Re: [PATCH v2 07/11] qapi: golang: Generate qapi's union types in Go

[Andrea Bolognani]

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