Sort s3 object versions by last_modified_on in descending order (#3588)

* Sort s3 object versions by last_modified_on in descending order

- addresses https://github.com/localstack/localstack/issues/3433

* fix for python 2

* fix lint

moto/s3/responses.py

@@ -6,7 +6,11 @@ import sys
 import six
 from botocore.awsrequest import AWSPreparedRequest
 
-from moto.core.utils import str_to_rfc_1123_datetime, py2_strip_unicode_keys
+from moto.core.utils import (
+    str_to_rfc_1123_datetime,
+    py2_strip_unicode_keys,
+    unix_time_millis,
+)
 from six.moves.urllib.parse import parse_qs, urlparse, unquote, parse_qsl
 
 import xmltodict
@@ -458,6 +462,8 @@ class ResponseObject(_TemplateEnvironmentMixin, ActionAuthenticatorMixin):
                 else:
                     delete_marker_list.append(version)
             template = self.response_template(S3_BUCKET_GET_VERSIONS)
+
+            key_list.sort(key=lambda r: (r.name, -unix_time_millis(r.last_modified)))
             return (
                 200,
                 {},

tests/test_s3/test_s3.py

@@ -873,12 +873,12 @@ def test_list_versions():
     versions.should.have.length_of(2)
 
     versions[0].name.should.equal("the-key")
-    versions[0].version_id.should.equal(key_versions[0])
-    versions[0].get_contents_as_string().should.equal(b"Version 1")
+    versions[0].version_id.should.equal(key_versions[1])
+    versions[0].get_contents_as_string().should.equal(b"Version 2")
 
     versions[1].name.should.equal("the-key")
-    versions[1].version_id.should.equal(key_versions[1])
-    versions[1].get_contents_as_string().should.equal(b"Version 2")
+    versions[1].version_id.should.equal(key_versions[0])
+    versions[1].get_contents_as_string().should.equal(b"Version 1")
 
     key = Key(bucket, "the2-key")
     key.set_contents_from_string("Version 1")
@@ -3700,6 +3700,10 @@ def test_boto3_list_object_versions():
     len(response["Versions"]).should.equal(2)
     keys = set([item["Key"] for item in response["Versions"]])
     keys.should.equal({key})
+
+    # the first item in the list should be the latest
+    response["Versions"][0]["IsLatest"].should.equal(True)
+
     # Test latest object version is returned
     response = s3.get_object(Bucket=bucket_name, Key=key)
     response["Body"].read().should.equal(items[-1])