Re: Hang while scrolling buffer with PNG images

[Ralf Angeli]

* Richard Stallman (2003-09-07,16:23) writes:

> Can you please try following the instructions given in
> etc/DEBUG for debugging an infinite loop?

Thank you for that pointer.  I was not aware of such a document.

Following the instructions I found that an infinite loop occurs at
three different points:

==========================

(gdb) finish
Run till exit from #0  move_it_vertically_backward (it=0xbfffe160,
dy=1)
    at xdisp.c:5862
try_scrolling (window=1213625160, just_this_one_p=1,
scroll_conservatively=0, 
    scroll_step=1, temp_scroll_step=0, last_line_misfit=-1073749664)
    at xdisp.c:10820
10820         scroll_margin_pos = it.current.pos;
(gdb) finish
Run till exit from #0  try_scrolling (window=1213625160,
just_this_one_p=1, 
    scroll_conservatively=0, scroll_step=1, temp_scroll_step=0, 
    last_line_misfit=-1073749664) at xdisp.c:10820

==========================

(gdb) finish
Run till exit from #0  start_display (it=0xbfffe160, w=0xbfffe160,
pos=
      {charpos = 2932, bytepos = 2949}) at xdisp.c:2203
try_scrolling (window=1213625160, just_this_one_p=1,
scroll_conservatively=0,
    scroll_step=1, temp_scroll_step=0, last_line_misfit=-1073749664)
    at xdisp.c:10819
10819         move_it_vertically (&it, - end_scroll_margin);
(gdb) finish
Run till exit from #0  try_scrolling (window=1213625160,
just_this_one_p=1,
    scroll_conservatively=0, scroll_step=1, temp_scroll_step=0,
    last_line_misfit=-1073749664) at xdisp.c:10819

==========================

(gdb) finish
Run till exit from #0  0x080725b8 in try_window (window=1213625160,
pos=
      {charpos = -1073750304, bytepos = 1055}) at xdisp.c:12034
0x0806eebf in try_scrolling (window=1213625160, just_this_one_p=1, 
    scroll_conservatively=0, scroll_step=1, temp_scroll_step=0, 
    last_line_misfit=1055) at xdisp.c:10937
10937     if (!try_window (window, startp))
Value returned is $58 = 1
(gdb) finish
Run till exit from #0  0x0806eebf in try_scrolling
(window=1213625160, 
    just_this_one_p=1, scroll_conservatively=0, scroll_step=1, 
    temp_scroll_step=0, last_line_misfit=1055) at xdisp.c:10937

==========================

Most of the time it is the last one.

I went through the last one with `step':

==========================

(gdb) finish
Run till exit from #0  0x080725b8 in try_window (window=1213625160,
pos=
      {charpos = -1073750304, bytepos = 1055}) at xdisp.c:12034
0x0806eebf in try_scrolling (window=1213625160, just_this_one_p=1, 
    scroll_conservatively=0, scroll_step=1, temp_scroll_step=0, 
    last_line_misfit=1055) at xdisp.c:10937
10937     if (!try_window (window, startp))
Value returned is $179 = 1
(gdb) step
10939     else if (w->cursor.vpos < 0)
(gdb) step
10947         if (!just_this_one_p
(gdb) step
10954         if (! make_cursor_line_fully_visible (w))
(gdb) step
make_cursor_line_fully_visible (w=0x41f) at xdisp.c:10659
10659   {
(gdb) step
10666     if (w->cursor.vpos < 0)
(gdb) step
10659   {
(gdb) step
10666     if (w->cursor.vpos < 0)
(gdb) step
10669     matrix = w->desired_matrix;
(gdb) step
10670     row = MATRIX_ROW (matrix, w->cursor.vpos);
(gdb) step
10673     if (!MATRIX_ROW_PARTIALLY_VISIBLE_P (row))
(gdb) step
971       struct frame *f = XFRAME (w->frame);
(gdb) 
972       int height = WINDOW_TOTAL_HEIGHT (w);
(gdb) 
982       if (WINDOW_WANTS_MODELINE_P (w))
(gdb) 
972       int height = WINDOW_TOTAL_HEIGHT (w);
(gdb) 
982       if (WINDOW_WANTS_MODELINE_P (w))
(gdb) 
985             = (w->current_matrix && w->current_matrix->rows
(gdb) 
988           if (ml_row && ml_row->mode_line_p)
(gdb) 
989             height -= ml_row->height;
(gdb) 
994       if (WINDOW_WANTS_HEADER_LINE_P (w))
(gdb) 
1004        }
(gdb) 
10679     if (row->height >= window_height)
(gdb) 
10721   }
(gdb) 
try_scrolling (window=1213625160, just_this_one_p=1,
scroll_conservatively=0, 
    scroll_step=1, temp_scroll_step=0, last_line_misfit=1055) at
    xdisp.c:10956
10956             clear_glyph_matrix (w->desired_matrix);
(gdb) 
10957             last_line_misfit = 1;
(gdb) 
10956             clear_glyph_matrix (w->desired_matrix);
(gdb) 
clear_glyph_matrix (matrix=0x858fe60) at dispnew.c:966
966       if (matrix)
(gdb) 
968           enable_glyph_matrix_rows (matrix, 0, matrix->nrows, 0);
(gdb) 
enable_glyph_matrix_rows (matrix=0x858fe60, start=0, end=54,
enabled_p=0)
    at dispnew.c:947
947       for (; start < end; ++start)
(gdb) 
948         matrix->rows[start].enabled_p = enabled_p != 0;
(gdb) 
947       for (; start < end; ++start)
(gdb) 
948         matrix->rows[start].enabled_p = enabled_p != 0;

[ many of these ]

948         matrix->rows[start].enabled_p = enabled_p != 0;
(gdb) 
947       for (; start < end; ++start)
(gdb) 
949     }
(gdb) 
clear_glyph_matrix (matrix=0x858fe60) at dispnew.c:969
969           matrix->no_scrolling_p = 0;
(gdb) 
971     }
(gdb) 
try_scrolling (window=1213625160, just_this_one_p=1,
scroll_conservatively=0, 
    scroll_step=1, temp_scroll_step=0, last_line_misfit=1) at
    xdisp.c:10958
10958             goto too_near_end;
(gdb) 
10812     CHARPOS (scroll_margin_pos) = XINT (window_end);
(gdb) 
10813     BYTEPOS (scroll_margin_pos) = CHAR_TO_BYTE (CHARPOS
(scroll_margin_pos));
(gdb) 
buf_charpos_to_bytepos (b=0x858d828, charpos=2932) at marker.c:140
140       if (charpos < BUF_BEG (b) || charpos > BUF_Z (b))
(gdb)
144       best_above_byte = BUF_Z_BYTE (b);
(gdb)
143       best_above = BUF_Z (b);
(gdb)
149       if (best_above == best_above_byte)
(gdb)
144       best_above_byte = BUF_Z_BYTE (b);
(gdb)
149       if (best_above == best_above_byte)
(gdb)
152       best_below = BEG;
(gdb)
164       CONSIDER (BUF_PT (b), BUF_PT_BYTE (b));
(gdb)
153       best_below_byte = BEG_BYTE;
(gdb)
164       CONSIDER (BUF_PT (b), BUF_PT_BYTE (b));
(gdb)
0x08107218      170         CONSIDER (cached_charpos, cached_bytepos);
(gdb)
165       CONSIDER (BUF_GPT (b), BUF_GPT_BYTE (b));
(gdb)
166       CONSIDER (BUF_BEGV (b), BUF_BEGV_BYTE (b));
(gdb)
0x081071ab      170         CONSIDER (cached_charpos, cached_bytepos);
(gdb)
167       CONSIDER (BUF_ZV (b), BUF_ZV_BYTE (b));
(gdb)
169       if (b == cached_buffer && BUF_MODIFF (b) == cached_modiff)
(gdb)
170         CONSIDER (cached_charpos, cached_bytepos);
(gdb)
172       for (tail = BUF_MARKERS (b); tail; tail = tail->next)
(gdb)
174           CONSIDER (tail->charpos, tail->bytepos);
(gdb)
179           if (best_above - best_below < 50)
(gdb)
172       for (tail = BUF_MARKERS (b); tail; tail = tail->next)
(gdb)
174           CONSIDER (tail->charpos, tail->bytepos);
(gdb)
179           if (best_above - best_below < 50)
(gdb)

[ many, many of these ]

(gdb) 
try_scrolling (window=1213625160, just_this_one_p=1,
scroll_conservatively=0, 
    scroll_step=1, temp_scroll_step=0, last_line_misfit=1) at
    xdisp.c:10815
10815     end_scroll_margin = this_scroll_margin + !!last_line_misfit;
(gdb) 
10816     if (end_scroll_margin)
(gdb) 
10818         start_display (&it, w, scroll_margin_pos);
(gdb) 
10819         move_it_vertically (&it, - end_scroll_margin);
(gdb) 
10818         start_display (&it, w, scroll_margin_pos);
(gdb) 
start_display (it=0x8567348, w=0x8567348, pos=
      {charpos = -1073749664, bytepos = -1}) at xdisp.c:2198

==========================

After that it goes on and on without producing some kind of special
pattern or loop I could identify.  You can sit there, hold the return
key for minutes and somewhere there will be something knew, breaking a
pattern I thought I could identify.  Am I missing here something?  I
thought that I would find a infinite loop somewhere while stepping
through it.  But this could be due to my limited knowledge of
debugging.  Any hints are welcome.  Hopefully this is useful
nevertheless.

Regards,
Ralf