[PATCH] A NSNetService implementation using Avahi

[Niels Grewe]

Hello guys,

here [0] you will find a patch (slighly to large for attachment) that
I've been cooking up for some time now. It augements the present
implementation of NSNetService and NSNetServiceBrowser in gnustep-base
with a second implementation using the Avahi API. Some words on why that
seems to be a good idea to me:

Our present implementation of NSNetServices makes use of the API that
Apple implemented in mDNSResponder [1] to provide zeroconf (=Bonjour)
services. That is, in principle, a good idea, because we can bet that
Apple also used it to implement NSNetServices on Mac OS X. The problem
is only that (way back) mDNSResponder was APSL licensed [2], which was
hindering adoption by some Linux distributions, so it is not available
on many Linux systems.
Fortunately, the Avahi project provides a compatibility layer for the
mDNSResponder API that can be used instead. Unfortunately, that
compatibility layer is only partitially implemented and (apparently) not
very well maintained. This means that presently our NSNetServices code
will run on most Linux distributions where avahi is available, but most
of the shiny stuff you would want in an useful application (e.g.
monitoring TXT records) wont work.

Since Avahi is probably going to stick on the Linux desktop (it provides
a DBUS interface that seems to be quite popular for reasons unknown to
me), I decided not to waste my time on fixing the Avahi compatability
layer, but to reimplement NSNetServices on top of the native Avahi API.
This was actually quite easy, because avahi provides some hooks that
allowed hooking the event-handling into NSRunLoop quite transparently.
This has the following ramifications:

1. NSNetService and NSNetServiceBrowser are now abstract superclasses
   that return concrete subclasses for the configured implementation.

2. The zeroconf API to use is now configurable by the
   --with-zeroconf-api configure switch. It can take the values "mdns",
   "avahi" and "any" (with "any" being the default). If both are
   installed, it will always use avahi, since the mDNS implementation is
   most certainly the broken compatibility-layer.

3. The new avahi-based implementation has some shiny additional features
   that make it more flexible then the Apple one. I.e. it allows you not
   only to browse for (or register) services and TXT records, but also
   arbitrary records. This is nice for use-cases like serverless
   XMPP-messaging, where you are supposed to publish a buddy-icon as a
   NULL record. While these features are not yet available with our
   mDNSResponder-based implementation, they are certainly possible, and
   I would be adding them eventually if there is sufficient interest.

I'm now soliciting feedback on this patch. It still has some
non-critical FIXMEs strewn over it that I plan to resolve, but in
general it seems to be quite usable. So if you have any code that uses
NSNetService and friends, please try it out and tell me when it breaks
and where it doesn't behave as expected. To this purpose, I have also
uploaded the patch to the Étoilé reviewboard [3].

Thanks,


Niels
--
[0] http://www.halbordnung.de/~thebeing/gnustep/NSNetServices+avahi.patch
[1] http://www.opensource.apple.com/source/mDNSResponder/mDNSResponder-214/
[2] It's Apache-licensed now.
[3] http://review.etoileos.com/r/137/

signature.asc
Description: Digital signature