5,6 +5,7 @@
 
 from __future__ import print_function
 
+import collections
 import glob
 import json
 import os
@@ -22,9 +23,11 @@
 script_dir = os.path.dirname(os.path.realpath(__file__))
 json_data_file = os.path.join(script_dir, 'win_toolchain.json')
 
-
-# Use MSVS2017 as the default toolchain.
-CURRENT_DEFAULT_TOOLCHAIN_VERSION = '2017'
+# VS versions are listed in descending order of priority (highest first).
+TOOLCHAIN_VERSIONS = collections.OrderedDict([
+  ('2017', '15.0'),
+  ('2019', '16.0'),
+])
 
 
 def SetEnvironmentAndGetRuntimeDllDirs():
@@ -129,9 +132,40 @@ def _RegistryGetValue(key, value):
 
 
 def GetVisualStudioVersion():
-  """Return GYP_MSVS_VERSION of Visual Studio.
+  """Return best available version of Visual Studio.
   """
-  return os.environ.get('GYP_MSVS_VERSION', CURRENT_DEFAULT_TOOLCHAIN_VERSION)
+
+  supported_versions = TOOLCHAIN_VERSIONS.keys()
+  supported_versions_str = ', '.join('{} ({})'.format(v,k)
+      for k,v in TOOLCHAIN_VERSIONS.items())
+  available_versions = []
+  for version in supported_versions:
+    for path in (
+        os.environ.get('vs%s_install' % version),
+        os.path.expandvars('%ProgramFiles(x86)%' +
+                           '/Microsoft Visual Studio/%s' % version)):
+      if path and os.path.exists(path):
+        available_versions.append(version)
+        break
+
+  if not available_versions:
+    if bool(int(os.environ.get('DEPOT_TOOLS_WIN_TOOLCHAIN', '1'))):
+      return TOOLCHAIN_VERSIONS.keys()[0]
+    raise Exception(('No supported Visual Studio can be found.'
+                     ' Supported versions are: %s.') % supported_versions_str)
+
+  env_version = os.environ.get('GYP_MSVS_VERSION')
+  if env_version:
+    if env_version not in supported_versions:
+      raise Exception(('Visual Studio version %s (from GYP_MSVS_VERSION)'
+                       ' is not supported. Supported versions are: %s.')
+                       % (env_version, supported_versions_str))
+    if env_version not in available_versions:
+      raise Exception(('Visual Studio version %s (from GYP_MSVS_VERSION)'
+                       ' is not available.') % env_version)
+    return env_version
+
+  return available_versions[0]
 
 
 def DetectVisualStudioPath():
@@ -141,14 +175,6 @@ def DetectVisualStudioPath():
   # Note that this code is used from
   # build/toolchain/win/setup_toolchain.py as well.
   version_as_year = GetVisualStudioVersion()
-  year_to_version = {
-      '2017': '15.0',
-      '2019': '16.0',
-  }
-  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())))
 
   # The VC++ >=2017 install location needs to be located using COM instead of
   # the registry. For details see:
@@ -267,7 +293,7 @@ def FindVCComponentRoot(component):
   version number part changes frequently so the highest version number found is
   used.
   """
-  assert GetVisualStudioVersion() in ['2017', '2019']
+
   SetEnvironmentAndGetRuntimeDllDirs()
   assert ('GYP_MSVS_OVERRIDE_PATH' in os.environ)
   vc_component_msvc_root = os.path.join(os.environ['GYP_MSVS_OVERRIDE_PATH'],