Clarify use of Multi as return type in RESTEasy Reactive

[geoand]

Description
Currently RESTEasy Reactive always tries to "stream" a result when Multi is used a return type of Resource method.

This can be surprising in cases like the following:

@Path("fruits")
@ApplicationScoped
@Produces("application/json")
@Consumes("application/json")
public class FruitMutinyResource {
    private static final Logger LOGGER = Logger.getLogger(FruitMutinyResource.class.getName());

    @Inject
    Mutiny.Session mutinySession;

    @GET
    public Multi<Fruit> get() {
        return mutinySession
                .createNamedQuery( "Fruits.findAll", Fruit.class ).getResults();
    }

}

where users expect to get a JSON array as a result, but instead end up with something like

{"id":2,"name":"Apple"}{"id":3,"name":"Banana"}{"id":1,"name":"Pineapple"}

We have the following options:

• Keep things as they are (as in Multi always "streams") and expect users to collect the results to Uni (using .collectItems().asList() on the Multi)
• Have RESTEasy Reactive automatically collect the result to Uni when the media type of resource method does not support streaming (for example application/json does not stream). This may not be bullet-proof however as there is no spec mandate about which media types can and can't stream.

Historical note

The discussion that lead to this is here

[ghost]

/cc @FroMage, @stuartwdouglas

[FroMage]

Technically, we can stream json arrays too, as long as we start with [ and separate them with , and end with ], with the application/json mime type: that's just raw HTTP streaming since the end result is a valid json value.

We could just do that and call it a day. Then it's up to the client to figure out if it wants to stream or not.

[cescoffier]

+1

If someone tells me how this should be done, I will be happy to implement it (well, reuse the code I wrote for reactive routes).

[FroMage]

#14214 should fix this.

[cescoffier]

Shouldn't we close that issue? PR #14214 has been merged.

@geoand @FroMage

[geoand]

Yes, let's close it