bug#6883: 23.2; `read-key' echos keys in the minibuffer

[Leo]

1. Define bookmark-read-search-input as this which inserts a (sit-for 1) to
demonstrate the bug easily:

(defun bookmark-read-search-input ()
  (let ((prompt       (propertize "Pattern: " 'face 'minibuffer-prompt))
        (tmp-list     ()))
    (while
        (let ((char (read-key (concat prompt bookmark-search-pattern))))
          (sit-for 1)
          (case char
            ((?\e ?\r) nil) ; RET or ESC break the search loop.
            (?\C-g (setq bookmark-quit-flag t) nil)
            (?\d (pop tmp-list) t) ; Delete last char of pattern with DEL
            (t
             (if (characterp char)
                 (push char tmp-list)
               (setq unread-command-events
                     (nconc (mapcar 'identity
                                    (this-single-command-raw-keys))
                            unread-command-events))
               nil))))
      (setq bookmark-search-pattern
            (apply 'string (reverse tmp-list))))))

2. and eval

  (let (bookmark-search-pattern) (bookmark-read-search-input))

3. then start typing away.

You should see each key is echoed. So for example if you hit backspace
you may see backspace or DEL in the echo area twice.

With the original definition back (ie without the sit-for) you can
observe a trail for each key you type by for example doing something
like this:

  1. C-x r l to list bookmarks
  2. M-g s and start typing away

That breaks the smooth flow of key strokes.

Thierry Volpiatto, YAMAMOTO Mitsuharu and I can observe this on all
three major platforms. On some platforms, it happens so fast that it is
hardly annoying, on others you can read the echoing literally.
`read-char' does not have this problem and thus Thierry's suggestion to
use it instead.

Leo