JSON marshalling produces invalid JSON when data is a zero length slice

[canni]

When event has data that's a not-nil, zero length slice, JSON marshalling produces malformed JSON.

POC, this code fails to unmarshall the event:

import (
	"log"

	ce "github.com/cloudevents/sdk-go/v2"
)

func main() {
	ev := ce.NewEvent(ce.VersionV1)
	ev.SetDataContentType(ce.ApplicationJSON)
	ev.SetType("type")
	ev.SetSource("source")
	ev.SetID("id")
	ev.DataEncoded = []byte{}

	if err := ev.Validate(); err != nil {
		log.Fatalf("failed to validate event: %s", err)
	}

	dta, err := ev.MarshalJSON()
	if err != nil {
		log.Fatalf("failed to marshal event: %s", err)
	}

	var res ce.Event
	err = res.UnmarshalJSON(dta)
	if err != nil {
		log.Fatalf("failed to unmarshal event: %s", err)
	}
}

[embano1]

Tbh, I don't think that field is supposed to be used directly (use SetData() instead) I'm not sure what the reasons were why this field was made public.

[canni]

In our case DataEncoded is set to non-nil empty byte array by gRPC decoder. Implementation detail.
IMO, no event state that returns empty Validate() err should produce malformed JSON. Code should either return validation error, or add check for this case.

[canni]

Issue can be replicated with valid usage of the API:

package main

import (
	"encoding/json"
	"log"

	format "github.com/cloudevents/sdk-go/binding/format/protobuf/v2"
	ce "github.com/cloudevents/sdk-go/v2"
)

func main() {
	ev := ce.NewEvent(ce.VersionV1)
	ev.SetDataContentType(ce.ApplicationJSON)
	ev.SetType("type")
	ev.SetSource("source")
	ev.SetID("id")

	if err := ev.Validate(); err != nil {
		log.Fatalf("failed to validate event: %s", err)
	}

	dta, err := format.Protobuf.Marshal(&ev)
	if err != nil {
		log.Fatalf("failed to proto marshal event: %s", err)
	}

	var res ce.Event
	err = format.Protobuf.Unmarshal(dta, &res)
	if err != nil {
		log.Fatalf("failed to proto unmarshal event: %s", err)
	}

	jsonDTA, err := res.MarshalJSON()
	if err != nil {
		log.Fatalf("failed to marshal event: %s", err)
	}

	var jsonEvent ce.Event
	err = json.Unmarshal(jsonDTA, &jsonEvent)
	if err != nil {
		log.Fatalf("failed to marshal event: %s", err)
	}
}