Need help understanding dbus-glib event loop

[John Palmieri]

----- "Glenn Schmottlach" <glenn.schmottlach at harman.com> wrote:

> > libdbus, the lower-level library used by dbus-glib, QtDBus etc., is
> likely to
> > be more thread-safe (various people have contributed thread-safety
> fixes to it,
> > I believe).
> 
> My experience has been quite the contrary. Perhaps I don't have all
> the necessary patches applied to ensure complete thread-safety but at
> one time the asynchronous (pending call) API was basically broken from
> a thread-safety standpoint (e.g. it had a bad race condition). It was
> ever sufficiently addressed for my tastes.
> 
> For me, the most successful approach has been to isolate the D-Bus
> main-loop (thread) from the rest of the system. All D-Bus calls should
> be made from this thread which subsequently makes the application
> appear as a single-threaded implementation (from the perspective of
> D-Bus). D-Bus requests issued from other threads are marshaled through
> some sort of barrier (queue) and dispatched from the main-loop. An
> internal pipe's file-descriptor can be monitored by the main-loop and
> used to detect when there is a new command to dispatch (rather than
> polling the queue). Basically, when you add a command to the "work"
> queue you write a byte to the pipe to "wake-up" the D-Bus main-loop. A
> "watch" handler deques the command and invokes the D-Bus method call.
> Incoming signals are delivered by the D-Bus main-loop to other threads
> so appropriate synchronization is necessary between these threads. 
> 
> Most users of D-Bus are GUI applications with the main-loop already
> baked into the GUI framework (QT, GLIB/GTK). It's a reactive system
> with button-clicks eventually invoking asynchronous D-Bus calls.
> Typically these applications are single threaded. That's the
> environment where D-Bus works best. Move it outside that comfort zone
> and things become more difficult . . . not impossible . . . but
> difficult.

Thread safety doesn't mean you don't need to know how threads work.  D-Bus is thread safe, you just can't have two threads accessing the same connection (one will starve the other).  Dispatching jumps you into user code, so yes the user must also make their code thread safe.  Thread safety only guarantees that you can use the library in one thread without effecting another thread.  As soon as you start jumping thread boundaries you need to understand how the code is going to interact.

--
John (J5) Palmieri
Software Engineer
Red Hat, Inc.