Fix an off by one error in the common backfill code

test/test_backfill_allocator.py

@@ -3,7 +3,7 @@ import logging
 import random
 
 from datetime import datetime, timedelta
-from util.migrate.allocator import CompletedKeys, NoAvailableKeysError
+from util.migrate.allocator import CompletedKeys, NoAvailableKeysError, yield_random_entries
 
 
 class CompletedTestCase(unittest.TestCase):
@@ -127,6 +127,31 @@ class CompletedTestCase(unittest.TestCase):
     self.assertGreater(iterations, 1024)
 
 
+class FakeQuery(object):
+  def __init__(self, result_list):
+    self._result_list = result_list
+
+  def limit(self, *args, **kwargs):
+    return self
+
+  def where(self, *args, **kwargs):
+    return self
+
+  def __iter__(self):
+    return self._result_list.__iter__()
+
+
+class QueryAllocatorTest(unittest.TestCase):
+  FAKE_PK_FIELD = 10  # Must be able to compare to integers
+
+  def test_no_work(self):
+    def create_empty_query():
+      return FakeQuery([])
+
+    for _ in yield_random_entries(create_empty_query, self.FAKE_PK_FIELD, 1, 10):
+      self.fail('There should never be any actual work!')
+
+
 if __name__ == '__main__':
   logging.basicConfig(level=logging.DEBUG)
   unittest.main()