Fix 500 error when no DisplayName in request body.

The original data of body from client to set acl like below:

<AccessControlPolicy xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <Owner>
    <ID>test:tester</ID>
    <DisplayName>test:tester</DisplayName>
  </Owner>
  <AccessControlList>
    ...
  </AccessControlList>
</AccessControlPolicy>

If the attribute "<DisplayName>" isn't exist, proxy will return
500 Internal Server Error.

Change-Id: Ia02af110109d5ecbcda05f707707b30e604af8f0
Closes-Bug: #1400367

swift3/subresource.py

@@ -412,7 +412,11 @@ class ACL(object):
         Convert an ElementTree to an ACL instance
         """
         id = elem.find('./Owner/ID').text
-        name = elem.find('./Owner/DisplayName').text
+        try:
+            name = elem.find('./Owner/DisplayName').text
+        except AttributeError:
+            name = id
+
         grants = [Grant.from_elem(e)
                   for e in elem.findall('./AccessControlList/Grant')]
         return cls(Owner(id, name), grants)

swift3/test/unit/test_subresource.py

@@ -182,6 +182,22 @@ class TestSwift3Subresource(unittest.TestCase):
         self.assertFalse(self.check_permission(acl, 'test:tester2',
                                                'WRITE_ACP'))
 
+    def test_acl_from_elem_by_id_only(self):
+        elem = ACLPrivate(Owner(id='test:tester',
+                                name='test:tester')).elem()
+        elem.find('./Owner').remove(elem.find('./Owner/DisplayName'))
+        acl = ACL.from_elem(elem)
+        self.assertTrue(self.check_permission(acl, 'test:tester', 'READ'))
+        self.assertTrue(self.check_permission(acl, 'test:tester', 'WRITE'))
+        self.assertTrue(self.check_permission(acl, 'test:tester', 'READ_ACP'))
+        self.assertTrue(self.check_permission(acl, 'test:tester', 'WRITE_ACP'))
+        self.assertFalse(self.check_permission(acl, 'test:tester2', 'READ'))
+        self.assertFalse(self.check_permission(acl, 'test:tester2', 'WRITE'))
+        self.assertFalse(self.check_permission(acl, 'test:tester2',
+                                               'READ_ACP'))
+        self.assertFalse(self.check_permission(acl, 'test:tester2',
+                                               'WRITE_ACP'))
+
     def test_decode_acl_container(self):
         access_control_policy = \
             {'Owner': 'test:tester',