Re: [Qemu-devel] [PATCH 2/2] QMP: Require 'use_unstable' arg for capabilities negotiation

[Markus Armbruster <armbru@redhat.com>]

Luiz Capitulino <lcapitulino@redhat.com> writes:

> On Fri, 09 Jul 2010 15:24:48 +0200
> Markus Armbruster <armbru@redhat.com> wrote:
>
>> Luiz Capitulino <lcapitulino@redhat.com> writes:
>> 
>> > On Fri, 09 Jul 2010 10:44:32 +0200
>> > Markus Armbruster <armbru@redhat.com> wrote:
>> >
>> >> Luiz Capitulino <lcapitulino@redhat.com> writes:
>> >> 
>> >> > This helps ensuring two things:
>> >> >
>> >> > 1. An initial warning on client writers playing with current QMP
>> >> > 2. Clients using unstable QMP will break when we declare QMP stable and
>> >> >    drop that argument
>> [...]
>> >> Is it really necessary to break all existing users of QMP?
>> >
>> > The protocol is going to change, they will break anyway.
>> 
>> Then why break them now in addition to then?
>
> Existing clients? Yes, you have a point. Although our only known existing
> client atm is libvirt.
>
>> >> What are we trying to accomplish by that?
>> >
>> > QMP in 0.13 is in usable state. I fear that people will start using it
>> > without noting/caring the protocol is going to be different in 0.14.
>> >
>> > The removal of this flag in 0.14 (assuming we'll have a stable QMP by then),
>> > makes clients break right away, instead of unexpected breaking in subtle ways.
>> >
>> > This makes it easy to identify what's wrong and the message will be: you
>> > should review your QMP usage, because the protocol has changed.
>> >
>> > That said, I'm not that strong about this particular solution. What I really
>> > would like to have is an easy way to identify old clients using a now
>> > stable version of QMP.
>> 
>> If we want obsolete clients to break when we release 0.14, then let's
>> break them then.  No need to break not-yet-obsolete clients now.
>> Especially not in a way that unbreaks them in 0.14, when they are
>> *really* obsolete.
>
> I don't think we have that many clients today, the only known one is
> libvirt. So, this patch takes only new clients in consideration, those
> who might start using QMP in 0.13.
>
> Again, I'm ok with a different solution. I just want to go beyond a README file
> warning.
>
> What about a "warning" key in the greeting message? Like:
>
>  "warning": "QMP is unstable, it will break soon!"
>
> Not sure it matters as our greeting message is a bit too verbose, but
> seems better than nothing.

Works for me.