[libvirt] (Dropping) OOM Handling in libvirt

[Andrea Bolognani]

On Mon, 2019-05-13 at 13:19 +0100, Daniel P. Berrangé wrote:
> On Mon, May 13, 2019 at 02:00:28PM +0200, Andrea Bolognani wrote:
> > One possible complication is that we would not be able to use any
> > of the GLib types in our public API... I think the way we should
> > approach this is to consider the current public API as if it were
> > yet another language binding, the language being plain C in this
> > case, and make sure we have a very well defined boundary between
> > them and everything else, basically treating them as a separate
> > project that just so happens to live in the same repository and be
> > developed in tandem. This should also make it easier for us to
> > switch to a different programming language in the future, should
> > we decide to.
> 
> I'm not sure why you say we can't use GLib types in our public API ?
> 
> I think we could use them, but I'd probably suggest we none the less
> choose not to use them in public API, only internally :-)
> 
> But I'm anticipating we could replace virObject, with GObject, and as
> such all the virXXXXXPtr types in our public API would become GObjects.
> I think we'd likely keep them as opaque types though, despite the fact
> that they'd be GObjects, to retain our freedom to change impl again
> later if we wish.
> 
> I won't think we need to change use of 'long long' to 'gint64', etc
> Not least because because GLib maintainers themselves are questioning
> whether to just mandate stdint.h types. This is fairly minor though.

I was mostly thinking about this latter example and other situations
along those lines. For example, we'll definitely need to start using
gchar* internally, and since we don't want that implementation detail
exposed in our plain C bindings, then we'll have to do at least some
very lightweight conversion (casting) between that and char*. This is
one of the examples where considering the existing API as a language
binding would IMHO result in a maintainable structure.

Another situation where the above model would help is error
reporting: if we start using GLib heavily, then it might make sense
to adopt GError as well, but doing so means we'd have to convert to
our existing error reporting facilities somewhere. If we consider the
plain C API to be a binding, then that's not different from what we
already do for Python and friends.

As for GObject, yeah, we want all public structures to be opaque
anyway; at the same time, we won't be able to turn existing
non-opaque public structures into GObjects. I'm not sure how big of
a deal that would be in practice, but I just thought I'd bring it up.

> > I also can't help but wonder what going in this direction would
> > mean for libvirt-glib and the projects built upon it...
> 
> I don't think it has a significant impact. libvirt-glib.so is just a
> glue to the GLib event loop. The libvirt built-in event loop might
> become the same thing.
> 
> Most of the code is in libvirt-gconfig though which is a mapping of
> XML docs into the GObject model which is all still relevant.
> 
> Likewise libvirt-gobject is a remapping of libvirt public API into
> GObject. If we don't expose the fact that our public API secretly
> uses GObject internally, then I think libvirt-gobject is also still
> valid.
> 
> Potentially we could merge libvirt-gobject into our public API
> officially exposing that its GObject based, but I don't think that's
> an important thing in the near future, possibly not even in the long
> term. Basically I'd be cautious with our public API to avoid tieing
> the public API to the internal impl choice.

Yeah, I guess we can figure that out at a later date.

-- 
Andrea Bolognani / Red Hat / Virtualization