Fixes an issue with the XMLEquals matcher

The matcher implementation would fail to match two documents that are
semantically equivalent, but sibling elements appear in different
document order.

Change-Id: I99dc6401e73be4c61bb265c3258b6245f2e7bb34
Closes-bug: #1347891

keystone/tests/matchers.py

@@ -12,8 +12,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import io
-
 from lxml import etree
 from testtools import matchers
 
@@ -30,24 +28,50 @@ class XMLEquals(object):
         return "%s(%r)" % (self.__class__.__name__, self.expected)
 
     def match(self, other):
+        def xml_element_equals(expected_doc, observed_doc):
+            """Tests whether two XML documents are equivalent.
+
+            This is a recursive algorithm that operates on each element in
+            the hierarchy. Siblings are sorted before being checked to
+            account for two semantically equivalent documents where siblings
+            appear in different document order.
+
+            The sorting algorithm is a little weak in that it could fail for
+            documents where siblings at a given level are the same, but have
+            different children.
+
+            """
+
+            if expected_doc.tag != observed_doc.tag:
+                return False
+
+            if expected_doc.attrib != observed_doc.attrib:
+                return False
+
+            def _sorted_children(doc):
+                return sorted(doc.getchildren(), key=lambda el: el.tag)
+
+            expected_children = _sorted_children(expected_doc)
+            observed_children = _sorted_children(observed_doc)
+
+            if len(expected_children) != len(observed_children):
+                return False
+
+            for expected_el, observed_el in zip(expected_children,
+                                                observed_children):
+                if not xml_element_equals(expected_el, observed_el):
+                    return False
+
+            return True
+
         parser = etree.XMLParser(remove_blank_text=True)
+        expected_doc = etree.fromstring(self.expected.strip(), parser)
+        observed_doc = etree.fromstring(other.strip(), parser)
 
-        def canonical_xml(s):
-            s = s.strip()
-
-            fp = io.BytesIO()
-            dom = etree.fromstring(s, parser)
-            dom.getroottree().write_c14n(fp)
-            s = fp.getvalue()
-
-            dom = etree.fromstring(s, parser)
-            return etree.tostring(dom, pretty_print=True).decode('utf-8')
-
-        expected = canonical_xml(self.expected)
-        other = canonical_xml(other)
-        if expected == other:
+        if xml_element_equals(expected_doc, observed_doc):
             return
-        return XMLMismatch(expected, other)
+
+        return XMLMismatch(self.expected, other)
 
 
 class XMLMismatch(matchers.Mismatch):
@@ -57,4 +81,11 @@ class XMLMismatch(matchers.Mismatch):
         self.other = other
 
     def describe(self):
-        return 'expected = %s\nactual = %s' % (self.expected, self.other)
+        def pretty_xml(xml):
+            parser = etree.XMLParser(remove_blank_text=True)
+            doc = etree.fromstring(xml.strip(), parser)
+            return (etree.tostring(doc, encoding='utf-8', pretty_print=True)
+                    .decode('utf-8'))
+
+        return 'expected =\n%s\nactual =\n%s' % (
+            pretty_xml(self.expected), pretty_xml(self.other))

keystone/tests/test_matchers.py

@@ -12,8 +12,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import textwrap
-
 from testtools.tests.matchers import helpers
 
 from keystone import tests
@@ -21,33 +19,38 @@ from keystone.tests import matchers
 
 
 class TestXMLEquals(tests.BaseTestCase, helpers.TestMatchersInterface):
-    matches_xml = b"""
-        <?xml version="1.0" encoding="UTF-8"?>
-        <test xmlns="http://docs.openstack.org/identity/api/v2.0">
-            <success a="a" b="b"/>
-        </test>
-    """
-    equivalent_xml = b"""
-        <?xml version="1.0" encoding="UTF-8"?>
-        <test xmlns="http://docs.openstack.org/identity/api/v2.0">
-            <success b="b" a="a"></success>
-        </test>
-    """
-    mismatches_xml = b"""
-        <?xml version="1.0" encoding="UTF-8"?>
-        <test xmlns="http://docs.openstack.org/identity/api/v2.0">
-            <nope_it_fails/>
-        </test>
-    """
-    mismatches_description = textwrap.dedent("""\
-        expected = <test xmlns="http://docs.openstack.org/identity/api/v2.0">
-          <success a="a" b="b"/>
-        </test>
+    matches_xml = b"""\
+<?xml version="1.0" encoding="UTF-8"?>
+<test xmlns="http://docs.openstack.org/identity/api/v2.0">
+    <first z="0" y="1" x="2"/>
+    <second a="a" b="b"></second>
+</test>
+"""
+    equivalent_xml = b"""\
+<?xml version="1.0" encoding="UTF-8"?>
+<test xmlns="http://docs.openstack.org/identity/api/v2.0">
+    <second a="a" b="b"/>
+    <first z="0" y="1" x="2"></first>
+</test>
+"""
+    mismatches_xml = b"""\
+<?xml version="1.0" encoding="UTF-8"?>
+<test xmlns="http://docs.openstack.org/identity/api/v2.0">
+    <nope_it_fails/>
+</test>
+"""
+    mismatches_description = """\
+expected =
+<test xmlns="http://docs.openstack.org/identity/api/v2.0">
+  <first z="0" y="1" x="2"/>
+  <second a="a" b="b"/>
+</test>
 
-        actual = <test xmlns="http://docs.openstack.org/identity/api/v2.0">
-          <nope_it_fails/>
-        </test>
-    """).lstrip()
+actual =
+<test xmlns="http://docs.openstack.org/identity/api/v2.0">
+  <nope_it_fails/>
+</test>
+"""
 
     matches_matcher = matchers.XMLEquals(matches_xml)
     matches_matches = [matches_xml, equivalent_xml]