Fix occasional RuntimeErrors in transaction.weakset on Python 3

One way to reproduce these in the wild was to run the ZODB test suite on
Python 3.3 in the py3 branch.  Example failure:

Error in test checkPackWhileWriting (ZODB.tests.testMVCCMappingStorage.MVCCMappingStorageTests)
Traceback (most recent call last):
  File "/usr/lib/python3.3/unittest/case.py", line 385, in _executeTestPart
    function()
  File "/home/mg/src/new-zope-order/ZODB/src/ZODB/tests/PackableStorage.py", line 281, in checkPackWhileWriting
    self._PackWhileWriting(pack_now=False)
  File "/home/mg/src/new-zope-order/ZODB/src/ZODB/tests/PackableStorage.py", line 208, in _PackWhileWriting
    transaction.commit()
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/_manager.py", line 111, in commit
    return self.get().commit()
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/_transaction.py", line 286, in commit
    self._synchronizers.map(lambda s: s.afterCompletion(self))
  File "/home/mg/.buildout/eggs/transaction-1.4.0-py3.3.egg/transaction/weakset.py", line 55, in map
    for wr in self.as_weakref_list():
RuntimeError: dictionary changed size during iteration

CHANGES.rst

@@ -7,6 +7,9 @@ Changes
 - Document that values returned by ``sortKey`` must be strings, in order
   to guarantee total ordering.
 
+- Fix occasional RuntimeError: dictionary changed size during iteration errors
+  in transaction.weakset on Python 3.
+
 1.4.0 (2013-01-03)
 ------------------
 

transaction/tests/test_weakset.py

@@ -58,7 +58,9 @@ class WeakSetTests(unittest.TestCase):
         w.add(dummy3)
         del dummy3
         gc.collect()
-        L = [x() for x in w.as_weakref_list()]
+        refs = w.as_weakref_list()
+        self.assertTrue(isinstance(refs, list))
+        L = [x() for x in refs]
         # L is a list, but it does not have a guaranteed order.
         self.assertTrue(list, type(L))
         self.assertEqual(set(L), set([dummy, dummy2]))

transaction/weakset.py

@@ -73,9 +73,11 @@ class WeakSet(object):
     # And so on.  Stress tests showed that it was easy to get into a state
     # where a WeakSet grows without bounds, despite that almost all its
     # elements are actually trash.  By returning a list of weakrefs instead,
-    # we avoid that, although the decision to use weakrefs is now# very
+    # we avoid that, although the decision to use weakrefs is now very
     # visible to our clients.
     def as_weakref_list(self):
         # We're cheating by breaking into the internals of Python's
         # WeakValueDictionary here (accessing its .data attribute).
-        return self.data.data.values()
+        # Python 3: be sure to freeze the list, to avoid RuntimeError:
+        # dictionary changed size during iteration.
+        return list(self.data.data.values())