bug#46933: Possible bugs in filepos-to-bufferpos / bufferpos-to-filepos

[Eli Zaretskii <eliz@gnu.org>]

> From: handa <handa@gnu.org>
> Cc: gregory@heytings.org, 46933@debbugs.gnu.org
> Date: Fri, 02 Apr 2021 00:14:02 +0900
> 
> In article <83tuovmivc.fsf@gnu.org>, Eli Zaretskii <eliz@gnu.org> writes:
> 
> > > Any coding system can have :post-read-conversion and
> > > :pre-write-conversion functions, it is not guaranteed that encoded byte
> > > length is greater than the number of characters.
> 
> > Agreed, but AFAICT, ISO-2022-JP doesn't have any of these attributes,
> > right?
> 
> Yes, but one can add them by coding-system-put.

Leaving the :pre-write/:post-read-conversion use case aside, do we
have some means of find where ISO-2022 shift-in/out sequence begins
and ends, so that we never try to decode a partial sequence (and
produce "characters" that are not really in the original buffer)?
If not, where can I find the description of every kind of such
sequences, i.e. sequences that modify the decoder state without
producing any characters?

(UTF-8 has the same issue, btw, but in that case we have a simpler
solution.)