[7.x] Ensure size is >=0 when using slices

Author: bk-equityzen

elasticsearch_dsl/search.py

@@ -344,7 +344,7 @@ def __getitem__(self, n):
             # Elasticsearch won't get all results so we default to size: 10 if
             # stop not given.
             s._extra['from'] = n.start or 0
-            s._extra['size'] = n.stop - (n.start or 0) if n.stop is not None else 10
+            s._extra['size'] = max(0, n.stop - (n.start or 0) if n.stop is not None else 10)
             return s
         else:  # This is an index lookup, equivalent to slicing by [n:n+1].
             # If negative index, abort.

test_elasticsearch_dsl/test_search.py

@@ -237,6 +237,7 @@ def test_slice():
     assert {'from': 0, 'size': 5} == s[:5].to_dict()
     assert {'from': 3, 'size': 10} == s[3:].to_dict()
     assert {'from': 0, 'size': 0} == s[0:0].to_dict()
+    assert {'from': 20, 'size': 0} == s[20:0].to_dict()
 
 def test_index():
     s = search.Search()