Make clients explicitly unpickleable.

Closes googleapis#3211.

core/google/cloud/client.py

@@ -14,6 +14,8 @@
 
 """Base classes for client used to interact with Google Cloud APIs."""
 
+from pickle import PicklingError
+
 import google.auth.credentials
 from google.oauth2 import service_account
 import google_auth_httplib2
@@ -126,6 +128,14 @@ def __init__(self, credentials=None, http=None):
             credentials, self.SCOPE)
         self._http_internal = http
 
+    def __getstate__(self):
+        """Explicitly state that clients are not pickleable."""
+
+        raise PicklingError('\n'.join([
+            'Pickling client objects is explicitly not supported.',
+            'Clients have non-trivial state that is local and unpickleable.',
+        ]))
+
     @property
     def _http(self):
         """Getter for object used for HTTP transport.

core/tests/unit/test_client.py

@@ -47,6 +47,13 @@ def _get_target_class():
     def _make_one(self, *args, **kw):
         return self._get_target_class()(*args, **kw)
 
+    def test_unpickleable(self):
+        import pickle
+
+        client = self._make_one()
+        with self.assertRaises(pickle.PicklingError):
+            pickle.dumps(client)
+
     def test_ctor_defaults(self):
         from google.cloud._testing import _Monkey
         from google.cloud import client