Avoid race between DbusIpcThread::close and DbusIpcThread::execute

...that caused Flatpak'ed LO to sometimes not terminate properly after
243d743dae127fe11ecd7f807121da8815af8f01 "solenv/flatpak-manifest.in:
incorporate upstream sandboxing improvements" had removed --socket=session-bus
and thus introduced flatpak-dbus-proxy into the mix:

> Oct 24 15:25:16 <sberg_> I'm not sure who's at fault there; on the LO side I
>  have a thread processing incoming dbus requests (DbusIpcThread::execute,
>  <https://opengrok.libreoffice.org/xref/core/desktop/source/app/officeipcthread.cxx#552>);
>  that thread is typically waiting within dbus_connection_read_write_dispatch
>  for new messages; if LO wants to close, it needs to get that thread out of
>  dbus_connection_read_write_dispatch, and the only way I found back then is
>  what's in DbusIpcThread::close (just following DbusIpcThread::execute), with
>  a big "this apparently needs a more DBus-y design anyway" testament to my
>  cluelesness re dbus; it calls dbus_bus_get_private to connect to itself, and
>  send a Close message that'll cause DbusIpcThread::execute to fall out of the
>  loop; the dbus_bus_get_private leads to a flatpak_proxy_incoming in the
>  proxy, which returns TRUE, and after that I see no further activity in the
>  proxy (but not sure what functions I should all put breakpoints on), and the
>  LO side is hung in dbus_bus_release_name a few lines futher down
> Oct 24 15:26:20 <alexlarsson> sberg_: i ran this:
>  https://github.com/sgh/dbus-examples/blob/master/dbus-signal.c
> Oct 24 15:26:27 <alexlarsson> sberg_: which calls dbus_bus_release_name
> Oct 24 15:26:30 <alexlarsson> and it seems to work for me
> Oct 24 15:26:39 <alexlarsson> If i --own-name=org.DBusTest.SignalTest
> Oct 24 15:27:50 <sberg_> so maybe it's related to my unorthodox way of trying
>  to quit the dbus loop there;  if anybody with a clue about dbus would be
>  willing to help me with the above, I'd be very grateful :)
> Oct 24 15:28:06 <alexlarsson> I don't see how the proxy should affect it tho
> Oct 24 15:28:11 <alexlarsson> other than maybe timing?
> Oct 24 15:29:06 <sberg_> maybe; I think I've seen things actually succeed once
>  when I stepped through the proxy somewhat manually
> Oct 24 15:31:01 <alexlarsson> sberg_: eeeentresting
> Oct 24 15:31:07 <alexlarsson> sberg_: so, it does an own-call
> Oct 24 15:31:13 <alexlarsson> sberg_: maybe that is what breaks?
> Oct 24 15:31:33 <sberg_> yeah, that's my dumb speculation
> Oct 24 15:34:38 <alexlarsson> So, you send the close from another thread, not
>  waiting for the reply
> Oct 24 15:34:56 <alexlarsson> then you start working on the main connection
>  immediately
> Oct 24 15:35:02 <alexlarsson> shouldn
> Oct 24 15:35:25 <alexlarsson> shouldn't you grab the mutex after that?
> Oct 24 15:36:35 <alexlarsson> This whole thing smells racy
> Oct 24 15:38:18 <alexlarsson> My guess is that with regular dbus, the flush
>  call will schedule dbus daemon which sends back the message, waking up the
>  other thread before continuing
> Oct 24 15:38:22 <sberg_> alexlarsson, ah, yeah, I see; apparently didn't
>  occur to me back then that I mustn't call dbus_bus_release_name when the
>  execute loop may still be busy
> Oct 24 15:38:25 <alexlarsson> But now you have two context switches
> Oct 24 15:38:37 <alexlarsson> so we continue before we get the reply
> Oct 24 15:38:44 <alexlarsson> eh, not reply
> Oct 24 15:38:46 <alexlarsson> the close message
> Oct 24 15:38:52 <alexlarsson> and then you race
> Oct 24 15:39:11 <alexlarsson> Honestly i don't remember the exact rules for
>  dbus threadedness
> Oct 24 15:39:58 <sberg_> ...so looks like something that can be corrected on
>  the LO side after all; I'll give it a try
> Oct 24 15:40:17 <alexlarsson> you're calling dbus_threads_init_default(), so
>  it should be nominally threadsafe
> Oct 24 15:40:29 <alexlarsson> still, I imagine it can still deadlock
> Oct 24 15:43:12 <alexlarsson> sberg_: i still don't see the exact deadlock
>  though
> Oct 24 15:43:32 <alexlarsson> i imagine working on the separate private
>  connection is threadsafe
> Oct 24 15:43:35 <alexlarsson> and we flush that
> Oct 24 15:44:08 <alexlarsson> then we call dbus_bus_release_name, which i
>  imagine would block if the dbus thread is in read_write_dispatch
> Oct 24 15:44:35 <alexlarsson> The question is, why is the close message not
>  delivered?
> Oct 24 15:44:44 <alexlarsson> I mean, we flushed it...
> Oct 24 15:45:48 <alexlarsson> sberg_: oh, i think i know
> Oct 24 15:45:53 <alexlarsson> sberg_: release_name probably sent some message,
>  but then the other thread pop:ed the reply
> Oct 24 15:46:04 <alexlarsson> sberg_: queue very long wait
> Oct 24 15:51:53 <sberg_> alexlarsson, yeah, sounds plausible; thanks!
> Oct 24 15:52:14 <sberg_> (now that I want to reproduce it, closing succeeds
>  cleanly every time I try, of course...)
> Oct 24 15:54:48 <alexlarsson> sberg_: obviously
> Oct 24 15:55:30 <alexlarsson> sberg_: that just makes the race theory more
>  valid tho
> Oct 24 15:57:31 <sberg_> alexlarsson, yup, after stopping the unrelated
>  background LO build that had put the machine under full load, I see it hang
>  again in release_name, while the execute loop is still happily running too;
>  that nicely confirms your theory
> Oct 24 15:58:28 <alexlarsson> sberg_: alternatively you could tell your users
>  to always build LO in the background
> Oct 24 15:59:04 <sberg_> I'll try to put that into the release notes; lets
>  see...

Change-Id: I2a8a58f9259d2854f42f4aa3db5bb232cf70845d

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