On Mon, Oct 16, 2023 at 05:27:00PM +0200, Victor Toso wrote:
> This patch handles QAPI union types and generates the equivalent data
> structures and methods in Go to handle it.
>
> The QAPI union type has two types of fields: The @base and the
> @Variants members. The @base fields can be considered common members
> for the union while only one field maximum is set for the @Variants.
>
> In the QAPI specification, it defines a @discriminator field, which is
> an Enum type. The purpose of the @discriminator is to identify which
> @variant type is being used.
>
> For the @discriminator's enum that are not handled by the QAPI Union,
> we add in the Go struct a separate block as "Unbranched enum fields".
> The rationale for this extra block is to allow the user to pass that
> enum value under the discriminator, without extra payload.
>
> The union types implement the Marshaler and Unmarshaler interfaces to
> seamless decode from JSON objects to Golang structs and vice versa.
>
> qapi:
> | { 'union': 'SetPasswordOptions',
> | 'base': { 'protocol': 'DisplayProtocol',
> | 'password': 'str',
> | '*connected': 'SetPasswordAction' },
> | 'discriminator': 'protocol',
> | 'data': { 'vnc': 'SetPasswordOptionsVnc' } }
>
> go:
> | type SetPasswordOptions struct {
> | Password string `json:"password"`
> | Connected *SetPasswordAction `json:"connected,omitempty"`
> | // Variants fields
> | Vnc *SetPasswordOptionsVnc `json:"-"`
> | // Unbranched enum fields
> | Spice bool `json:"-"`
> | }
Instead of using bool for these, can we denote a special type? For
example
type Empty struct{}
We could then do
u := SetPasswordOptions{
Password: "...",
Spice: &Empty{},
}
The benefit I have in mind is that you'd be able to check which
variant field is set consistently:
if u.Vnc != nil {
...
}
if u.Spice != nil {
...
}
Additionally, this would allow client code that *looks* at the union
to keep working even if actual data is later added to the branch;
client code that *creates* the union would need to be updated, of
course, but that would be the case regardless.
--
Andrea Bolognani / Red Hat / Virtualization
