[vlc-devel] dbus mini-summit at Desktop Summit 2011

Rémi Denis-Courmont remi at remlab.net
Wed Aug 10 19:10:14 CEST 2011

Le mercredi 10 août 2011 19:12:38 Will Thompson, vous avez écrit :
> • Declare that using (eg) _send_with_reply_and_block() in >1 thread is
> unsupported. We can't actually remove the connection-locking code
> because QtDBus depends on it working (inasmuch as it works). VLC (for
> instance) uses one private connection per thread, which should continue
> to be supported.

You got that from the MPRIS people?

I am afraid someone confused dbus_connection_read_write(_dispatch)() with 

VLC indeed uses a private connection to implement the MPRIS service. It is not 
for the sake of reinventing the wheel: dbus_connection_read_write() is not 
cancellation-safe (at least it's not specified), so the poll() loop had to be 
reinvented just so we could destroy the DBus connection cleanly.

VLC does however use dbus_connection_send_with_reply_and_block() where it acts 
as a DBus "client", i.e. the Telepathy "NowPlaying" thingy and the power 
management inhibitor.

libdbus has in any case never been really thread-safe because:
 - it modifies the environment variables, and
 - dbus_threads_init_default() is not re-entrant.

If you want to achieve true thread-safety, you should maybe consider depending 
on pthread-stubs from the XCB guys.

Rémi Denis-Courmont

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