Can't return responses with empty arrays from http ingress verb

[mistermoe]

Repro

package offerings

import (
	"context"
	"ftl/builtin"

	"github.com/TBD54566975/ftl/go-runtime/ftl"
)

type GetOfferingsResponse struct {
	Data []string `json:"data"`
}

type ApiError struct {
	Error string `json:"error"`
}

//ftl:export
//ftl:ingress GET /offerings
func GetOfferings(ctx context.Context, req builtin.HttpRequest[builtin.Empty]) (builtin.HttpResponse[GetOfferingsResponse, ApiError], error) {
	r := GetOfferingsResponse{
		Data: make([]string, 0),
	}
	return builtin.HttpResponse[GetOfferingsResponse, ApiError]{
		Status: 200,
		Body:   ftl.Some(r),
	}, nil
}

❯ curl -v 'http://localhost:8892/ingress/offerings'
*   Trying [::1]:8892...
* connect to ::1 port 8892 failed: Connection refused
*   Trying 127.0.0.1:8892...
* Connected to localhost (127.0.0.1) port 8892
> GET /ingress/offerings HTTP/1.1
> Host: localhost:8892
> User-Agent: curl/8.4.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: application/json; charset=utf-8
< Date: Mon, 08 Apr 2024 00:54:57 GMT
< Content-Length: 2
<
* Connection #0 to host localhost left intact
{}

Expected Output

❯ curl -v 'http://localhost:8892/ingress/offerings'
*   Trying [::1]:8892...
* connect to ::1 port 8892 failed: Connection refused
*   Trying 127.0.0.1:8892...
* Connected to localhost (127.0.0.1) port 8892
> GET /ingress/offerings HTTP/1.1
> Host: localhost:8892
> User-Agent: curl/8.4.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: application/json; charset=utf-8
< Date: Mon, 08 Apr 2024 00:54:57 GMT
< Content-Length: 2
<
* Connection #0 to host localhost left intact
{"data": []}

Workaround

switching the response body type to []byte at the cost of losing strong types e.g.

package offerings

import (
	"context"
	"encoding/json"
	"ftl/builtin"

	"github.com/TBD54566975/ftl/go-runtime/ftl"
)

type GetOfferingsResponse struct {
	Data []string `json:"data"`
}

type ApiError struct {
	Error string `json:"error"`
}

//ftl:export
//ftl:ingress GET /offerings
func GetOfferings(ctx context.Context, req builtin.HttpRequest[builtin.Empty]) (builtin.HttpResponse[[]byte, ApiError], error) {
	r := GetOfferingsResponse{
		Data: make([]string, 0),
	}

	b, _ := json.Marshal(r)

	return builtin.HttpResponse[[]byte, ApiError]{
		Status: 200,
		Body:   ftl.Some(b),
	}, nil
}

Other Thoughts

potentially has something to do with how marshaling ftl.Some handles zero values

[alecthomas]

The fields are omitted by the internal encoder here, but the bug is with the ingress marshaler in that it shouldn't be skipping them.