Pruning virtualenv from environment in mb.py.

Bug: 996624
Change-Id: Ibdaf0267017ff652431822bd53bf1d982b254401
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1795757Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Karen Qian <karenqian@google.com>
Cr-Commit-Position: refs/heads/master@{#695426}

build/toolchain/win/setup_toolchain.py

@@ -28,15 +28,21 @@ def _ExtractImportantEnvironment(output_of_set):
   """Extracts environment variables required for the toolchain to run from
   a textual dump output by the cmd.exe 'set' command."""
   envvars_to_save = (
+      'cipd_cache_dir', # needed by vpython
+      'homedrive', # needed by vpython
+      'homepath', # needed by vpython
       'goma_.*', # TODO(scottmg): This is ugly, but needed for goma.
       'include',
       'lib',
       'libpath',
+      'luci_context', # needed by vpython
       'path',
       'pathext',
       'systemroot',
       'temp',
       'tmp',
+      'userprofile', # needed by vpython
+      'vpython_virtualenv_root' # needed by vpython
       )
   env = {}
   # This occasionally happens and leads to misleading SYSTEMROOT error messages

tools/mb/mb.py

@@ -36,8 +36,31 @@ sys.path = [os.path.join(CHROMIUM_SRC_DIR, 'build')] + sys.path
 
 import gn_helpers
 
+def PruneVirtualEnv():
+  # Set by VirtualEnv, no need to keep it.
+  os.environ.pop('VIRTUAL_ENV', None)
+
+  # Set by VPython, if scripts want it back they have to set it explicitly.
+  os.environ.pop('PYTHONNOUSERSITE', None)
+
+  # Look for "activate_this.py" in this path, which is installed by VirtualEnv.
+  # This mechanism is used by vpython as well to sanitize VirtualEnvs from
+  # $PATH.
+  os.environ['PATH'] = os.pathsep.join([
+    p for p in os.environ.get('PATH', '').split(os.pathsep)
+    if not os.path.isfile(os.path.join(p, 'activate_this.py'))
+  ])
+
 
 def main(args):
+  # Prune all evidence of VPython/VirtualEnv out of the environment. This means
+  # that we 'unwrap' vpython VirtualEnv path/env manipulation. Invocations of
+  # `python` from GN should never inherit the gn.py's own VirtualEnv. This also
+  # helps to ensure that generated ninja files do not reference python.exe from
+  # the VirtualEnv generated from depot_tools' own .vpython file (or lack
+  # thereof), but instead reference the default python from the PATH.
+  PruneVirtualEnv()
+
   mbw = MetaBuildWrapper()
   return mbw.Main(args)