bug#53207: 28.0.91; create-lockfiles nil breaks file change detection

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Wed, 12 Jan 2022 13:13:40 -0500
> Cc: michael.albinus@gmx.de, 53207@debbugs.gnu.org
> 
> Probably a consequence of 9ce6541ac9, specifically:
> 
>   * src/filelock.c (lock_file): Don't check create_lockfiles.

Actually, the more relevant part is this:

    (Flock_file): Check create_lockfiles.

which did

  -  CHECK_STRING (file);
  -  lock_file (file);
  +#ifndef MSDOS
  +  /* Don't do locking if the user has opted out.  */
  +  if (create_lockfiles)
  +    {
  +      CHECK_STRING (file);
  +      lock_file (file);
  +    }
  +#endif /* MSDOS */

> which would seem to mean that ask-user-about-supersession-threat
> is no longer called when create-lockfiles is nil.
> Was this intentional?

Michael, can you please chime in?  The long discussion we had back
then doesn't seem to mention this aspect, or maybe I'm missing
something?

We should either document this change (if we think what we have now is
the intended behavior), or we should move the call to
userlock--ask-user-about-supersession-threat into Flock_file if it's
unintended.

Personally, I prefer the former, since lock_file accesses the lock
file, which doesn't make a lot of sense if the user opted out of the
feature.  But that's me.

Lars, WDYT?

Thanks.