bug#34138: 27.0.50; Delayed display of PDF file images

[Eli Zaretskii]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: rudalics@gmx.at,  34138@debbugs.gnu.org,  politza@hochschule-trier.de,  
> tsdh@gnu.org
> Date: Sun, 20 Jan 2019 18:14:13 +0100
> 
> > It means, during the time of the delay, type from the shell prompt:
> >
> >   $ gdb -p PID
> >
> > where PID is the numeric process-id of the Emacs process; you should
> > find that in advance, e.g. by running "ps".  Once GDB starts and shows
> > its prompt, "(gdb)", type:
> >
> >   (gdb) thread apply all bt
> >
> > This should show the C-level backtrace.  Then:
> >
> >   (gdb) source /path/to/emacs/src/.gdbinit
> >   (gdb) xbacktrace
> >
> > The last command should show the Lisp-level backtrace.
> 
> Ah, thanks.  I started gdb from the path of the emacs executable and the
> Lisp backtrace was shown after typing "thread apply all bt";

Please start GDB from a directory other that the Emacs src directory,
I'd like to see the C backtrace that way.

> FWIW, while the backtrace was being shown in the shell, the raw PDF
> changed to the image display in Emacs.

I don't understand how could Emacs be running while GDB was showing
the backtrace.  It shouldn't happen, because the executable to which
GDB is attached is supposed to be stopped in its tracks.  What does
this show, after you attach GDB and GDB shows its prompt?

  (gdb) show non-stop

It should say that non-stop mode is OFF.

> Lisp Backtrace:
> "image-size" (0x70824720)
> 0x354bf60 PVEC_COMPILED
> "gethash" (0x70820b38)
> "pdf-cache--data-get" (0x70820d40)
> "pdf-cache-number-of-pages" (0x70820f10)
> ---Type <return> to continue, or q <return> to quit---
> "terminal-live-p" (0x70820ea8)
> "framep-on-display" (0x708210b8)
> "overlayp" (0x70824718)
> 0x92591e8 PVEC_COMPILED
> "overlayp" (0x70824a48)
> 0x92591e8 PVEC_COMPILED
> "redisplay--update-region-highlight" (0x70824fd0)
> "run-hook-with-args" (0x70824fc8)
> "ignore" (0x70825310)
> "apply" (0x70825308)
> 0x9420638 PVEC_COMPILED
> "redisplay_internal (C function)" (0x0)

This Lisp backtrace contradicts the C backtrace of the main thread:

> Thread 1 (Thread 0x7f9109a29bc0 (LWP 5077)):
> #0  0x00007f910c590291 in __pselect (nfds=16, readfds=0x7ffc70826670, 
> writefds=0x7ffc708265f0, exceptfds=0x0, timeout=<optimized out>, 
> sigmask=<optimized out>)
>     at ../sysdeps/unix/sysv/linux/pselect.c:69
> #1  0x00000000005bca5d in really_call_select (arg=arg@entry=0x7ffc70826130)
>     at /mnt/data/steve/git/emacs-master/src/thread.c:580
> #2  0x0000000000542d88 in flush_stack_call_func (func=func@entry=0x5bca12 
> <really_call_select>, arg=arg@entry=0x7ffc70826130)
>     at /mnt/data/steve/git/emacs-master/src/alloc.c:5229
> #3  0x00000000005bd24f in thread_select (func=<optimized out>, 
> max_fds=max_fds@entry=16, rfds=rfds@entry=0x7ffc70826670, wfds=<optimized 
> out>, efds=efds@entry=0x0, timeout=timeout@entry=0x7ffc708268c0, sigmask=0x0)
>     at /mnt/data/steve/git/emacs-master/src/thread.c:610
> #4  0x00000000005d7c64 in xg_select (fds_lim=16, 
> rfds=rfds@entry=0x7ffc70826960,---Type <return> to continue, or q <return> to 
> quit---
>  wfds=0x7ffc708268e0, efds=efds@entry=0x0, 
> timeout=timeout@entry=0x7ffc708268c0, sigmask=sigmask@entry=0x0)
>     at /mnt/data/steve/git/emacs-master/src/xgselect.c:117
> #5  0x000000000059daf3 in wait_reading_process_output 
> (time_limit=time_limit@entry=97, nsecs=nsecs@entry=0, read_kbd=-1, 
> do_display=do_display@entry=true, wait_for_cell=wait_for_cell@entry=XIL(0), 
> wait_proc=wait_proc@entry=0x0, just_wait_proc=0) at 
> /mnt/data/steve/git/emacs-master/src/process.c:5405
> #6  0x0000000000424e63 in sit_for (timeout=timeout@entry=make_number(97), 
> reading=reading@entry=true, display_option=display_option@entry=1)
>     at /mnt/data/steve/git/emacs-master/src/lisp.h:1056
> #7  0x00000000004f769e in read_char (commandflag=1, 
> map=map@entry=XIL(0x35f3093), prev_event=XIL(0), 
> used_mouse_menu=used_mouse_menu@entry=0x7ffc70826d4b, 
> end_time=end_time@entry=0x0) at 
> /mnt/data/steve/git/emacs-master/src/lisp.h:751
> #8  0x00000000004f81ec in read_key_sequence 
> (keybuf=keybuf@entry=0x7ffc70826e10, prompt=prompt@entry=XIL(0), 
> dont_downcase_last=dont_downcase_last@entry=false, 
> can_return_switch_frame=can_return_switch_frame@entry=true, 
> fix_current_buffer=fix_current_buffer@entry=true, 
> prevent_redisplay=prevent_redisplay@entry=false)
>     at /mnt/data/steve/git/emacs-master/src/lisp.h:1386
> #9  0x00000000004f9730 in command_loop_1 ()
>     at /mnt/data/steve/git/emacs-master/src/lisp.h:1056

The C backtrace says we are idling, waiting for some input inside
pselect, while the Lisp backtrace says we are in the image-size
function, which was somehow invoked from pre-redisplay-function.  I'm
confused...

Thanks.