Vert.x JSON wrapped in a Multi is not working as expected

[nweike]

Describe the bug
(Describe the problem clearly and concisely.)
When the JSON content is wrapped in a 'Multi', it is not recognised as a Quarkus web resource, instead it embeds the entire json object in a new object called 'map'.

Here is the code:

@Path("/users/v1")
@Produces(MediaType.APPLICATION_JSON)
public class UserResource {
@GET
@Path("{name}/multi")
@Produces(MediaType.APPLICATION_JSON)
public Multi<JsonObject> multi(@PathParam String name) {
    Multi<JsonObject> multi = Multi.createFrom().items(new JsonObject().put("Hello", name));
    return multi;
}
@GET
@Path("{name}/uni")
@Produces(MediaType.APPLICATION_JSON)
public Uni<JsonObject> uni(@PathParam String name) {
    Uni<JsonObject> multi = Uni.createFrom().item(new JsonObject().put("Hello", name));
    return multi;
}
}

when I access this end point:
localhost:82/users/v1/all/uni
I
it returns a nicely formated json like this:

{
    "Hello": "all"
}

however, when i access localhost:82/users/v1/all/multi

[
    {
        "empty": false,
        "map": {
            "Hello": "all"
        }
    }
]

Expected behavior
(Describe the expected behavior clearly and concisely.)
It is expected to return this :

{
    "Hello": "all"
}

Actual behavior
(Describe the actual behavior clearly and concisely.)

It returns this:

[
    {
        "empty": false,
        "map": {
            "Hello": "all"
        }
    }
]

To Reproduce
Steps to reproduce the behavior:

1. Create a JAX-RS resource method that returns a Multi
2. Annotate the method with @produces(MediaType.APPLICATION_JSON) on the methos
3. Add Multi multi = Multi.createFrom().items(new JsonObject().put("Hello", name));
   return multi; to the bosy of the method

Configuration

# Add your application.properties here, if applicable.

Screenshots
(If applicable, add screenshots to help explain your problem.)

Environment (please complete the following information):

• Output of uname -a or ver:
• Output of java -version:
• GraalVM version (if different from Java):
• Quarkus version or git rev:
• Build tool (ie. output of mvnw --version or gradlew --version):

Additional context
(Add any other context about the problem here.)

[gsmet]

/cc @cescoffier

[cescoffier]

When returning a Multi, RestEasy accumulates the items in an ArrayList. This ArrayList is then serialised using the ArrayList serialiser which is JSON-B. Items included in the list are also serialised using JSON-B.
So, we would need a JSON-B “serialiser” for the Vert.x JSONObject.

So it's more a Resteasy issue.

[cescoffier]

See #8848