Fix response of DELETE Multiple Objects without bucket write permission

S3 returns 200 status code and following xml body, even if it does not have
bucket write permission.

<?xml version="1.0" encoding="UTF-8"?>
<DeleteResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <Error>
    <Key>sample1</Key>
    <Code>AccessDenied</Code>
    <Message>Access Denied</Message>
  </Error>
  <Error>
    <Key>sample2</Key>
    <Code>AccessDenied</Code>
    <Message>Access Denied</Message>
  </Error>
</DeleteResult>

Change-Id: I75d5543ee35d108c9b3d037c8dfb3a96fc65b634

swift3/controllers/multi_delete.py

@@ -17,7 +17,7 @@ from swift3.controllers.base import Controller, bucket_operation
 from swift3.etree import Element, SubElement, fromstring, tostring, \
     XMLSyntaxError, DocumentInvalid
 from swift3.response import HTTPOk, S3NotImplemented, NoSuchKey, \
-    ErrorResponse, MalformedXML, UserKeyMustBeSpecified
+    ErrorResponse, MalformedXML, UserKeyMustBeSpecified, AccessDenied
 from swift3.cfg import CONF
 from swift3.utils import LOGGER
 
@@ -29,6 +29,19 @@ class MultiObjectDeleteController(Controller):
     Handles Delete Multiple Objects, which is logged as a MULTI_OBJECT_DELETE
     operation in the S3 server log.
     """
+    def _gen_error_body(self, error, elem, delete_list):
+        for key, version in delete_list:
+            if version is not None:
+                # TODO: delete the specific version of the object
+                raise S3NotImplemented()
+
+            error_elem = SubElement(elem, 'Error')
+            SubElement(error_elem, 'Key').text = key
+            SubElement(error_elem, 'Code').text = error.__class__.__name__
+            SubElement(error_elem, 'Message').text = error._msg
+
+        return tostring(elem)
+
     @bucket_operation
     def POST(self, req):
         """
@@ -45,9 +58,6 @@ class MultiObjectDeleteController(Controller):
 
                 yield key, version
 
-        # check bucket existence
-        req.get_response(self.app, 'HEAD')
-
         try:
             xml = req.xml(MAX_MULTI_DELETE_BODY_SIZE, check_md5=True)
             elem = fromstring(xml, 'Delete')
@@ -71,6 +81,13 @@ class MultiObjectDeleteController(Controller):
 
         elem = Element('DeleteResult')
 
+        # check bucket existence
+        try:
+            req.get_response(self.app, 'HEAD')
+        except AccessDenied as error:
+            body = self._gen_error_body(error, elem, delete_list)
+            return HTTPOk(body=body)
+
         for key, version in delete_list:
             if version is not None:
                 # TODO: delete the specific version of the object