Re: [sqlsmith] Unpinning error in parallel worker

On Fri, Apr 20, 2018, at 00:42, Thomas Munro wrote:
> On Wed, Apr 18, 2018 at 11:43 AM, Jonathan Rudenberg
> <jonathan(at)titanous(dot)com> wrote:
> > On Tue, Apr 17, 2018, at 19:31, Thomas Munro wrote:
> >> On Wed, Apr 18, 2018 at 11:01 AM, Jonathan Rudenberg
> >> <jonathan(at)titanous(dot)com> wrote:
> >> > Yep, I think I know approximately what it looked like, I've attached a lightly redacted plan. All of the hung queries were running some variant of this plan as far as I can tell.
> >>
> >> Hmm, that isn't a parallel query. I was expecting to see "Gather" and
> >> "Parallel" in there.
> >
> > Oops, I'm really sorry about that. I only have the first part of the hung queries, and there are a few variants. Here's one that's parallel.
>
> I spent some time trying to reproduce this failure without any luck,
> using query plans similar to your Gather plan fragment, and using some
> test harness code for the allocator stuff in isolation.
>
> I had an idea that (1) freeing a large object that releases and unpins
> a segment in one backend and then (2) freeing it again in another
> backend (illegally) might produce this effect with sufficiently bad
> luck. I'm still trying to reproduce that without any success, but I
> get other kinds of failures which I think you'd be seeing too if that
> hunch were right. Still looking...

Thanks for investigating! We have monitoring in place and a plan to grab stack traces if this happens again. Is there anything else that would be useful for us to try to get in addition to a stack trace from the process processing the stuck query?