On Sat, Apr 02, 2022 at 12:40:58AM +0200, Victor Toso wrote:
> This patch handles QAPI alternate types and generates data structures
> in Go that handles it.
>
> At this moment, there are 5 alternates in qemu/qapi, they are:
> * BlockDirtyBitmapMergeSource
> * Qcow2OverlapChecks
> * BlockdevRef
> * BlockdevRefOrNull
> * StrOrNull
>
> Alternate types are similar to Union but without a discriminator that
> can be used to identify the underlying value on the wire. It is needed
> to infer it. That can't be easily mapped in Go.
I don't buy that. Given this example:
type BlockdevRef struct {
// Options are:
// * definition (BlockdevOptions): defines a new block device inline
// * reference (string): references the ID of an existing block device
Value Any
}
What is the problem with having this Go struct:
type BlockdevRef struct {
Definition *BlockdevOptions
Reference *string
}
when deserializing from JSON, we know exactly which one of these two
fields to populate. The programmer consuming this can look at which
field is non-nil.
When serializing to JSON, we serialize which ever field is non-nil.
If both fields are non-nil that's a programmer bug. Either ignore it
and only serialize the first non-nil field, or raise an error.
>
> For each Alternate type, we will be using a Any type to hold the
> value. 'Any' is an alias type for interface{} (similar to void* in C).
>
> Similarly to the Enum types (see previous commit), we will implement
> Marshaler and Unmarshaler interfaces for the Alternate types and in
> those MarshalJSON() and UnmarshalJSON() methods is where we are going
> to put the logic to read/set alternate's value.
>
> Note that on UnmarshalJSON(), a helper function called StrictDecode()
> will be used. This function is the main logic to infer if a given JSON
> object fits in a given Go struct. Because we only have 5 alternate
> types, it is not hard to validate the unmarshaling logic but we might
> need to improve it in the future if Alternate with branches that have
> similar fields appear.
>
> Examples:
> * BlockdevRef
> ```go
> // Data to set in BlockdevOptions
> qcow2 := BlockdevOptionsQcow2{}
> // BlockdevRef using a string
> qcow2.File = BlockdevRef{Value: "/some/place/my-image"}
> opt := BlockdevOptions{}
> opt.Driver = BlockdevDriverQcow2
> opt.Value = qcow2
>
> b, _ := json.Marshal(data.s)
> // string(b) == `{"driver":"qcow2","file":"/some/place/my-image"}`
> ```
>
> Signed-off-by: Victor Toso <victort...@redhat.com>
> ---
> scripts/qapi/golang.py | 157 ++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 155 insertions(+), 2 deletions(-)
With regards,
Daniel
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