Re: crashes due to setting max_parallel_workers=0

On Sat, Mar 25, 2017 at 7:01 PM, Peter Eisentraut <
peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:

> On 3/25/17 09:01, David Rowley wrote:
> > On 25 March 2017 at 23:09, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>
> wrote:
> >> Also another point which I think we should fix is, when someone set
> >> max_parallel_workers = 0, we should also set the
> >> max_parallel_workers_per_gather
> >> to zero. So that way it we can avoid generating the gather path with
> >> max_parallel_worker = 0.
> > I see that it was actually quite useful that it works the way it does.
> > If it had worked the same as max_parallel_workers_per_gather, then
> > likely Tomas would never have found this bug.
>
> Another problem is that the GUC system doesn't really support cases
> where the validity of one setting depends on the current value of
> another setting. So each individual setting needs to be robust against
> cases of related settings being nonsensical.
>

Okay.

About the original issue reported by Tomas, I did more debugging and
found that - problem was gather_merge_clear_slots() was not returning
the clear slot when nreader is zero (means nworkers_launched = 0).
Due to the same scan was continue even all the tuple are exhausted,
and then end up with server crash at gather_merge_getnext(). In the patch
I also added the Assert into gather_merge_getnext(), about the index
should be less then the nreaders + 1 (leader).

PFA simple patch to fix the problem.

Thanks,
Rushabh Lathia
www.Enterprisedb.com