Re: add-hook sets hook globally if buffer-local hook is nil

[Stefan Monnier]

>     @@ -856,9 +870,7 @@
>                 (set (make-local-variable hook) (list t)))
>        ;; Detect the case where make-local-variable was used on a hook
>        ;; and do what we used to do.
>     -    (when (and (local-variable-p hook)
>     -        (not (and (consp (symbol-value hook))
>     -                  (memq t (symbol-value hook)))))
>     +    (unless (and (consp (symbol-value hook)) (memq t (symbol-value 
> hook)))
>          (setq local t)))
>        (let ((hook-value (if local (symbol-value hook) (default-value hook))))
>        ;; If the hook value is a single function, turn it into a list.

> i dont think this change causes the problem.
> this code detects case where hook var has a local binding
> and it wasnt made with add-hook.
> i think thye change makes it safer.

I disagree.  The old code (the above diff is reversed, BTW, sorry) set
`local' to t more often, which makes it less likely that add-hook will
change the global value by mistake.
More specifically, the old code set `local' to t even if the variable is
not local-variable-p but is local-variable-if-set-p.
Thus with a variable that has been make-variable-buffer-local'd, the new
code will fail to set `local' to t and will thus incorrectly modify the global
part of the hook.  That's the bug we're seeing.

A better fix than the one you provided is to replace in the above code
`local-variable-p' with `local-variable--if-set-p', but I still have no
idea why the additional check would be needed in the first place.


        Stefan