bug#64311: [PATCH] Fix shell-dirtrack-mode showing up as enabled in unrelated buffers

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

FWIW, my take on the root cause is that I made a mistake in commit
05327ca9724287cc3da4c625f180da5ab11be998 where I forgot to remove the
`defvar` of `shell-dirtrackp` (and I swapped the args to `defvaralias`).

It's fundamentally wrong to define a variable at 2 places.

> diff --git a/lisp/shell.el b/lisp/shell.el
> index 5cf108b..4bbd295 100644
> --- a/lisp/shell.el
> +++ b/lisp/shell.el
> @@ -348,8 +348,10 @@ shell-dirstack
>  
>  (defvaralias 'shell-dirtrack-mode 'shell-dirtrackp)
>  
> -(defvar shell-dirtrackp t
> -  "Non-nil in a shell buffer means directory tracking is enabled.")
> +(defvar-local shell-dirtrackp nil
> +  "Non-nil in a shell buffer means directory tracking is enabled.
> +Directory tracking (`shell-dirtrack-mode') is automatically enabled
> +when `shell-mode' is activated.")
>  
>  (defvar shell-last-dir nil
>    "Keep track of last directory for ksh `cd -' command.")
> @@ -1129,6 +1131,7 @@ shell-extract-num
>  
>  (define-minor-mode shell-dirtrack-mode
>    "Toggle directory tracking in this shell buffer (Shell Dirtrack mode).
> +This assigns a buffer-local non-nil value to `shell-dirtrackp'.
>  
>  The `dirtrack' package provides an alternative implementation of
>  this feature; see the function `dirtrack-mode'.  Also see

This aligns the `defvar` of `shell-dirtrackp` with that of
`shell-dirtrack-mode`, which reduces the harm of the duplication, so
it fixes the worst part of my mess, thanks.

I'd prefer the original patch, tho, which should give basically the same
end result but without this weird duplicate definition.

I assume we're discussing this patch for `master`, not `emacs-29`, right?


        Stefan