[Python-Dev] Issue 14417: consequences of new dict runtime error

[R. David Murray]

On Sun, 01 Apr 2012 03:03:13 +1000, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On Sun, Apr 1, 2012 at 2:09 AM, Guido van Rossum <guido at python.org> wrote:
> > Here's a different puzzle. Has anyone written a demo yet that provokes
> > this RuntimeError, without cheating? (Cheating would be to mutate the
> > dict from *inside* the __eq__ or __hash__ method.) If you're serious
> > about revisiting this, I'd like to see at least one example of a
> > program that is broken by the change. Otherwise I think the status quo
> > in the 3.3 repo should prevail -- I don't want to be stymied by
> > superstition.
> 
> I attached an attempt to *deliberately* break the new behaviour to the
> tracker issue. It isn't actually breaking for me, so I'd like other
> folks to look at it to see if I missed something in my implementation,
> of if it's just genuinely that hard to induce the necessary bad timing
> of a preemptive thread switch.

Thanks, Nick.  It looks reasonable to me, but I've only given it a quick
look so far (I'll try to think about it more deeply later today).

If it is indeed hard to provoke, then I'm fine with leaving the
RuntimeError as a signal that the application needs to add some locking.
My concern was that we'd have working production code that would start
breaking.  If it takes a *lot* of threads or a *lot* of mutation to
trigger it, then it is going to be a lot less likely to happen anyway,
since such programs are going to be much more careful about locking
anyway.

--David