fix: query.map() and query.map_async() hanging with empty result set.

google/cloud/ndb/query.py

@@ -2031,7 +2031,11 @@ def map_async(self, callback, **kwargs):
                 mapped = future
             futures.append(mapped)
 
-        mapped_results = yield futures
+        if futures:
+            mapped_results = yield futures
+        else:
+            mapped_results = []
+
         raise tasklets.Return(mapped_results)
 
     @_query_options

tests/system/test_query.py

@@ -1323,6 +1323,18 @@ def get_other_foo(thing):
     assert query.map(get_other_foo) == foos
 
 
+@pytest.mark.usefixtures("client_context")
+def test_map_empty_result_set(dispose_of):
+    class SomeKind(ndb.Model):
+        foo = ndb.StringProperty()
+
+    def somefunc(x):
+        raise Exception("Shouldn't be called.")
+
+    query = SomeKind.query()
+    assert query.map(somefunc) == []
+
+
 @pytest.mark.usefixtures("client_context")
 def test_gql(ds_entity):
     for i in range(5):

tests/unit/test_query.py

@@ -1877,6 +1877,25 @@ def callback(result):
         query = query_module.Query()
         assert query.map(callback) == (1, 2, 3, 4, 5)
 
+    @staticmethod
+    @pytest.mark.usefixtures("in_context")
+    @mock.patch("google.cloud.ndb._datastore_query")
+    def test_map_empty_result_set(_datastore_query):
+        class DummyQueryIterator:
+            def __init__(self, items):
+                self.items = list(items)
+
+            def has_next_async(self):
+                return utils.future_result(bool(self.items))
+
+        _datastore_query.iterate.return_value = DummyQueryIterator(())
+
+        def callback(result):  # pragma: NO COVER
+            raise Exception("Shouldn't get called.")
+
+        query = query_module.Query()
+        assert query.map(callback) == []
+
     @staticmethod
     @pytest.mark.usefixtures("in_context")
     @mock.patch("google.cloud.ndb._datastore_query")