Re: [Qemu-devel] [PATCH 00/13] RFC: luks/encrypted qcow2 key management

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2019-08-21 at 13:31 +0200, Markus Armbruster wrote:
> Maxim Levitsky <mlevitsk@redhat.com> writes:
> 
> > On Thu, 2019-08-15 at 10:00 -0500, Eric Blake wrote:
> > > On 8/15/19 9:44 AM, Maxim Levitsky wrote:
> > > 
> > > > > > > Does the idea of a union type with a default value for the discriminator
> > > > > > > help?  Maybe we have a discriminator which defaults to 'auto', and add a
> > > > > > > union branch 'auto':'any'.  During creation, if the "driver":"auto"
> > > > > > > branch is selected (usually implicitly by omitting "driver", but also
> > > > > > > possible explicitly), the creation attempt is rejected as invalid
> > > > > > > regardless of the contents of the remaining 'any'.  But during amend
> > > > > > > usage, if the 'auto' branch is selected, we then add in the proper
> > > > > > > "driver":"xyz" and reparse the QAPI object to determine if the remaining
> > > > > > > fields in 'any' still meet the specification for the required driver branch.
> > > > > > > 
> > > > > > > This idea may still require some tweaks to the QAPI generator, but it's
> > > > > > > the best I can come up with for a way to parse an arbitrary JSON object
> > > > > > > with unknown validation, then reparse it again after adding more
> > > > > > > information that would constrain the parse differently.
> > > > > > 
> > > > > > Feels like this would be a lot of code just to allow the client to omit
> > > > > > passing a value that it knows anyway. If this were a human interface, I
> > > > > > could understand the desire to make commands less verbose, but for QMP I
> > > > > > honestly don't see the point when it's not trivial.
> > > > > 
> > > > > Seconded.
> > > > 
> > > > 
> > > > But what about my suggestion of adding something like:
> > > > 
> > > > { 'union': 'BlockdevAmendOptions',
> > > > 
> > > >   'base': {
> > > >       'node-name':         'str' },
> > > > 
> > > >   'discriminator': { 'get_block_driver(node-name)' } ,
> > > 
> > > Not worth it. It makes the QAPI generator more complex (to invoke
> > > arbitrary code instead of a fixed name) just to avoid a little bit of
> > > complexity in the caller (which is assumed to be a computer, and thus
> > > shouldn't have a hard time providing a sane 'driver' unconditionally).
> > > An HMP wrapper around the QMP command can do whatever magic it needs to
> > > omit driver, but making driver mandatory for QMP is just fine.
> > 
> > All right! I kind of not agree with that, since I think even though QMP is a machine language,
> > it still should be consistent since humans still use it, even if this is humans that code some
> > tool that use it.
> > 
> > I won't argue with you though, let it be like that.
> 
> Software's fundamental limit is complexity.  We need to pick what we use
> it for.  Sometimes, that means saying no to things that would be nice to
> have.

I fully agree with that and that is usually the exact reason I argue about such things:
Sometimes avoiding complexity in one place, and/or breaking consistency can introduce complexity in other place (like libvirt).

In this particular case, I won't argue about this, but still it kind of feels like
it is a precedent of requiring the user to supply redundant data.

Of all issues all of you pointed out (thanks!!) this is probably the least important one that I should be arguing about, 
so let it be like you say.

Best regards,
	Maxim Levitsky