Re: VC, read-only buffers for writable files

[Stefan Monnier]

> > Currently VC sometimes makes a file read-only because it thinks it's
> > better that way for the user.  More specifically, it makes it read-only
> > if the file seems to be locked by another user.
> 
> I have talked this over with Stefan several times, and we have reached
> no agreement.

We did agree to postpone the decision to after 21.1, tho ;-)

> > This special behavior used to apply to SCCS, RCS and CVS under Emacs-20
> > and was silently changed in Emacs-21 to only apply to RCS and SCCS (it was
> > changed as a side effect of another change).  Nobody complained about
> > this change and I think we should go further and remove this hack
> > altogether so it doesn't aply to RCS and SCCS either.
> 
> The behavior wasn't changed accidentally for CVS.  There are no locks

Maybe it wasn't accidentally, but it was changed.  In Emacs-20 you
could end up in CVS with a read-only buffer because the owner of the
locally modified file was different from the current user (or
seemed that way to VC).  That's because VC considered a locally modified
CVS file as "locked by the owner of the file".

> under CVS, and Emacs-21 handles that correctly by not assuming that a
> file could be "locked" by anybody.  RCS and SCCS do have locks, and
> Emacs-21 deliberately chooses to enforce the locking.  This is not a
> "hack".

Sorry for using "hack".  I tried to be somewhat "neutral" in my post,
but my bias took precedence.  The reason why I think it's a hack
is because such things *should* be enforced with file permissions
and are enforced that way on all the systems I've used.

> Here is the relevant portion of code from vc-hooks.el, since the
> comments may make it clearer to everybody.
> 
>     ;; If the file is locked by some other user, make
>     ;; the buffer read-only.  Like this, even root
>     ;; cannot modify a file that someone else has locked.
>     (and (equal file (buffer-file-name))
>          (stringp (vc-state file))
>        (setq buffer-read-only t))
>     ;; If the user is root, and the file is not owner-writable,
>     ;; then pretend that we can't write it
>     ;; even though we can (because root can write anything).
>     ;; This way, even root cannot modify a file that isn't locked.
>     (and (equal file (buffer-file-name))
>        (not buffer-read-only)
>        (zerop (user-real-uid))
>        (zerop (logand (file-modes (buffer-file-name)) 128))
>        (setq buffer-read-only t)))

Note that I'd gladly remove the second half as well, but I'm mostly
interested in the first half (the behavior of Emacs when run as root
is not of much concern to me).


        Stefan