Re: BUG #18711: Attempting a connection with a database name longer than 63 characters now fails

On Thu, Nov 21, 2024 at 11:09:14AM -0600, Nathan Bossart wrote:
> On Thu, Nov 21, 2024 at 11:44:44AM -0500, Bruce Momjian wrote:
> > On Thu, Nov 21, 2024 at 09:14:23AM -0600, Nathan Bossart wrote:
> >> Tom provided a concise explanation upthread [0]. My understanding is the
> >> same as Bertrand's, i.e., this is an easy way to rule out a bunch of cases
> >> where we know that we couldn't possibly have truncated in the middle of a
> >> multi-byte character. This allows us to avoid doing multiple pg_database
> >> lookups.
> >
> > Where does Tom mention anything about checking two bytes?
>
> Here [0]. And he further elaborated on this idea here [1].
>
> > He is
> > basically saying remove all trailing high-bit characters until you get a
> > match, because once you get a match, you are have found the point of
> > valid truncation for the encoding.
>
> Yes, we still need to do that if it's possible the truncation wiped out
> part of a multi-byte character. But it's not possible that we truncated
> part of a multi-byte character if the NAMEDATALEN-1'th or NAMEDATALEN-2'th
> byte is ASCII, in which case we can avoid doing extra lookups.

Why would you check for two characters at the end rather than just a
normal check in the main loop?

> > needs to be fixed, at a minimum, specifically, "So if IS_HIGHBIT_SET is
> > true for both NAMEDATALEN-1 and NAMEDATALEN-2, we know we're in the
> > middle of a multibyte character."
>
> Agreed, the second-to-last sentence should be adjusted to something like
> "we might be in the middle of a multibyte character." We don't know for
> sure.
>
> >> * Try to do multibyte-aware truncation (the patch at hand).
> >
> > Yes, I am fine with that, but we need to do more than the patch does to
> > accomplish this, unless I am totally confused.
>
> What more do you think is required?

I think the IS_HIGHBIT_SET needs to be integrated into the 'for' loop
more clearly; the 'if' check plus the comment above it is just
confusing.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

When a patient asks the doctor, "Am I going to die?", he means
"Am I going to die soon?"