Re: pg_trgm comparison bug on cross-architecture replication due to different char implementation

On Wed, Oct 2, 2024 at 10:02 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Tue, Oct 1, 2024 at 8:57 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >
> > Noah Misch <noah(at)leadboat(dot)com> writes:
> > > On Tue, Oct 01, 2024 at 11:55:48AM -0700, Masahiko Sawada wrote:
> > >> Considering that the population of database cluster signedness will
> > >> converge to signedness=true in the future, we can consider using
> > >> -fsigned-char to prevent similar problems for the future. We need to
> > >> think about possible side-effects as well, though.
> >
> > > It's good to think about -fsigned-char. While I find it tempting, several
> > > things would need to hold for us to benefit from it:
> >
> > > - Every supported compiler has to offer it or an equivalent.
> > > - The non-compiler parts of every supported C implementation need to
> > > cooperate. For example, CHAR_MIN must change in response to the flag. See
> > > the first comment in cash_in().
> > > - Libraries we depend on can't do anything incompatible with it.
> >
> > > Given that, I would lean toward not using -fsigned-char. It's unlikely all
> > > three things will hold. Even if they do, the benefit is not large.
> >
> > I am very, very strongly against deciding that Postgres will only
> > support one setting of char signedness. It's a step on the way to
> > hardware monoculture, and we know where that eventually leads.
> > (In other words, I categorically reject Sawada-san's assertion
> > that signed chars will become universal. I'd reject the opposite
> > assertion as well.)
>
> Thank you for pointing this out. I agree with both of you.
>

I've attached PoC patches for the idea Noah proposed. Newly created
clusters unconditionally have default_char_signedness=true, and the
only source of signedness=false is pg_upgrade. To update the
signedness in the controlfile, pg_resetwal now has a new option
--char-signedness, which is used by pg_upgrade internally. Feedback is
very welcome.

Regards,

--
Masahiko Sawada
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