Date: Thu, 21 Nov 2024 09:47:56 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Nathan Bossart <nathandbossart@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 02:35:50PM +0000, Bertrand Drouvot wrote:
> On Thu, Nov 21, 2024 at 09:21:16AM -0500, Bruce Momjian wrote:
> > I don't understand this logic.  Why are two bytes important?  If we knew
> > it was UTF8 we could check for non-first bytes always starting with
> > bits 10, but we can't know that.
> 
> I think this is because this is a reliable way to detect if the truncation happened
> in the middle of a character, without needing to know the specifics of the encoding.
> 
> My understanding is that the key insight is that in any multibyte encoding, all
> bytes within a multibyte character will have their high bits set.
> 
> That's just my understanding from the code and Tom's previous explanations:  I
> might be wrong as not an expert in this area.

But the logic doesn't make sense.  Why would two bytes be any different
than one?  I assumed you would just remove all trailing high-bit bytes
and stop and the first non-high-bit byte.  Also, do we really expect
there to be trailing multi-byte characters and then some ASCII before
it? Isn't it likely it will be all ASCII or all multi-byte characters? 
I guess for Latin1, it would work fine, but I assume for Asian
languages, it will be almost all multi-byte characters.  I guess digits
would be ASCII.  This all just seems very unfocused.

-- 
  Bruce Momjian  <bruce@momjian.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?"