using dbus in the platform

[Alp Toker]

Havoc Pennington wrote:
> Hi,
>
> Alp Toker wrote:
>>> 2) GTK+ has a dependency on dbus, on X11 only, for "desktop
>>> integration" features to work. (See list of examples above.) By
>>> "dependency" I mean specifically:
>>>  - dbus.h is not included in gtk.h
>>>  - gtk or gdk contains interfaces such as settings, notifications,
>>> single instance, launch help viewer; on X11, these are implemented
>>> using libdbus (suggest a cut-and-paste of hippo-dbus-helper.[hc] or
>>> equivalent is used internal to gtk)
>>
>> I believe that we've already discussed this possibility and duly 
>> dismissed it as unworkable.
>
> Who is "we"? I certainly did not agree.

The ideas being thrown about involved pre-loading a module that would 
provide IPC functionality, rather than a hard-coded dependency on D-Bus.

>
> I define GTK as an application framework[1]; it should support what 
> apps need to do. This means talking to the desktop, and dbus is how 
> that is done on Linux (and unix in general, of course).
>
> For GTK to do this it needs some dbus implementation.
>
> Note the "dbus.h is not included in gtk.h" point I made. i.e. We are 
> not talking about libdbus in the gtk API; we're talking about using it 
> internally. In theory another dbus implementation could be swapped in 
> sometime, though I don't think that is a worthwhile or near-term 
> likely goal. In practice, we will probably want to swap something else 
> in right away on Quartz and Windows ports.
>
> [1] some may define GTK as a UI toolkit only, but in practice it has 
> already gone far beyond that, and IMO it's right to do so because nice 
> APIs really require a broader "application framework"

I appreciate that you didn't propose exposing libdbus directly in the 
API. The problems I've described exist even if D-Bus is to be used 
behind the scenes by GTK+. They are not hypothetical, but real problems 
that will appear if ever GTK+ starts to open D-Bus connections.

>
>>   * .NET, Java, Ruby (and soon Python) applications each make use of 
>> their own D-Bus implementations. There has even been talk of a new C 
>> implementation. There is little sense in imposing the dead weight of 
>> libdbus on GTK+ applications using these platforms.
>
> It is not "dead weight," it's needed to provide functionality. gvfs 
> already relies on it, which all gnome apps will already be using. KDE 
> apps also already rely on it.

The functionality is welcome, but it should be provided in a separate 
library, just like gvfs.

Is there any technical reason why it needs to be included in GTK+? Does 
it make use of GTK+ internals? I suspect not.

>
> Some implementation is needed in the C stack; I don't see a new one 
> that's close to ready (nor do I think a new C implementation makes 
> sense for free desktop purposes[2], but that's another debate).
>
> We aren't going to use the .NET, Python, or Ruby bindings in C 
> libraries like GTK or gvfs, so they are irrelevant here.

Nobody suggested this. This is a straw man argument.

>
> [2] it might make sense for embedded or something, since libdbus is 
> optimized for flexibility and security, at some cost in both size and 
> speed
>
>>   * Moreover, multiple implementations of the D-Bus client in one 
>> process will lead to multiple connections to each of the system and 
>> session bus daemons.
>
> That is one reason why libdbus is *designed* for language binding, and 
> for sharing the DBusConnection object among object system mappings, 
> and it is not encouraged to reimplement DBusConnection.

libdbus was not well-suited to language binding when I reviewed its 
documentation. It lacked necessary entry points for 
marshalling/demarshalling data, ABORTed under recoverable error 
conditions and permitted, even encouraged consumers to disconnect shared 
connections.

>
> A basic assumption of the GTK+ and freedesktop.org "stack" to date is 
> that shared code among languages / projects is written in C.
>
> As I've said, I think it's a worthwhile tradeoff[3] in .NET or Java to 
> reimplement, because there is value on those platforms to a 
> zero-native-code solution. However, reimplementing DBusConnection in 
> the languages that already heavily rely on native code (C, C++, Ruby, 
> Python) is simply a bad idea[4], for the reason you give among others.
>
> The shared code that leads to a single connection *must be* in C, 
> because we need to use it in gvfs, GTK, and other elements of the C 
> stack. This has been known for years, and that's why libdbus is 
> designed the way it is.
>
> [3] Even in the managed languages like .NET and Java, it's a tradeoff, 
> though a worthwhile one. One price of avoiding native code is multiple 
> bus connections - this could potentially be dynamically avoided at 
> runtime, though, if libdbus is found to be present. Another major 
> price is making it harder to add a new feature, such as a new auth 
> mechanism, in a way that spans all apps / the whole desktop. This 
> price is higher the more reimplementations we have.
>
> [4] Let me qualify this, I think it could be interesting to have 
> reimplementations in these languages for use in some contexts, but I 
> think it would be better if the bindings dynamically decided to push 
> messages into the shared DBusConnection when libdbus is available. We 
> can optimize this as needed to avoid extra copies of the message, 
> which has been discussed. But the point is for a typical Linux desktop 
> it is optimal to have all the apps going through the DBusConnection in 
> libdbus.

If "DBusConnection" is so easily shared, why are various language 
bindings today opening private, unshared connections to the bus? It is 
evident that reality has not matched expectations here.

In my experience with libdbus, I've found that it sets low standards for 
interoperability and fails to achieve them.

Given these realities, I cannot see how libdbus in its current state 
could be an appropriate dependency for GTK+.

>
>>  >  - on Windows and OS X, these interfaces are implemented in some
>>  > native way (though it could make sense to rely on a dbus port in some
>>  > cases, I don't know)
>>
>>   * The lack of portability you mention is an artificial limitation 
>> (or perhaps one imposed by libdbus).
>
> There is no problem making libdbus portable. The unportable part is 
> the stuff *on* the bus. i.e. if we have a "Screensaver" interface 
> defined for the free desktops (GNOME, KDE) that is not going to be 
> present on Windows unless someone goes to some effort, and it's 
> unclear to me that writing a dbus service to talk to the Windows 
> screensaver API makes more sense than just having GTK talk to the 
> Windows screensaver API directly. (For example.)
>
>> Modularity is one thing we've been getting right all these years. The 
>> functionality that you've discussed is definitely welcome as an 
>> optional part of the platform, but is not appropriate for integration 
>> with the UI toolkit, any more than, say, Bonobo would have been 
>> suitable for GTK+ 1.2.
>
> This isn't why Bonobo did not go in (to 2.0, it was 2.0-era).
>
> (Well, Bonobo *did* go in, in the same not-in-the-API optional sense 
> I'm proposing D-Bus, since it's used by the a11y stuff that generally 
> gets loaded at runtime. But leave that aside for a moment.)
>
> You're right there was a period of time where at least some people 
> thought there should be a libgnomeui layer that was the app framework, 
> while GTK was only the UI toolkit (i.e. only widgets). I don't think 
> this is the theory or the practice today, though.
>
> Aside from that debate, the major problem with Bonobo was in the 
> specifics, not the idea of IPC. Example specifics were neglect of 
> lifecycle issues; the famous "recursive main loop" bug; 
> widget-embedding and app framework interfaces that didn't make sense 
> with the GTK API; and being GNOME-specific instead of 
> least-common-denominator.
>
> Having IPC and IO (dbus and gvfs) below the app framework is the 
> correct layering, because the app framework needs to use this stuff. 
> The app framework does not necessarily need to expose this stuff in 
> the API, though, especially when it needs to be (or should be) done 
> differently per-platform.

Leaving D-Bus use out of GTK+ will provide everyone with more choice and 
modularity -- the features you have in mind belong elsewhere in the 
platform. Developing them outside of GTK+ will let you prototype, test 
and deploy your own code faster whilst leaving others free to experiment 
with different approaches.

Keep in mind that not everyone uses GTK+ in the same way that we do in 
GNOME.