Setting cursor-type does not trigger redisplay of cursor

[Hrvoje Niksic]

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In GNU Emacs 21.4.1 (i386-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2005-03-17 on trouble, modified by Debian
configured using `configure '--build=i386-linux' '--host=i386-linux' 
'--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib' 
'--localstatedir=/var/lib' '--infodir=/usr/share/info' 
'--mandir=/usr/share/man' '--with-pop=yes' '--with-x=yes' 
'--with-x-toolkit=athena' 'CFLAGS=-DDEBIAN -g -O2' 'build_alias=i386-linux' 
'host_alias=i386-linux''
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I wanted to set cursor-type off post-command-hook with the intention
of changing the cursor appearance depending on whether the point is at
the end of line/buffer or not.  (This emulates an XEmacs feature of
showing a thin cursor at the end of line.)  Something like:

(defun set-cursor-adaptive ()
  (setq cursor-type (if (eq (char-after (point)) ?\n) '(bar . 5) t)))
(add-hook 'post-command-hook 'set-cursor-adaptive)

However, it turns out that this doesn't work as well as it should --
apparently changing `cursor-type' does not affect the shape of the
cursor until after it is redrawn, either by changing the position of
point, or by redrawing the frame.  I can work around that by calling
(sit-for 0), but it seems wasteful to invoke redisplay after every
single command.  I ended up checking whether the cursor would actually
change, and calling (sit-for 0) then, like this:

(defun set-cursor-adaptive ()
  (let ((type (let ((c (char-after (point))))
                (if (or (eq c ?\n) (null c))
                    '(bar . 5)
                  t))))
    (if (not (equal cursor-type type))
        (progn
          (setq cursor-type type)
          (sit-for 0)))))
(add-hook 'post-command-hook 'set-cursor-adaptive)

To summarize, changing the cursor type should IMO automatically
refresh the cursor when the event loop is reentered, without the user
having to do anything special.  This would be consistent with the rest
of Emacs, which doesn't require Lisp code to explicitly trigger
redisplay -- it simply happens when necessary.