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

Hi,

On Thu, Nov 21, 2024 at 09:21:16AM -0500, Bruce Momjian wrote:
> 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.

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.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com