Fix bug in WebDAV/HEAD requests caused by traversal order change

doc/CHANGES.txt

@@ -8,6 +8,9 @@ Zope Changes
 
     Bugs Fixed
 
+      - Traversal order changes were causing WebDAV requests which used
+        acquisition to fail.
+
       - Collector #2157: Expose name of broken class in SystemError raised
         from '__getstate__' of a broken instance.
 

lib/python/OFS/Traversable.py

@@ -26,6 +26,7 @@ from Acquisition import Acquired, aq_inner, aq_parent, aq_base
 from zExceptions import NotFound
 from ZODB.POSException import ConflictError
 from OFS.interfaces import ITraversable
+import webdav
 
 from zope.interface import implements, Interface
 from zope.component import queryMultiAdapter
@@ -165,6 +166,7 @@ class Traversable:
         else:
             obj = self
 
+        resource = _marker
         try:
             while path:
                 name = path_pop()
@@ -237,6 +239,13 @@ class Traversable:
                         else:
                             try:
                                 next = obj[name]
+                                # The item lookup may return a NullResource,
+                                # if this is the case we save it and return it
+                                # if all other lookups fail.
+                                if isinstance(next,
+                                              webdav.NullResource.NullResource):
+                                    resource = next
+                                    raise KeyError(name)
                             except AttributeError:
                                 # Raise NotFound for easier debugging
                                 # instead of AttributeError: __getitem__
@@ -267,6 +276,9 @@ class Traversable:
                                 next = getattr(obj, name, _marker)
                         except AttributeError:
                             raise e
+                        if next is _marker:
+                            # If we have a NullResource from earlier use it.
+                            next = resource
                             if next is _marker:
                                 # Nothing found re-raise error
                                 raise e

lib/python/OFS/tests/testTraverse.py

@@ -597,6 +597,20 @@ def test_view_doesnt_shadow_attribute():
       >>> self.folder.ftf.unrestrictedTraverse('mouse')()
       u'The mouse has been eaten by the eagle'
 
+    Head requests have some unusual behavior in Zope 2, in particular, a failed
+    item lookup on an ObjectManager returns a NullResource, rather
+    than raising a KeyError.  We need to make sure that this doesn't
+    result in acquired attributes being shadowed by the NullResource,
+    but that unknown names still give NullResources:
+
+      >>> self.app.REQUEST.maybe_webdav_client = True
+      >>> self.app.REQUEST['REQUEST_METHOD'] = 'HEAD'
+      >>> self.folder.ftf.unrestrictedTraverse('mouse')()
+      u'The mouse has been eaten by the eagle'
+      >>> self.folder.ftf.unrestrictedTraverse('nonsense')
+      <webdav.NullResource.NullResource object at ...>
+
+
     Clean up:
 
       >>> from zope.app.testing.placelesssetup import tearDown