Re: BUG #14825: enum type: unsafe use?

I wrote:
> Pushed; sorry for the delay.

... and the buildfarm's not too happy. It looks like force_parallel_mode
breaks all the regression test cases around unsafe enums; which on
reflection is unsurprising, because parallel workers will not have access
to the parent's blacklist hash, so they will think unsafe values are safe.

Now, as long as parallel workers are read-only, perhaps this matters
little; they would not be allowed to write unsafe values into tables
anyway. I'm concerned though about whether it might be possible for a
parallel worker to return an unsafe value to the parent (in OID form)
and then the parent writes it into a table. If we can convince ourselves
that's not possible, it might be okay to just turn off force_parallel_mode
for these test cases.

A safer answer would be to mark enum_in() and other callers of
check_safe_enum_use() as parallel-restricted. That'd require a
post-RC1 catversion bump, which seems pretty unpleasant, but
none of the other answers are nice either.

Transmitting the blacklist hash to workers would be a good long-term
answer, but I don't want to try to shoehorn it in for v10.

Another idea is that maybe the existence of a blacklist hash should
be enough to turn off parallel mode altogether ... but ugh.

Or maybe we're back to "revert the whole feature, go back to 9.6
behavior".

Thoughts?

regards, tom lane