RE: [bug?] Missed parallel safety checks, and wrong parallel safety

> > Sometimes this is actually quite useful. You might know that, while
> > the function is in general volatile, it is immutable in the particular
> > way that you are using it. Or, perhaps, you are using the volatile
> > function incidentally and it doesn't affect the output of your
> > function at all. Or, maybe you actually want to build an index that
> > might break, and then it's up to you to rebuild the index if and when
> > that is required. Users do this kind of thing all the time, I think,
> > and would be unhappy if we started checking it more rigorously than we
> > do today.
> >
> > Now, I don't see why the same idea can't or shouldn't apply to
> > parallel-safety. If you call a parallel-unsafe function in a parallel
> > context, it's pretty likely that you are going to get an error, and so
> > you might not want to do it. If the function is written in C, it could
> > even cause horrible things to happen so that you crash the whole
> > backend or something, but I tried to set things up so that for
> > built-in functions you'll just get an error. But on the other hand,
> > maybe the parallel-unsafe function you are calling is not
> > parallel-unsafe in all cases. If you want to create a wrapper function
> > that is labelled parallel-safe and try to make that it only calls the
> > parallel-unsafe function in the cases where there's no safety problem,
> > that's up to you!
> >
>
> I think it is difficult to say for what purpose parallel-unsafe function got called in
> parallel context so if we give an error in cases where otherwise it could lead to
> a crash or caused other horrible things, users will probably appreciate us.
> OTOH, if the parallel-safety labeling is wrong (parallel-safe function is marked
> parallel-unsafe) and we gave an error in such a case, the user can always change
> the parallel-safety attribute by using Alter Function.
> Now, if adding such a check is costly or needs some major re-design then
> probably it might not be worth whereas I don't think that is the case for
> non-built-in function invocation.

Temporarily, Just in case someone want to take a look at the patch for the safety check.
I splited the patch into 0001(parallel safety check for user define function), 0003(parallel safety check for builtin function)
and the fix for testcases.

IMO, With such a check to give an error when detecting parallel unsafe function in parallel mode,
it will be easier for users to discover potential threats(parallel unsafe function) in parallel mode.

I think users is likely to invoke parallel unsafe function inner a parallel safe function unintentionally.
Such a check can help they detect the problem easier.

Although, the strict check limits some usages(intentionally wrapper function) like Robert-san said.
To mitigate the effect of the limit, I was thinking can we do the safety check conditionally, such as only check the top function invoke and/or
introduce a guc option to control whether do the strict parallel safety check? Thoughts ?

Best regards,
houzj