Bug#1064982: gnuplot-qt: gnuplot displays a window with nothing in it

[Vincent Lefevre]

Control: clone -1 -2
Control: reassign -1 gnuplot-qt,gnuplot-x11
Control: affects -1 - gnuplot-qt gnuplot-x11
Control: retitle -1 gnuplot: with GNUTERM=wxt, "gnuplot -persist" sometimes hangs with Pango 1.52+ (race condition)
Control: tags -1 patch
Control: retitle -2 libpango-1.0-0: Pango 1.52+ API+ABI breakage (affecting gnuplot in particular)

I've reassigned the initial bug instead of the cloned bug as I had
reported the initial bug against gnuplot.

On 2024-10-30 13:14:05 +0100, Vincent Lefevre wrote:
> On 2024-09-17 14:55:47 +0200, Vincent Lefevre wrote:
> > Control: found -1 1.54.0+ds-2
> > 
> > And this still occurs with the latest libpango-1.0-0 version
> > when testing with
> > 
> >   echo 'set terminal wxt; plot x' | gnuplot -persist
> 
> I think I've found the cause of the issue: strace shows that gnuplot
> does an "exit_group(0)", which causes a Pango thread to terminate
> unexpectedly.

This is due to the fork in src/wxterminal/wxt_gui.cpp and the exit
of the parent process.

According to Matthias Clasen,

  pango_cairo_font_map_set_default(NULL);

is needed before the fork to avoid the issue with the new Pango 1.52
code (which has changed the threading method). See

  https://gitlab.gnome.org/GNOME/pango/-/issues/784#note_2269462
  https://gitlab.gnome.org/GNOME/pango/-/issues/784#note_2269810

I've tried the attached patch, and it seems to solve the issue,
without any drawback.

Concerning the Pango 1.52+ API+ABI breakage, the breakage is rather
uncommon as it probably occurs only for code like with a fork + exit,
but a soname bump seems to be needed anyway (this seems to be the
same kind of issue as in libxml2, where Debian requested a soname
bump - see https://gitlab.gnome.org/GNOME/libxml2/-/issues/751
about slight ABI changes).

-- 
Vincent Lefèvre <vincent at vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
-------------- next part --------------
Index: b/src/wxterminal/wxt_gui.cpp
===================================================================
--- a/src/wxterminal/wxt_gui.cpp
+++ b/src/wxterminal/wxt_gui.cpp
@@ -4277,8 +4277,10 @@ void wxt_atexit()
 	/* fork */
 	pid_t pid;
 
-	if (openwindows > 0)
+	if (openwindows > 0) {
+		pango_cairo_font_map_set_default(NULL);
 		pid = fork();
+	}
 	else
 		pid = -1;