Date: Thu, 21 Nov 2024 11:09:14 -0600
From: Nathan Bossart <nathandbossart@gmail.com>
To: Bruce Momjian <bruce@momjian.us>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>,
	Tom Lane <tgl@sss.pgh.pa.us>, adam@labkey.com,
	pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #18711: Attempting a connection with a database name longer
 than 63 characters now fails
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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.

> This text:
> 
>                * If the original name is too long and we see two consecutive bytes
>                * with their high bits set at the truncation point, we might have
>                * truncated in the middle of a multibyte character. In multibyte
>                * encodings, every byte of a multibyte character has its high bit
>                * set. 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. We need to try truncating one more byte back to find the
>                * start of the next character.
> 
> 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?

[0] https://postgr.es/m/3976665.1732057784%40sss.pgh.pa.us
[1] https://postgr.es/m/158506.1732120196%40sss.pgh.pa.us

-- 
nathan