How to avoid 'Mutiny had to drop the following exception'

[ennishol]

Hello!

if I run the following test, it is green but Mutiny logs [-- Mutiny had to drop the following exception --] which is thrown by Uni<String> a

Is there a way to properly cancel other Unis if the first one was already received, so that no dropped exceptions are reported? It seems that it happens only if the unis run on an executor.

Thanks!

The test:

import io.smallrye.mutiny.Uni;
import io.smallrye.mutiny.infrastructure.Infrastructure;
import org.junit.jupiter.api.Test;

import java.util.concurrent.TimeUnit;

import static org.assertj.core.api.Assertions.assertThat;

class UniFirstTest {

    @Test
    void uni() throws Exception {
        var bean = new Bean();
        Uni<String> a = Uni.createFrom().item(() -> bean.getValue("f")).runSubscriptionOn(Infrastructure.getDefaultWorkerPool());
        Uni<String> b = Uni.createFrom().item(() -> bean.getValue("test")).runSubscriptionOn(Infrastructure.getDefaultWorkerPool());
        var subscribe = Uni.join().first(a, b).withItem().subscribe();
        assertThat(subscribe.asCompletionStage().get(10, TimeUnit.SECONDS)).isEqualTo("testtest");
    }

    private static class Bean {

        public String getValue(String test) {
            if (test == null || test.length() < 3) {
                try {
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                }
                throw new IllegalArgumentException("too short");
            }
            return test + test;
        }
    }
}

[jponge]

UniJoinFirst actually cancels the remaining Uni instances when the first one emits an item.

What you are seeing here is actually normal, the upstream Uni is running (blocking) code in getValue. So ultimately an exception is thrown after cancellation, and this is still being reported because creating from an item should not throw, and because the code in getValue has no way to cancel (Mutiny cannot automagically cancel code).

If you want to run some code that can either emit an item or report an exception, you might use Uni.createFrom().emitter(...).