bug#74208: 31.0.50; minibuffer read-file-name-default mutates global value of default-directory incorrectly

[Eli Zaretskii]

> Date: Tue, 05 Nov 2024 07:39:24 +0530 (IST)
> From: Madhu <enometh@meer.net>
> 
> minibuffer.el: (read-file-name-default) has the following code:
> ```
>                      (minibuffer-with-setup-hook
>                          (lambda ()
>                            (setq default-directory dir)
>                  ...
>                         (set-syntax-table minibuffer-local-filename-syntax))
> ```
> 
> This mutates the global binding of default-directory which is incorrect.
> 
> To demonstrate the problem, in emacs -Q
> 
> (insert "http://example.com";)
> (setq enable-recursive-minibuffers t)
> 
> position point at in the middle of the string  http://example.com
> M-: (ffap)
> 
> ;; while waiting for input
> M-: default-directory
> ;; => default-directory is bound to "http://example.com";
> M-: (shell-command "echo foo")
> ; ;=>
> ;; Debugger entered--Lisp error: (file-missing "Setting current directory" 
> "No such file or directory" "http://example.com/";)
> 
> I think this can be addressed by binding default-directory before
> modifying it.
> 
> ```
> diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
> --- a/lisp/minibuffer.el
> +++ b/lisp/minibuffer.el
> @@ -3671,6 +3671,7 @@ read-file-name-default
>                                (expand-file-name dir))))
>                      (minibuffer-with-setup-hook
>                          (lambda ()
> +                         (let ((default-directory default-directory))
>                            (setq default-directory dir)
>                            ;; When the first default in `minibuffer-default'
>                            ;; duplicates initial input `insdef',
> @@ -3689,7 +3690,7 @@ read-file-name-default
>                                   (with-current-buffer
>                                       (window-buffer 
> (minibuffer-selected-window))
>                                  (read-file-name--defaults dir initial))))
> -                       (set-syntax-table minibuffer-local-filename-syntax))
> +                       (set-syntax-table minibuffer-local-filename-syntax)))
>                        (completing-read prompt 'read-file-name-internal
>                                         pred require-match insdef
>                                         'file-name-history default-filename)))
> ```
> 
> [I'm sending this before I forget so it can be reviewed, but I can
> send this patch as as attachment if needed, and it'll take some more
> time until I figure out how to write a test for it and test it.]

Stefan, any comments?