🚀 go-pugleaf

Bug: selective display can cause core dumps.
Version: 18.52, Sun3 / SunOS4.0.3

Repeat by: Insert the following text into *scratch*.

(defun event::handler ()
  (let*
    (
      (when (and callback (functionp callback))
	    (unwind-protect
		;; BODY
		(funcall callback event::type event::value event::screen)
	      ;; CLEAN-UP
	      (when (null epoch::event-handler) ;something got hosed
		    (setq epoch::event-handler 'event::handler) ;reinstall me
		    (ignore-event event::type)	;inhibit the handler
		    )
	      )
Set selective-display to 16. Scroll left 36. Refresh display. Emacs aborts.

Cause: [ Warning - this gets technical! ]
In display_text_line(), hpos can be a large negative number, if the window is
horizontally scrolled. p1 is set to be line->body + hpos, which means that
(p1 < line->body) can be true. Another variable startp is set to line->body
+ w->width, and in most cases p1 is checked against startp to verify that it
is inside the body of the display_line struct. Also, p1prev is set to be p1
at the start of the main display loop, so that p1prev < line->body can also
be true (if it is true for p1). Selective display does not use p1, but p1prev,
and _does not check the value against startp_. This means that if the " ..."
is scrolled off the left edge of the window far enough, the " ..." will
overwrite parts of the display_line structure, an obviously unhealthy thing
to do.

Fix: None yet, but shouldn't be too hard. The key question is, if the " ..."
is scrolled off the left, should it
1. Be blown off and not plotted at all?
2. Be truncated on the left (and the non-scrolled portion plotted?)
3. Always be plotted in its entirety?
Answer this question, and I'll generate a fix.

Alan M. Carroll                Barbara/Marilyn in '92 :
carroll@cs.uiuc.edu            + This time, why not choose the better halves?
Epoch Development Team         
CS Grad / U of Ill @ Urbana    ...{ucbvax,pur-ee,convex}!cs.uiuc.edu!carroll