forward-patch from release21-maint branch:

anthonybaxter · anthonybaxter · commit ed9057083bcd · 2001-12-21T05:13:37.000Z
Make dumbdbm merely "dumb", rather than "terminally broken". Without this
  patch, it's almost impossible to use dumbdbm _without_ causing horrible
  datalossage. With this patch, dumbdbm passes my own horrible torture test,
  as well as the roundup test suite.

  dumbdbm really could do with a smidgin of a rewrite or two, but that's not
  suitable for the release21-maint branch.
diff --git a/Lib/dumbdbm.py b/Lib/dumbdbm.py
@@ -143,9 +143,15 @@ def __len__(self):
         return len(self._index)
 
     def close(self):
+        self._commit()
         self._index = None
         self._datfile = self._dirfile = self._bakfile = None
 
+    def __del__(self):
+        if self._index is not None:
+            self._commit()
+  
+
 
 def open(file, flag=None, mode=0666):
     # flag, mode arguments are currently ignored
