Assert using realpath when confirming "env".

recipe_engine/env.py

 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """Sets up recipe engine Python environment."""
 
 import contextlib
 import os
 import sys
 
@@ skipping 41 matching lines @@
   # In a temporary environment where "sys.path" consists solely of our
   # "third_party" directory, register the google namespace. By restricting the
   # options in "sys.path", "pkg_resources" will not cache or prefer system
   # resources for this namespace or its derivatives.
   sys.path = [THIRD_PARTY]
   pkg_resources.declare_namespace('google')
   pkg_resources.fixup_namespace_packages(THIRD_PARTY)
 
 # From here on out, we're back to normal imports. Let's assert that the we're
 # using the correct protobuf package, though.
+#
+# We use "realpath" here because the importer may resolve the path differently
+# based on symlinks, and we want to make sure our calculated path matches the
+# impoter's path regardless.
 import google.protobuf
-assert THIRD_PARTY in google.protobuf.__path__[0]
+assert (os.path.realpath(THIRD_PARTY) in
+        os.path.realpath(google.protobuf.__path__[0]))