Re: [RFC PATCH v1 4/8] qapi: golang: Generate qapi's union types in Go

[Victor Toso]

Hi,

On Tue, May 10, 2022 at 11:26:56AM +0100, Daniel P. Berrangé wrote:
> On Sat, Apr 02, 2022 at 12:41:00AM +0200, Victor Toso wrote:
> > This patch handles QAPI union types and generates the equivalent data
> > structures and methods in Go to handle it.
> > 
> > At the moment of this writing, it generates 67 structures.
> > 
> > The QAPI union type can be summarized by its common members that are
> > defined in a @base struct and a @value. The @value type can vary and
> > depends on @base's field that we call @discriminator. The
> > @discriminator is always a Enum type.
> > 
> > Golang does not have Unions. The generation of QAPI union type in Go
> > with this patch, follows similar approach to what is done for QAPI
> > struct types and QAPI alternate types.
> 
> The common way to approach unions in Go is to just use a struct
> where each union case is an optional field, and declare that
> only one field must ever be set. ie
> 
>   type SocketAddressLegacy struct {
>         // Value based on @type, possible types:
>         Inet *InetSocketAddressWrapper
>         Unix *UnixSocketAddressWrapper
>         VSock *VsockSocketAddressWrapper
>         FD *StringWrapper
>   }

Like Alternates, I like this better.

> When deserializing from JSON we populate exactly one of the
> optional fields.
> 
> When serializing to JSON process the first field that is
> non-nil.
> 
> Note, you don't actually need to include the discriminator as a
> field at all, since it is implicitly determined by whichever
> case is non-nil.  Introducing the discriminator as a field just
> provides the possibility for the programmer to make
> inconsistent settings, for no gain.

Sounds reasonable. We still need to implement Marshal/Unmarshal
for unknow types (e.g: a new Type for SocketAddressLegacy was
introduced in 7.1 and we should be able to know that current
qapi-go version can't understand it).

Cheers,
Victor

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