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From: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David E. Wheeler <david@justatheory.com>
Cc: Christophe Pettus <christophe.pettus@pgexperts.com>
Cc: pgsql-bugs@postgresql.org
Cc: pgsql-hackers@postgresql.org
Subject: Re: BUG #14825: enum type: unsafe use?
Date: Tue, 26 Sep 2017 15:33:03 -0400
Message-ID: <38835cdd-c378-8d7a-013b-fd733c78c649@2ndQuadrant.com> (raw)
In-Reply-To: <329.1506451066@sss.pgh.pa.us>
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On 09/26/2017 02:37 PM, Tom Lane wrote:
> 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?


I think I would mark enum_in and friends as parallel-restricted. Yes I
know it would involve a cat version bump, so I'll understand if that's
not acceptable, but it seems to me the best of a bad bunch of choices.
Second choice might be turning off parallel mode if the hash exists, but
I'm unclear how that would work.

cheers

andrew

-- 

Andrew Dunstan                https://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services



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