Re: Leader backend hang on IPC/ParallelFinish when LWLock held at parallel query start

On Thu, Nov 07, 2024 at 12:17:21PM -0500, Tom Lane wrote:
> Noah Misch <noah@leadboat.com> writes:
> > On Wed, Sep 18, 2024 at 12:23:42AM -0400, Tom Lane wrote:
> >> I do not have much faith in this patch.  It assumes that the
> >> condition "interrupts can be processed" is the same at plan time and
> >> execution time.  For plans extracted from the plan cache, there seems
> >> little reason to assume that must be true.  The proposed test case
> >> cannot trigger that (today anyway) because SQL-language functions
> >> don't deal in cached plans, but I suspect it could be broken with a
> >> test case using a plpgsql function instead.
> 
> > Good point.  I missed that.
> 
> While working on the release notes, I noticed that nothing further
> got done about this concern.  What do you think of adding a test
> somewhere early in executor startup, to the effect of
> 
>     if (!INTERRUPTS_CAN_BE_PROCESSED())
>         ereport(ERROR,
>                 (errmsg("cannot start a query with interrupts disabled")));
> 
> It's definitely a band-aid, but it seems better than leaving
> things at the status quo.

That would fire in queries that don't error or hang today, so I think that
would be worse than the status quo.

This thread's previous commit just forced a serial plan.  The executor
counterpart would look like having the executor do what it does when there are
no free worker slots.