bug#27668: 26.0.50; Crash with display-line-numbers t

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Date: Fri, 14 Jul 2017 10:59:17 +0200
> 
> xdisp.c:21012: Emacs fatal error: assertion failed: it->glyph_row == NULL || it->glyph_row->used[TEXT_AREA] == 0
> 
> Thread 1 "emacs" hit Breakpoint 1, terminate_due_to_signal (sig=sig@entry=6, 
>     backtrace_limit=backtrace_limit@entry=2147483647) at emacs.c:363
> 363	{
> (gdb) bt
> #0  terminate_due_to_signal (sig=sig@entry=6, backtrace_limit=backtrace_limit@entry=2147483647) at emacs.c:363
> #1  0x00000000005a4ed4 in die (msg=msg@entry=0x67aec0 "it->glyph_row == NULL || it->glyph_row->used[TEXT_AREA] == 0", 
>     file=file@entry=0x678e68 "xdisp.c", line=line@entry=21012) at alloc.c:7348
> #2  0x000000000045befb in maybe_produce_line_number (it=it@entry=0x7fffffff8130) at xdisp.c:21012
> #3  0x0000000000465335 in display_line (it=it@entry=0x7fffffff8130, cursor_vpos=cursor_vpos@entry=9) at xdisp.c:21227
> #4  0x000000000046728d in try_window (window=..., window@entry=XIL(0x146e6c5), pos=..., flags=flags@entry=1)
>     at xdisp.c:17544
> #5  0x000000000047f8fb in redisplay_window (window=XIL(0x146e6c5), just_this_one_p=just_this_one_p@entry=false)
>     at xdisp.c:16991

In this frame #5, in redisplay_window, which of the conditions in the
following 'if' clause are true?

      /* Try to redisplay starting at same place as before.
         If point has not moved off frame, accept the results.  */
      if (!current_matrix_up_to_date_p
	  /* Don't use try_window_reusing_current_matrix in this case
	     because a window scroll function can have changed the
	     buffer.  */
	  || !NILP (Vwindow_scroll_functions)
	  || MINI_WINDOW_P (w)
	  || !(used_current_matrix_p
	       = try_window_reusing_current_matrix (w)))
	{
	  IF_DEBUG (debug_method_add (w, "1"));
	  if (try_window (window, startp, TRY_WINDOW_CHECK_MARGINS) < 0)

It cannot be MINI_WINDOW_P, so it's either current_matrix_up_to_date_p
is false, or your window-scroll-functions is non-nil, or we called
try_window_reusing_current_matrix.  In the latter case, I'd expect my
recent change to fix the problem, so I guess that function wasn't
called, and some other condition caused us to call try_window.  Or
maybe I'm missing something here.

> I was looking at a commit in Magit, and had just hit enter in a diff
> hunk to view the source file containing the change, so it's happening
> when we switch from non-line-number to line-number mode.

How do you "switch from non-line-number to line-number mode"?  I tried
several ways I could think about, but couldn't reproduce the issue.  I
wonder what's different in your case that you hit this problem so
frequently.

Did the previous times also happened when switching from a Magit
buffer to a buffer under display-line-numbers?