forward-patch from release21-maint branch:

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.