Re: [Qemu-devel] [PATCH 02/13] docs/qapi: Document optional discriminators

[Eric Blake <eblake@redhat.com>]

On 05/09/2018 11:55 AM, Max Reitz wrote:
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>   docs/devel/qapi-code-gen.txt | 21 +++++++++++++++++++--
>   1 file changed, 19 insertions(+), 2 deletions(-)
> 

I might have squashed this in with patch 1, but I can live with it separate.

> @@ -502,6 +502,23 @@ the enum).  In the resulting generated C data types, a flat union is
>   represented as a struct with the base members included directly, and
>   then a union of structures for each branch of the struct.
>   
> +If the discriminator points to an optional member of the base struct,
> +its default value must be specified as a 'default-variant'.  In the
> +following example, the above BlockDriver struct is changed so it
> +defaults to the 'file' driver if that field is omitted on the wire:
> +
> + { 'union': 'BlockdevOptions',
> +   'base': { '*driver': 'BlockdevDriver', '*read-only': 'bool' },
> +   'discriminator': 'driver',
> +   'default-variant': 'file',
> +   'data': { 'file': 'BlockdevOptionsFile',
> +             'qcow2': 'BlockdevOptionsQcow2' } }
> +
> +Now the 'file' JSON object can be abbreviated to:
> +
> + { "read-only": "true",
> +   "filename": "/some/place/my-image" }
> +

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org