Repair hermetic_environment_as for C-level environment variable acce…

…ss. (#5898)

Relates to #5893

src/python/pants/util/contextutil.py

@@ -76,15 +76,31 @@ def setenv(key, val):
       setenv(key, val)
 
 
+def _purge_env():
+  # N.B. Without the use of `del` here (which calls `os.unsetenv` under the hood), subprocess32
+  # invokes or other things that may access the environment at the C level may not see the
+  # correct env vars (i.e. we can't just replace os.environ with an empty dict).
+  # See https://docs.python.org/2/library/os.html#os.unsetenv for more info.
+  for k in os.environ.keys():
+    del os.environ[k]
+
+
+def _restore_env(env):
+  for k, v in env.items():
+    os.environ[k] = v
+
+
 @contextmanager
 def hermetic_environment_as(**kwargs):
   """Set the environment to the supplied values from an empty state."""
-  old_environment, os.environ = os.environ, {}
+  old_environment = os.environ.copy()
+  _purge_env()
   try:
     with environment_as(**kwargs):
       yield
   finally:
-    os.environ = old_environment
+    _purge_env()
+    _restore_env(old_environment)
 
 
 @contextmanager

tests/python/pants_test/util/test_contextutil.py

@@ -64,6 +64,16 @@ def test_hermetic_environment(self):
     with hermetic_environment_as(**{}):
       self.assertNotIn('USER', os.environ)
 
+  def test_hermetic_environment_subprocesses(self):
+    self.assertIn('USER', os.environ)
+    with hermetic_environment_as(**dict(AAA='333')):
+      output = subprocess.check_output('env', shell=True)
+      self.assertNotIn('USER=', output)
+      self.assertIn('AAA', os.environ)
+      self.assertEquals(os.environ['AAA'], '333')
+    self.assertIn('USER', os.environ)
+    self.assertNotIn('AAA', os.environ)
+
   def test_simple_pushd(self):
     pre_cwd = os.getcwd()
     with temporary_dir() as tempdir: