Re: [Qemu-devel] [PATCH v3 3/6] block QMP: Drop query-block member "type" (type= in info block)

[Markus Armbruster <armbru@redhat.com>]

Luiz Capitulino <lcapitulino@redhat.com> writes:

> On Fri, 13 May 2011 10:26:38 +0200
> Markus Armbruster <armbru@redhat.com> wrote:
>
>> Luiz Capitulino <lcapitulino@redhat.com> writes:
>> 
>> > On Thu, 12 May 2011 19:54:40 +0200
>> > Markus Armbruster <armbru@redhat.com> wrote:
>> >
>> >> Luiz Capitulino <lcapitulino@redhat.com> writes:
>> >> 
>> >> > On Thu, 12 May 2011 19:12:56 +0200
>> >> > Markus Armbruster <armbru@redhat.com> wrote:
>> >> >
>> >> >> Luiz Capitulino <lcapitulino@redhat.com> writes:
[...]
>> >> >> > Also, we can't just drop it from QMP. We should first note it's deprecated.
>> >> >> 
>> >> >> Would you accept a change to the more honest value "unknown" for the
>> >> >> deprecation period?
>> >> >
>> >> > We have to avoid breaking the protocol. Changing something that has always
>> >> > been reported as 'cdrom' to 'unknown' will likely cause as many as damages
>> >> > as dropping the command.
>> >> 
>> >> I can cause damage only if somebody is using it.  Which I doubt.
>> >
>> > Me too and I'd agree with this patch if I was 100% sure. But it's impossible
>> > to be sure, unless we do it by trial and error which is harmful.
>> >
>> >> Remember, the value is unreliable.  It's a *lie*.  We can stop lying in
>> >> two ways: shut up (drop member "type"), or tell the truth (change the
>> >> value to "unknown", which is a documented value of "type").
>> >
>> > Can we set it to 'unknown' when if=none?
>> 
>> Maybe.  Makes query-block mix up host and guest information again.
>
> It's temporary, just to respect our deprecation policy and give us time
> to provide a viable alternative.
>
>> Purging guest information from block.c is the point of this series.
>> Therefore, query-block can't be done in block.c anymore.  It needs to
>> move to blockdev.c, where the mixed-up-by-design DriveInfo is available.
>> It could move back when we finally clean up query-block.
>> 
>> Even with such compatibility gymnastics, it could still break your
>> hypothetical client.
>
> Our deprecation policy is not hypothetical. There isn't case by case. Either
> we respect it or we don't.

My point is: even your deprecation policy cannot protect your
hypothetical client.

The only way to ensure not even the silliest hypothetical client breaks
is not to change anything.  Which means we cannot fix query-block not to
lie about the type, period.  Which means we have to deprecate
query-block wholesale to fix that bug.

Kevin, please apply patches 1+2+6.  Feel free to drop 3-5.

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