The deadlocks [was: Announcing new version of the Qt4 bindings]

[Thiago Macieira]

Thiago Macieira wrote:
>I think I can solve this inside the binding without need for anything
>special inside libdbus. I'll let you know in a while if my idea worked.
>
>Currently, the process stack goes like this:
>event loop -> socket notifier -> dbus_connection_dispatch -> filter ->
>user function
>
>If the user function recurses into another loop, it'll try to acquire
> the dispatcher again and deadlock (since the mutex is non-reentrant).
>
>My solution will be to rely on the behaviour that
> dbus_connection_dispatch pops never more than one message from the
> input queue. That way, we'll have:
>
>event loop -> socket notifier -> dbus_connection_dispatch -> filter
>
>The filter will just prepare the call, but not execute it. When
>dbus_connection_dispatch exits, the socket notifier code will realise
>there's a pending call to be delivered and will deliver from there. This
>way, the DBus dispatcher will be unlocked for further calls.

This is just to let you guys know that the idea worked.

When the filter is invoked from dbus_connection_dispatch, it verifies 
whether the message is one that it can handle. If it is, it'll prepare 
the call but not actually make it. Then we return HANDLED to the 
dispatcher.

When dbus_connection_dispatch returns, we execute the calls that were 
prepared. This has now allowed for unlimited reentrancy inside Qt-based 
applications.

However, this also means that the Qt bindings are uncooperative to anyone 
else using the same DBusConnection. If our filter is run while another 
function called dbus_connection_dispatch, we could end up queueing calls 
that may never be executed.

-- 
Thiago José Macieira - thiago.macieira AT trolltech.com
Trolltech AS - Sandakerveien 116, NO-0402 Oslo, Norway
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