[Gtk2-perl-bugs] [Bug 620099] Make it safe to call into Glib from foreign threads

[gnome-perl (bugzilla.gnome.o. <bug...@gn...>]

https://bugzilla.gnome.org/show_bug.cgi?id=620099
  gnome-perl | Glib | unspecified

--- Comment #11 from Torsten Schoenfeld <kaf...@gm...> 2012-02-19 14:39:14 UTC ---
(In reply to comment #10)
> I managed to reproduce it in this small hang.pl script that simply play a file
> given on the command line.
> I tested this on a xubuntu 11.04, the hanging happens every time with the more
> elaborated notify callback and with the line setting the volume to 1. If any of
> these are removed, everything works correctly.

I can reproduce the hang, and that's why I've been holding off on this patch
for so long.  The basic outline of this kind of hang is the following:

• There are two threads involved: the main thread running the event loop and
all Perl code, and an auxiliary thread constructed by gstreamer to manage the
audio stream.

• The call "$player->set(volume => 1)" reconfigures part of the pipeline and
causes the auxiliary thread to change the "volume" property of the internal
sink element while holding a lock.  Among other things this triggers the
"notify::volume" signal, for which we have a handler.  But since this is in a
non-main thread, the patch shovels the invocation over to the main loop and
/blocks the auxiliary thread until the signal handler has returned/.

• The signal handler calls get("volume") which ends up in the sink element's
volume accessor, which also tries to acquire the lock mentioned above. 
Deadlock.

A way to avoid the deadlock is to wrap in Glib::Idle->add those parts of the
signal handler that access object properties:

  $player->signal_connect(notify => sub {
    my ($object,$property)=@_;
    Glib::Idle->add (sub {
      my $name=$property->get_name;
      warn "$object $name : ".$object->get($name)."\n";
      Glib::SOURCE_REMOVE;
    });
  });

Since this workaround exists, the patch fixes otherwise unsolvable problems
while introducing new but solvable problems.  So I'll try to get the patch into
Glib today.

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