Single.never not singleton

Signed-off-by: Daniel Kec <daniel.kec@oracle.com>

common/reactive/src/main/java/io/helidon/common/reactive/Single.java

@@ -228,7 +228,7 @@ public interface Single<T> extends Subscribable<T>, CompletionStage<T>, Awaitabl
      * @return Single
      */
     static <T> Single<T> never() {
-        return SingleNever.instance();
+        return new SingleNever<T>();
     }
 
     /**

common/reactive/src/main/java/io/helidon/common/reactive/SingleNever.java

@@ -21,24 +21,16 @@ import java.util.concurrent.Flow.Subscriber;
  * Implementation of {@link Single} that never invokes
  * {@link Subscriber#onComplete()} or
  * {@link Subscriber#onError(java.lang.Throwable)}.
+ *
+ * @param <T> item type
  */
-final class SingleNever extends CompletionSingle<Object> {
+final class SingleNever<T> extends CompletionSingle<T> {
 
-    /**
-     * Singleton instance.
-     */
-    private static final SingleNever INSTANCE = new SingleNever();
-
-    private SingleNever() {
+    SingleNever() {
     }
 
     @Override
-    public void subscribe(Subscriber<? super Object> actual) {
+    public void subscribe(Subscriber<? super T> actual) {
         actual.onSubscribe(EmptySubscription.INSTANCE);
     }
-
-    @SuppressWarnings("unchecked")
-    static <T> Single<T> instance() {
-        return (Single<T>) INSTANCE;
-    }
 }

common/reactive/src/test/java/io/helidon/common/reactive/SingleTest.java

@@ -157,6 +157,22 @@ public class SingleTest {
         assertThat(subscriber.getItems(), is(empty()));
     }
 
+    @Test
+    public void testNeverIsNotSingleton() throws InterruptedException, TimeoutException, ExecutionException {
+        CompletableFuture<Void> cf1 = new CompletableFuture<>();
+        CompletableFuture<Void> cf2 = new CompletableFuture<>();
+        Single<Object> never1 = Single.never();
+        Single<Object> never2 = Single.never();
+
+        never1.onCancel(() -> cf1.complete(null));
+        never2.onCancel(() -> cf2.complete(null));
+        never1.cancel();
+
+        cf1.get(100, TimeUnit.MILLISECONDS);
+        assertThat("First Single.never should be cancelled!", cf1.isDone());
+        assertThat("Other Single.never should NOT be cancelled!", !cf2.isDone());
+    }
+
     @Test
     public void testMap() {
         SingleTestSubscriber<String> subscriber = new SingleTestSubscriber<>();