fix: Address queries not fully satisfying requested offset

[crwilcox]

WIP, submitting tests to run on CI

Currently Failing Test: https://github.com/googleapis/python-datastore/pull/18/files#diff-8a7c6f378fc6b49d53199fcf42bcfe0fR518

Related to #20

[crwilcox]

Test Added. Not really confirming the case. Will need to further flesh out.

def test_query_paginate_with_large_offset(self):
        page_query = self._base_query()
        page_query.order = "appearances"
        offset = 1000 # This is greater than the number of entries.
        limit = 3

        # Offset beyond items. Verify no items found
        iterator = page_query.fetch(limit=limit, offset=offset)
        page = six.next(iterator.pages)
        entities = list(page)
        self.assertEqual(len(entities), 0)

        # Offset beyond items, explicit start_cursor
        new_iterator = page_query.fetch(limit=limit, offset=offset, start_cursor=None)
        entities = list(new_iterator)
        self.assertEqual(len(entities), 0)

        # Offset beyond items, use cursor one item in.
        new_iterator = page_query.fetch(limit=1, offset=0)
        cursor = new_iterator.next_page_token
        new_iterator = page_query.fetch(limit=limit, offset=offset, start_cursor=cursor)
        entities = list(new_iterator)
        self.assertEqual(len(entities), 0)

[crwilcox]

10 tests fail both with and without my changes

FAILED tests/system/test_system.py::TestDatastoreQuery::test_ancestor_query - AssertionError: 4 != 8
FAILED tests/system/test_system.py::TestDatastoreQuery::test_limit_queries - AssertionError: 4 != 5
FAILED tests/system/test_system.py::TestDatastoreQuery::test_ordered_query - AssertionError: 4 != 8
FAILED tests/system/test_system.py::TestDatastoreQuery::test_projection_query - AssertionError: 4 != 9
FAILED tests/system/test_system.py::TestDatastoreQuery::test_query_distinct_on - AssertionError: 1 != 2
FAILED tests/system/test_system.py::TestDatastoreQuery::test_query_key_filter - AssertionError: 0 != 1
FAILED tests/system/test_system.py::TestDatastoreQuery::test_query_multiple_filters - AssertionError: 3 != 4
FAILED tests/system/test_system.py::TestDatastoreQuery::test_query_paginate_with_offset - AssertionError: 2 != 3
FAILED tests/system/test_system.py::TestDatastoreQuery::test_query_paginate_with_start_cursor - AssertionError: 2 != 3
FAILED tests/system/test_system.py::TestDatastoreQuery::test_query_simple_filter - AssertionError: 4 != 6

[crwilcox]

Update: after refreshing test dataset, all tests are passing.

[crwilcox]

Bypassing kokoro. Only failures are lint and docs. On areas not touched by this change.