Add two tests cases the verify the pickles look like old pickles

src/ZODB/tests/testPersistentMapping.py

+"""Verify that PersistentMapping works with old versions of Zope.
+
+The comments in PersistentMapping.py address the issue in some detail.
+The pickled form of a PersistentMapping must use _container to store
+the actual mapping, because old versions of Zope used this attribute.
+If the new code doesn't generate pickles that are consistent with the
+old code, developers will have a hard time testing the new code.
+"""
+
+import unittest
+
+import ZODB
+from ZODB.MappingStorage import MappingStorage
+from ZODB.Transaction import Transaction
+import cPickle
+import cStringIO
+import sys
+
+# This pickle contains a persistent mapping pickle created from the
+# old code.
+pickle = ('((U\x0bPersistenceq\x01U\x11PersistentMappingtq\x02Nt.}q\x03U\n'
+          '_containerq\x04}q\x05U\x07versionq\x06U\x03oldq\x07ss.\n')
+
+class PMTests(unittest.TestCase):
+
+    def checkOldStyleRoot(self):
+        # insert the pickle in place of the root
+        s = MappingStorage()
+        t = Transaction()
+        s.tpc_begin(t)
+        s.store('\000' * 8, None, pickle, '', t)
+        s.tpc_vote(t)
+        s.tpc_finish(t)
+        
+        db = ZODB.DB(s)
+        # If the root can be loaded successfully, we should be okay.
+        r = db.open().root()
+        # But make sure it looks like a new mapping
+        self.assert_(hasattr(r, 'data'))
+        self.assert_(not hasattr(r, '_container'))
+
+    def checkNewPicklesAreSafe(self):
+        s = MappingStorage()
+        db = ZODB.DB(s)
+        r = db.open().root()
+        r[1] = 1
+        r[2] = 2
+##        r[3] = r
+        get_transaction().commit()
+        # MappingStorage stores serialno + pickle in its _index.
+        root_pickle = s._index['\000' * 8][8:]
+
+        f = cStringIO.StringIO(root_pickle)
+        u = cPickle.Unpickler(f)
+        klass_info = u.load()
+        klass = find_global(*klass_info[0])
+        inst = klass()
+        state = u.load()
+        inst.__setstate__(state)
+
+        self.assert_(hasattr(inst, '_container'))
+        self.assert_(not hasattr(inst, 'data'))
+
+def find_global(modulename, classname):
+    """Helper for this test suite to get special PersistentMapping"""
+
+    print modulename, classname
+    if classname == "PersistentMapping":
+        class PersistentMapping:
+            def __setstate__(self, state):
+                self.__dict__.update(state)
+        return PersistentMapping
+    else:
+        __import__(modulename)
+        mod = sys.modules[modulename]
+        return getattr(mod, classname)
+
+def test_suite():
+    return unittest.makeSuite(PMTests, 'check')
+
+if __name__ == "__main__":
+    loader = unittest.TestLoader()
+    loader.testMethodPrefix = "check"
+    unittest.main(testLoader=loader)