Fixed a bug in commits with savepoints and changes since savepoints.

Once we start using savepoints, we need to make sure that all data are
committed through the savepoints. Otherwise, things can get committed
in the wrong order, leading to conflicts.

src/ZODB/Connection.py

@@ -63,7 +63,6 @@ class Connection(ExportImport, object):
 
     _storage = _normal_storage = _savepoint_storage = None
 
-    _tmp = None
     _code_timestamp = 0
 
     ##########################################################################
@@ -437,9 +436,17 @@ class Connection(ExportImport, object):
         """Commit changes to an object"""
 
         if self._savepoint_storage is not None:
+
+            # We first checkpoint the current changes to the savepoint
+            self.savepoint()
+
+            # then commit all of the savepoint changes at once
             self._commit_savepoint(transaction)
 
-        self._commit(transaction)
+            # No need to call _commit since savepoint did.
+            
+        else:
+            self._commit(transaction)
 
     def _commit(self, transaction):
         """Commit changes to an object"""

src/ZODB/tests/testConnectionSavepoint.py

@@ -33,6 +33,21 @@ def testAddingThenModifyThenAbort():
     >>> transaction.abort()
   
 """
+
+def testModifyThenSavePointThenModifySomeMoreThenCommit():
+    """
+    >>> import ZODB.tests.util
+    >>> db = ZODB.tests.util.DB()
+    >>> connection = db.open()
+    >>> root = connection.root()
+    >>> sp = transaction.savepoint()
+    >>> root['a'] = 1
+    >>> sp = transaction.savepoint()
+    >>> root['a'] = 2
+    >>> transaction.commit()
+  
+"""
+
 def test_suite():
     return unittest.TestSuite((
         doctest.DocFileSuite('testConnectionSavepoint.txt'),