Set next_page_token to none if there are no more result (#4349)

* Update _process_query_results_done Datastore test

Response ``end_cursor`` might be non-empty as reported in #4347.

* Set next_page_token to None using MoreResults enum (#4347)

Depending on empty value of ``response_pb.batch.end_cursor`` is not
reliable as the behaviour is not documented. It's better to check API
response field ``moreResults``, see
https://cloud.google.com/datastore/docs/reference/rpc/google.datastore.v1#google.datastore.v1.QueryResultBatch.

datastore/google/cloud/datastore/query.py

@@ -26,9 +26,10 @@
 
 
 _NOT_FINISHED = query_pb2.QueryResultBatch.NOT_FINISHED
+_NO_MORE_RESULTS = query_pb2.QueryResultBatch.NO_MORE_RESULTS
 
 _FINISHED = (
- query_pb2.QueryResultBatch.NO_MORE_RESULTS,
+ _NO_MORE_RESULTS,
  query_pb2.QueryResultBatch.MORE_RESULTS_AFTER_LIMIT,
  query_pb2.QueryResultBatch.MORE_RESULTS_AFTER_CURSOR,
 )
@@ -470,7 +471,7 @@ def _process_query_results(self, response_pb):
  """
  self._skipped_results = response_pb.batch.skipped_results
 
- if response_pb.batch.end_cursor == b'': # Empty-value for bytes.
+ if response_pb.batch.more_results == _NO_MORE_RESULTS:
  self.next_page_token = None
  else:
  self.next_page_token = base64.urlsafe_b64encode(

datastore/tests/unit/test_query.py

@@ -467,7 +467,7 @@ def test__process_query_results_done(self):
  entity_pbs = [
  _make_entity('World', 1234, 'PROJECT'),
  ]
- cursor_as_bytes = b''
+ cursor_as_bytes = b'\x9ai\xe7'
  skipped_results = 44
  more_results_enum = query_pb2.QueryResultBatch.NO_MORE_RESULTS
  response_pb = _make_query_response(