The kdbuswreck [LWN.net]

The kdbuswreck

Posted Apr 25, 2015 13:34 UTC (Sat) by mchapman (subscriber, #66589)
In reply to: The kdbuswreck by lsl
Parent article: The kdbuswreck

> How would one implement the concept "I want this functionality but I don't care who provides it" in dbus?

I would say that is *exactly* what D-Bus provides now. There is nothing in D-Bus's policy configuration that locks a service to a particular binary. A service name can be claimed by any process that matches that service's policy (for system services this is typically just a check that the connection was authenticated as root). Of course, only one D-Bus connection can own a service name at any particular time.

Service activation is a bit different, as D-Bus (or systemd, if it's doing the activation) needs to know which binary to launch when the service is requested. I suppose you could use something like alternatives to cater for multiple implementations of that service.

The kdbuswreck

Posted Apr 27, 2015 12:16 UTC (Mon) by javispedro (guest, #83660) [Link] (3 responses)

> I would say that is *exactly* what D-Bus provides now. There is nothing in D-Bus's policy configuration that locks a service to a particular binary. A service name can be claimed by any process that matches that service's policy (for system services this is typically just a check that the connection was authenticated as root). Of course, only one D-Bus connection can own a service name at any particular time.

You actually _cannot_ do what is being asked with current D-Bus, and it is one of my major gripes with it (which is why I prefer anything else over it). The "using the service name as what every other IPC would call interface name" idea works well until you realize you cannot register more than one service with the same name. Thus, you start seeing ugly tricks such as what MPRIS does, which require greping over the service names, etc.

The kdbuswreck

Posted Apr 27, 2015 12:54 UTC (Mon) by mchapman (subscriber, #66589) [Link] (2 responses)

> The "using the service name as what every other IPC would call interface name" idea works well until you realize you cannot register more than one service with the same name. Thus, you start seeing ugly tricks such as what MPRIS does, which require greping over the service names, etc.

I don't think it's necessarily an ugly trick. The difference between getting a list of service names with some prefix and in getting a list of services providing some object (assuming this were even possible with D-Bus) is mostly superficial.

If efficiency is the problem, getting a list of service names for things like MPRIS could be optimized by extending the protocol slightly, e.g. by having org.freedesktop.DBus.ListNames take a prefix as an argument.

But allowing a service name to be owned by at most one connection is essential for lsl's use case. You can't sanely dispatch "to an implementation that makes sense accorrding to local system configuration" if more than one such implementation is on the bus at the same time.

The kdbuswreck

Posted Apr 27, 2015 13:28 UTC (Mon) by MrWim (subscriber, #47432) [Link]

I don't think it's necessarily an ugly trick. The difference between getting a list of service names with some prefix and in getting a list of services providing some object (assuming this were even possible with D-Bus) is mostly superficial.

If efficiency is the problem, getting a list of service names for things like MPRIS could be optimized by extending the protocol slightly, e.g. by having org.freedesktop.DBus.ListNames take a prefix as an argument.

Indeed, this is the purpose of the arg0namespace match rule. You register for NameOwnerChanged events with some prefix, call ListNames (or ListActivatableNames) filtering on the prefix and then you can efficiently and asynchronously keep your local list of remote names up-to-date.

The kdbuswreck

Posted Apr 28, 2015 8:10 UTC (Tue) by javispedro (guest, #83660) [Link]

It introduces a bunch of additional problems. For example, what if you want the same process to expose more than one instance of the service? You will be hit by the fact that despite having multiple service names you still have one object namespace only... and no way to setup different policies, etc.

The MPRIS trick obviously works, but it puts the design of DBus upside down.