[Libguestfs] OpenStack output - server_id

[Richard W.M. Jones]

On Mon, Sep 24, 2018 at 10:00:21AM +0200, Fabien Dupont wrote:
> Hi,

Hi Fabien, sorry I didn't respond to this earlier as I was doing some
work.  If you CC me on emails then you can usually get a quicker
response.

> I've read the virt-v2v OpenStack output code to understand how it works and
> I've seen this:
> 
> >   (* The server name or UUID of the conversion appliance where
> >    * virt-v2v is currently running.  In future we may be able
> >    * to make this optional and derive it from the OpenStack
> >    * metadata service instead.
> >    *)
> >   server_id : string;
> 
> Indeed, it can be derived from OpenStack metadata service. The following
> URL called from within the conversion appliance will return the metadata:
> http://169.254.169.254/openstack/latest/meta_data.json. As you can see, the
> IP address is 169.254.169.254, which will is the metadata service. The JSON
> body contains a uuid entry that is the current appliance UUID, hence the
> server_id used by virt-v2v.

We certainly do want to do this, although there was some concern about
whether the metadata service is enabled on every OpenStack instance
out there.  (Also there are two different types of metadata service IIRC?)

In fact I tried fetching that URL from the internal OpenStack test
instance that we have set up, and it doesn't work, so I suppose the
metadata service isn't running.  (Unfortunately the connection hung
for minutes instead of timing out quickly, which is not great.)

This is fine however.  We should try the metadata service only if the
‘-oo server-id=...’ was not supplied.

> I don't know how to code that in OCaml, so I defer implementation to anyone
> with such skills. And by reading the patch, I may learn how to write small
> pieces of OCaml and contribute further changes :)

No real need to do it in OCaml.  Easiest to string together the Curl &
JSON_parser modules.

https://github.com/libguestfs/libguestfs/blob/master/common/mltools/curl.mli
https://github.com/libguestfs/libguestfs/blob/master/common/mltools/JSON_parser.mli

What does the meta_data.json file actually look like?

Rich.

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