Date: Thu, 21 Nov 2024 09:21:16 -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 07:27:22AM +0000, Bertrand Drouvot wrote:
> +		/*
> +		 * 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.
> +		 */
...
> +				/*
> +				 * If we've hit a byte with high bit clear (an ASCII byte), we
> +				 * know we can't be in the middle of a multibyte character,
> +				 * because all bytes of a multibyte character must have their
> +				 * high bits set. Any following byte must therefore be the
> +				 * start of a new character, so we can stop looking for
> +				 * earlier truncation points.
> +				 */

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.

-- 
  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?"