update unit test and fix StartAfter

moto/s3/responses.py

@@ -451,15 +451,14 @@ class ResponseObject(_TemplateEnvironmentMixin, ActionAuthenticatorMixin):
         continuation_token = querystring.get('continuation-token', [None])[0]
         start_after = querystring.get('start-after', [None])[0]
 
+        # sort the combination of folders and keys into lexicographical order
+        all_keys = result_keys + result_folders
+        all_keys.sort(key=self._get_name)
+
         if continuation_token or start_after:
             limit = continuation_token or start_after
-            if not delimiter:
-                result_keys = self._get_results_from_token(result_keys, limit)
-            else:
-                result_folders = self._get_results_from_token(result_folders, limit)
+            all_keys = self._get_results_from_token(all_keys, limit)
 
-        all_keys = result_keys + result_folders
-        all_keys.sort(key=self._get_key_name)
         truncated_keys, is_truncated, next_continuation_token = self._truncate_result(all_keys, max_keys)
         result_keys, result_folders = self._split_truncated_keys(truncated_keys)
 
@@ -480,7 +479,7 @@ class ResponseObject(_TemplateEnvironmentMixin, ActionAuthenticatorMixin):
         )
 
     @staticmethod
-    def _get_key_name(key):
+    def _get_name(key):
         if isinstance(key, FakeKey):
             return key.name
         else:

tests/test_s3/test_s3.py

@@ -1396,16 +1396,20 @@ def test_boto3_list_objects_v2_fetch_owner():
 def test_boto3_list_objects_v2_truncate_combined_keys_and_folders():
     s3 = boto3.client('s3', region_name='us-east-1')
     s3.create_bucket(Bucket='mybucket')
-    s3.put_object(Bucket='mybucket', Key="A/a", Body="folder/a")
-    s3.put_object(Bucket='mybucket', Key="A/aa", Body="folder/aa")
-    s3.put_object(Bucket='mybucket', Key="A/B/a", Body="nested/folder/a")
-    s3.put_object(Bucket='mybucket', Key="c", Body="plain c")
+    s3.put_object(Bucket='mybucket', Key='1/2', Body='')
+    s3.put_object(Bucket='mybucket', Key='2', Body='')
+    s3.put_object(Bucket='mybucket', Key='3/4', Body='')
+    s3.put_object(Bucket='mybucket', Key='4', Body='')
 
-    resp = s3.list_objects_v2(Bucket='mybucket', Prefix="A/", MaxKeys=1, Delimiter="/")
-    assert "Prefix" in resp
-    assert "Delimiter" in resp
-    assert resp["IsTruncated"] is True
-    assert resp["KeyCount"] != 0
+    resp = s3.list_objects_v2(Bucket='mybucket', MaxKeys=2, Delimiter='/')
+    assert 'Delimiter' in resp
+    assert resp['IsTruncated'] is True
+    assert resp['KeyCount'] == 2
+
+    last_tail = resp['NextContinuationToken']
+    resp = s3.list_objects_v2(Bucket='mybucket', MaxKeys=2, Delimiter='/', StartAfter=last_tail)
+    assert resp['KeyCount'] == 2
+    assert resp['IsTruncated'] is False
 
 
 @mock_s3