[ZODB-Dev] preventing a transaction from committing within app code

[Chris McDonough]

Is there or should there be an API within ZODB which would allow app
code to manually mark a transaction (or a connection) as
"uncommittable"?  I have a need for this sort of thing for a variety of
reasons (at least I believe I do, I'd like to hear from others that also
think they do ;-), and one way to do it is to manually insert something
into a Connection's _conflicts dictionary (the same thing that
Connection._is_invalidated does).  Of course that's wrong and nasty. 
Any better way?

- C