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Re: [libvirt] [PATCH libguestfs 0/4] Add a libvirt backend to libguestfs.



On Mon, Jul 23, 2012 at 11:21:37AM +0100, Daniel P. Berrange wrote:
> On Mon, Jul 23, 2012 at 11:02:41AM +0100, Richard W.M. Jones wrote:
> > It's using NULL and expecting libvirt to choose the appropriate
> > connection URI, which does appear to work.
> 
> Apps should only rely on NULL, if they are able to work with any
> possible hypervisor. If you have specific requirements for QEMU
> you should always request QEMU explicitly. A local sysadmin may
> well have set a different default URI using an env variable or
> $HOME/.libvirt/libvirt.conf which will give you an unexpected
> choice.

Currently the administrator can set the attach-method to one of:

  "appliance" (default): run qemu directly
  "libvirt": run libvirt with conn URI = NULL
  "libvirt:URI": run libvirt with conn URI = "URI"

We could make "libvirt" mean "choose qemu:///session or
qemu:///system".

Then if they want NULL, we could have "libvirt:" (colon followed by
empty string) or "libvirt:NULL" or something else.

> Alternatively you could just say to hell with it, and require
> the application to pass in a pre-opened virConnectPtr that
> you use. This is actually quite desirable, since it will avoid
> the user having to authenticate multiple times, when the app
> already has an open connection

Unfortunately this is hard to implement.  Specifically we cannot
generally convert a language-specific object (eg. a Perl Sys::Virt
object) into a virConnectPtr.  Been discussed before a few years back
when we wanted to pass virDomainPtr to a libguestfs call.  There is
even non-working support in the generator for this ...

In true garbage-collected languages it's even harder to get it right.
How would you stop the connection object 'conn' from getting collected
and closed too early in OCaml code such as:

  conn = Libvirt.Connect.connect_readonly ()
  g#attach_libvirt conn;
  (* as far as OCaml is concerned, conn is unreferenced from here onwards *)
  g#launch ();
  g#do_lots_of_stuff ();
  g#close ()

The language bindings would have to model the lifetime of every object
that could potentially be attached to the libguestfs handle.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.
http://et.redhat.com/~rjones/virt-top


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