Windows tests exposed a bug in MVCCMappingStorage: the transaction ID

did not necessarily increase for every transaction.  Fixed and tested.

src/ZODB/MappingStorage.py

@@ -280,7 +280,11 @@ class MappingStorage(object):
         self._transaction = transaction
         self._tdata = {}
         if tid is None:
-            tid = ZODB.utils.newTid(self._ltid)
+            if self._transactions:
+                old_tid = self._transactions.maxKey()
+            else:
+                old_tid = None
+            tid = ZODB.utils.newTid(old_tid)
         self._tid = tid
 
     # ZODB.interfaces.IStorage

src/ZODB/tests/MVCCMappingStorage.py

@@ -50,7 +50,10 @@ class MVCCMappingStorage(MappingStorage):
     def poll_invalidations(self):
         """Poll the storage for changes by other connections.
         """
-        new_tid = self._transactions.maxKey()
+        if self._transactions:
+            new_tid = self._transactions.maxKey()
+        else:
+            new_tid = ''
 
         if self._polled_tid:
             if not self._transactions.has_key(self._polled_tid):

src/ZODB/tests/testMVCCMappingStorage.py

@@ -45,16 +45,10 @@ class MVCCTests:
             r2 = c2.root()
             self.assert_('myobj' not in r2)
 
-            old_tid = c1._storage._polled_tid
             c1.transaction_manager.commit()
-            new_tid = c1._storage._polled_tid
-
-            self.assertNotEqual(new_tid, old_tid)
-            self.assertEqual(c2._storage._polled_tid, old_tid)
-
             self.assert_('myobj' not in r2)
+
             c2.sync()
-            self.assertEqual(new_tid, c2._storage._polled_tid)
             self.assert_('myobj' in r2)
             self.assert_(r2['myobj'] == 'yes')
         finally:
@@ -153,6 +147,26 @@ class MVCCMappingStorageTests(
         pass # we don't support undo yet
     checkUndoZombie = checkLoadBeforeUndo
 
+    def checkTransactionIdIncreases(self):
+        import time
+        from ZODB.utils import newTid
+        from ZODB.TimeStamp import TimeStamp
+        t = transaction.Transaction()
+        self._storage.tpc_begin(t)
+        self._storage.tpc_vote(t)
+        self._storage.tpc_finish(t)
+
+        # Add a fake transaction
+        transactions = self._storage._transactions
+        self.assertEqual(1, len(transactions))
+        fake_timestamp = 'zzzzzzzy'  # the year 5735 ;-)
+        transactions[fake_timestamp] = transactions.values()[0]
+
+        # Verify the next transaction comes after the fake transaction
+        t = transaction.Transaction()
+        self._storage.tpc_begin(t)
+        self.assertEqual(self._storage._tid, 'zzzzzzzz')
+
 
 def test_suite():
     suite = unittest.makeSuite(MVCCMappingStorageTests, 'check')