Re: crashes due to setting max_parallel_workers=0

On 25 March 2017 at 13:10, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> while working on a patch I ran into some crashes that seem to be caused by
> inconsistent handling of max_parallel_workers - queries still seem to be
> planned with parallel plans enabled, but then crash at execution.
>
> The attached script reproduces the issue on a simple query, causing crashed
> in GatherMerge, but I assume the issue is more general.

I had a look at this and found a bunch of off by 1 errors in the code.

I've attached a couple of patches, one is the minimal fix, and one
more complete one.

In the more complete one I ended up ditching the
GatherMergeState->nreaders altogether. It was always the same as
nworkers_launched anyway, so I saw no point in it.
Here I've attempted to make the code a bit more understandable, to
prevent further confusion about how many elements are in each array.
Probably there's more that can be done here. I see GatherState has
nreaders too, but I've not looked into the detail of if it suffers
from the same weirdness of nreaders always matching nworkers_launched.

--
David Rowley http://www.2ndQuadrant.com/
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