Re: [PATCH 04/14] qapi: add a 'command-features' pragma

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Jul 12, 2024 at 10:50:54AM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > On Fri, Jul 12, 2024 at 10:07:34AM +0200, Markus Armbruster wrote:
> >> Daniel P. Berrangé <berrange@redhat.com> writes:
> >> 
> >> > The 'command-features' pragma allows for defining additional
> >> > special features that are unique to a particular QAPI schema
> >> > instance and its implementation.
> >> 
> >> So far, we have special features (predefined, known to the generator and
> >> treated specially), and normal features (user-defined, not known to the
> >> generator).  You create a new kind in between: user-defined, not known
> >> to the generator, yet treated specially (I guess?).  Hmm.
> >> 
> >> Could you at least hint at indented use here?  What special treatment do
> >> you have in mind?
> >
> > Essentially, these features are a way to attach metadata to commands that
> > the server side impl can later query. This eliminates the need to hardcode
> > lists of commands, such as in QGA which hardcodes a list of commands which
> > are safe to use when filesystems are frozen. This is illustrated later in
> > this series.
> 
> Please update docs/devel/qapi-code-gen.rst section "Pragma directives",
> and maybe section "Features".
> 
> I'm not sure conflating the new kind of feature with existing special
> features is a good idea.  I need to review more of the series before I
> can make up my mind.

I originally implemented a completely separate 'tags' concept in the
QAPI parser, before deciding I was just re-inventing 'features' for
no obvious benefit.

The other nice thing about using features is that these are exposed
in the schema and docs. With the 'fsfreeze' restriction in code,
there's no formal docs of what commands are allowed when frozen, and
this is also not exposed in QAPI schema to apps. Using 'features'
we get all that as standard.

With regards,
Daniel
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