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

On Fri, Nov 22, 2024 at 02:23:47PM -0500, Tom Lane wrote:
> +	if (strlen(dbname) < NAMEDATALEN)
> +	{
> +		/* Typical, easy case: no truncation needed */
> +		tuple = GetDatabaseTupleInternal(relation, dbname);
> +	}
> +	else if (!IS_HIGHBIT_SET(dbname[NAMEDATALEN - 1]) ||
> +			 !IS_HIGHBIT_SET(dbname[NAMEDATALEN - 2]))
> +	{
> +		/* Also easy: truncation at NAMEDATALEN - 1 cannot break an MB char */
> +		memcpy(tname, dbname, sizeof(tname));
> +		tname[NAMEDATALEN - 1] = '\0';
> +		tuple = GetDatabaseTupleInternal(relation, tname);

I had some time to think about this issue and I now realize the test
above is correct, but I couldn't figure out why it was correct before.
If we want to use this test, which I now think is fine, I suggest the
following comment:

	If we put a NULL byte at byte offset NAMEDATALEN - 1, we don't want
	to break a multi-byte character when doing this.  If byte offset
	NAMEDATALEN - 1 does not have its high bit set, we can be sure
	we will not break a multi-byte character during the overwrite.
	Also, if NAMEDATALEN - 2 does not have its high bit set, then
	NAMEDATALEN - 1 is either a single-byte non-ASCII character,
	or is the _start_ of a multi-byte character, so it is also safe
	to overwrite it without breaking a multi-byte character.

Sorry I did not figure this out earlier.

-- 
  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?"