S3: Return ETag in 304 responses from get_object (#8535)

moto/s3/responses.py

@@ -1501,14 +1501,20 @@ def _key_response_get(self, query: Dict[str, Any], key_name: str) -> TYPE_RESPON
     def get_object(self) -> TYPE_RESPONSE:
         key, not_modified = self._get_key()
         response_headers = self._get_cors_headers_other()
-        if not_modified:
-            return 304, response_headers, "Not Modified"
 
         if key.version_id != "null":
             response_headers["x-amz-version-id"] = key.version_id
 
-        response_headers.update(key.metadata)
         response_headers.update(key.response_dict)
+
+        if not_modified:
+            # Real S3 omits any content-* headers for a 304
+            for header in list(response_headers.keys()):
+                if header.startswith("content-"):
+                    response_headers.pop(header)
+            return 304, response_headers, "Not Modified"
+
+        response_headers.update(key.metadata)
         response_headers.update({"Accept-Ranges": "bytes"})
 
         part_number = self._get_int_param("partNumber")

tests/test_s3/test_s3.py

@@ -2127,6 +2127,7 @@ def test_get_object_if_none_match():
         s3_client.get_object(Bucket=bucket_name, Key=key, IfNoneMatch=etag)
     err_value = err.value
     assert err_value.response["Error"] == {"Code": "304", "Message": "Not Modified"}
+    assert err_value.response["ResponseMetadata"]["HTTPHeaders"]["etag"] == etag
 
 
 @mock_aws
@@ -2227,6 +2228,7 @@ def test_head_object_if_none_match():
         s3_client.head_object(Bucket=bucket_name, Key=key, IfNoneMatch=etag)
     err_value = err.value
     assert err_value.response["Error"] == {"Code": "304", "Message": "Not Modified"}
+    assert err_value.response["ResponseMetadata"]["HTTPHeaders"]["etag"] == etag
 
 
 @mock_aws