bug#16576: Binding `standard-output' to a function doesn't work -- function sometimes called with garbage

[Eli Zaretskii]

> Date: Tue, 28 Jan 2014 10:09:08 +0100
> From: Anders Lindgren <andlind@gmail.com>
> 
>     emacs -Q
>     Eval the following:
> 
> (defvar bug-bind-output-str nil)
> 
> (defun bug-bind-output-function (char)
>   (setq bug-bind-output-str
>         (concat bug-bind-output-str (list char))))
> 
> (defun bug-bind-output-test ()
>   (interactive)
>   (let ((bug-bind-output-str "")
>         (standard-output 'bug-bind-output-function)
>         (s
> "(\\(co\\(?:mbine-after-change-calls\\|nd\\(?:ition-case\\(?:-unless-debug\\)?\\)?\\)\\|eval-\\(?:a\\(?:fter-load\\|nd-compile\\)\\|next-after-load\\|when\\(?:-compile\\)?\\)\\|i\\(?:f\\|nline\\)\\|l\\(?:ambda\\|et\\(?:\\*\\|rec\\)?\\)\\|p\\(?:case\\(?:-let\\*?\\)?\\|rog[*12nv]?\\)\\|save-\\(?:current-buffer\\|excursion\\|match-data\\|restriction\\|selected-window\\|window-excursion\\)\\|track-mouse\\|unwind-protect\\|w\\(?:hile\\(?:-no-input\\)?\\|ith-\\(?:c\\(?:a\\(?:\\(?:se\\|tegory\\)-table\\)\\|urrent-buffer\\)\\|demoted-errors\\|electric-help\\|local-quit\\|no-warnings\\|output-to-\\(?:string\\|temp-buffer\\)\\|s\\(?:elected-\\(?:frame\\|window\\)\\|ilent-modifications\\|yntax-table\\)\\|t\\(?:emp-\\(?:buffer\\|\\(?:fil\\|messag\\)e\\)\\|imeout\\(?:-handler\\)?\\)\\|wrapper-hook\\)\\)\\)\\>")
>         s0)
>     (prin1 s)
>     bug-bind-output-str))
> 
> Type this a number of times:
> 
>     M-x (bug-bind-output-test) RET

I think there's a bug in your test program.  You let-bind
bug-bind-output-str, and print that local binding after you call
prin1.  But your print function concatenates each character onto the
_global_ binding of bug-bind-output-str, so the result is not in your
local binding, it's in the global one.

If you change your program like this:

  (defun bug-bind-output-test ()
    (interactive)
    (setq bug-bind-output-str "")
    (let ((standard-output 'bug-bind-output-function)
          (s
  
"(\\(co\\(?:mbine-after-change-calls\\|nd\\(?:ition-case\\(?:-unless-debug\\)?\\)?\\)\\|eval-\\(?:a\\(?:fter-load\\|nd-compile\\)\\|next-after-load\\|when\\(?:-compile\\)?\\)\\|i\\(?:f\\|nline\\)\\|l\\(?:ambda\\|et\\(?:\\*\\|rec\\)?\\)\\|p\\(?:case\\(?:-let\\*?\\)?\\|rog[*12nv]?\\)\\|save-\\(?:current-buffer\\|excursion\\|match-data\\|restriction\\|selected-window\\|window-excursion\\)\\|track-mouse\\|unwind-protect\\|w\\(?:hile\\(?:-no-input\\)?\\|ith-\\(?:c\\(?:a\\(?:\\(?:se\\|tegory\\)-table\\)\\|urrent-buffer\\)\\|demoted-errors\\|electric-help\\|local-quit\\|no-warnings\\|output-to-\\(?:string\\|temp-buffer\\)\\|s\\(?:elected-\\(?:frame\\|window\\)\\|ilent-modifications\\|yntax-table\\)\\|t\\(?:emp-\\(?:buffer\\|\\(?:fil\\|messag\\)e\\)\\|imeout\\(?:-handler\\)?\\)\\|wrapper-hook\\)\\)\\)\\>")
          s0)
      (prin1 s)
      bug-bind-output-str))

i.e., work with the global binding, then the program works as
expected, AFAICS.