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

[Vincent Lefevre]

Control: reassign -1 libpango-1.0-0 1.52.0+ds-1
Control: retitle -1 libpango-1.0-0: hangs in pango_fc_patterns_get_font_pattern (echo 'set terminal wxt; plot x' | gnuplot -persist)
Control: forwarded -1 https://gitlab.gnome.org/GNOME/pango/-/issues/784
Control: affects -1 gnuplot-qt gnuplot-x11

On 2024-02-29 16:10:06 +0100, Vincent Lefevre wrote:
> I've identified the "problematic" commit in Pango, and I would tend
> to say that this is the reason:
> 
> https://gitlab.gnome.org/GNOME/pango/-/commit/89442dae443eba2aa0f0a526b4d6d39c0c9b13c6
> 
> According to the commit message, a new thread was created "for every
> single fontconfig call". Now "a single fontconfig thread per fontmap"
> is used. So I suppose that this makes Pango faster, triggering the
> race condition in gnuplot.

(now a single thread using a queue instead of a thread per request)

> I've updated the upstream gnuplot bug and opened a bug in Pango,
> hoping confirmation:
> 
> https://gitlab.gnome.org/GNOME/pango/-/issues/784

The backtrace I posted in the gnuplot and pango bugs shows that
the hang occurs in pango_fc_patterns_get_font_pattern, i.e. in
pango, in the source file changed by the above commit. So it is
probably a bug in this new pango code, which is visible only
under some conditions.

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