bug#70587: More buffers that ought to be read-only

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Dan Jacobson <jidanni@jidanni.org>,  70587@debbugs.gnu.org
> Date: Sat, 27 Apr 2024 23:02:16 -0400
> 
> > Stefan, this seems to be due to your changes in b2e4481906ec: you
> > added '(setq buffer-read-only t)' which was not there before (in fact,
> > the code before those changes did the opposite:
> >
> > --- a/lisp/vc/diff.el
> > +++ b/lisp/vc/diff.el
> > @@ -131,37 +147,32 @@
> > -        (thisdir default-directory)
> > -        proc)
> > -    (save-excursion
> > -      (display-buffer buf)
> > -      (set-buffer buf)
> > -      (setq buffer-read-only nil)
> > +    (with-current-buffer buf
> > +      (setq buffer-read-only t)
> >
> > So I suggest to remove the setting of buffer-read-only there'
> > diff-mode will do TRT according to diff-default-read-only.
> 
> There is no right or wrong answer here.  The current design reflects the
> behavior I think makes more sense, but that's largely just my
> personal preference.  The above patch was no mistake.

If your personal preference is that diff-mode buffers should be
read-only, then you (and those who agree with you) should customize
diff-default-read-only to a non-nil value.  What we have now is a
mess: some cases which invoke diff-mode behave according to
diff-default-read-only, while others behave differently
unconditionally.

However, if by "no mistake" you mean something else, please elaborate
why it was no mistake.

> But if we install the above patch, then *vc-diff* will suddenly not be
> read-only any more, so we should arguably change
> `diff-default-read-only` at the same time, which will in turn cause
> another behavior change when people open a `.diff` file.

Why should *vc-diff* be read-only regardless of
diff-default-read-only?  And why visiting a .diff file is different in
this regard?