Re: Parallel worker hangs while handling errors.

Thanks for reviewing and adding your thoughts, My comments are inline.

On Fri, Jul 17, 2020 at 1:21 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> The same hang issue can occur(though I'm not able to back it up with a
> use case), in the cases from wherever the EmitErrorReport() is called
> from "if (sigsetjmp(local_sigjmp_buf, 1) != 0)" block, such as
> autovacuum.c, bgwriter.c, bgworker.c, checkpointer.c, walwriter.c, and
> postgres.c.
>

I'm not sure if this can occur in other cases.

> >
> > One of the fixes could be to call BackgroundWorkerUnblockSignals just
> > after sigsetjmp. I'm not sure if this is the best solution.
> > Robert & myself had a discussion about the problem yesterday. We felt
> > this is a genuine problem with the parallel worker error handling and
> > need to be fixed.
> >
>
> Note that, in all sigsetjmp blocks, we intentionally
> HOLD_INTERRUPTS(), to not cause any issues while performing error
> handling, I'm concerned here that now, if we directly call
> BackgroundWorkerUnblockSignals() which will open up all the signals
> and our main intention of holding interrupts or block signals may go
> away.
>
> Since the main problem for this hang issue is because of blocking
> SIGUSR1, in sigsetjmp, can't we just only unblock only the SIGUSR1,
> instead of unblocking all signals? I tried this with parallel copy
> hang, the issue is resolved.
>

On putting further thoughts on this, I feel just unblocking SIGUSR1
would be the right approach in this case. I'm attaching a new patch
which unblocks SIGUSR1 signal. I have verified that the original issue
with WIP parallel copy patch gets fixed. I have made changes only in
bgworker.c as we require the parallel worker to receive this signal
and continue processing. I have not included the changes for other
processes as I'm not sure if this scenario is applicable for other
processes.

Regards,
Vignesh
EnterpriseDB: http://www.enterprisedb.com