Add tests for EventTask#resumeWhenComplete and clarify Javadoc.

api/src/main/java/com/velocitypowered/api/event/EventTask.java

@@ -75,7 +75,7 @@ public interface EventTask {
   }
 
   /**
-   * Creates an continuation based {@link EventTask} from the given {@link Consumer}. The task isn't
+   * Creates a continuation based {@link EventTask} from the given {@link Consumer}. The task isn't
    * guaranteed to be executed asynchronously ({@link #requiresAsync()} always returns
    * {@code false}).
    *
@@ -99,8 +99,9 @@ public interface EventTask {
   }
 
   /**
-   * Creates an continuation based {@link EventTask} for the given {@link CompletableFuture}. The
-   * continuation will be notified once the given future is completed.
+   * Creates a continuation based {@link EventTask} for the given {@link CompletableFuture}. The
+   * continuation is resumed upon completion of the given {@code future}, whether it is completed
+   * successfully or not.
    *
    * @param future The task to wait for
    * @return The event task

proxy/src/test/java/com/velocitypowered/proxy/event/EventTaskTest.java

@@ -0,0 +1,118 @@
+package com.velocitypowered.proxy.event;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.velocitypowered.api.event.Continuation;
+import com.velocitypowered.api.event.EventTask;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
+import java.util.function.Consumer;
+import org.junit.jupiter.api.Test;
+
+public class EventTaskTest {
+
+  @Test
+  public void testResumeWhenCompleteNormal() {
+    WitnessContinuation continuation = new WitnessContinuation();
+    CompletableFuture<Void> completed = CompletableFuture.completedFuture(null);
+    EventTask.resumeWhenComplete(completed).execute(continuation);
+    assertTrue(continuation.completedSuccessfully(), "Completed future did not "
+        + "complete successfully");
+  }
+
+  @Test
+  public void testResumeWhenCompleteException() {
+    WitnessContinuation continuation = new WitnessContinuation();
+    CompletableFuture<Void> failed = CompletableFuture.failedFuture(new Throwable());
+    EventTask.resumeWhenComplete(failed).execute(continuation);
+    assertTrue(continuation.completedWithError(), "Failed future completed successfully");
+  }
+
+  @Test
+  public void testResumeWhenCompleteFromOtherThread() throws InterruptedException {
+    WitnessContinuation continuation = new WitnessContinuation();
+    CountDownLatch latch = new CountDownLatch(1);
+    continuation.onComplete = (ignored) -> latch.countDown();
+    CompletableFuture<Void> async = CompletableFuture.supplyAsync(() -> null);
+    EventTask.resumeWhenComplete(async).execute(continuation);
+    latch.await();
+
+    assertTrue(continuation.completedSuccessfully(), "Completed future did not "
+        + "complete successfully");
+  }
+
+  @Test
+  public void testResumeWhenFailFromOtherThread() throws InterruptedException {
+    WitnessContinuation continuation = new WitnessContinuation();
+    CountDownLatch latch = new CountDownLatch(1);
+    continuation.onComplete = (ignored) -> latch.countDown();
+    CompletableFuture<Void> async = CompletableFuture.supplyAsync(() -> {
+      throw new RuntimeException();
+    });
+    EventTask.resumeWhenComplete(async).execute(continuation);
+    latch.await();
+
+    assertTrue(continuation.completedWithError(), "Failed future completed successfully");
+  }
+
+  @Test
+  public void testResumeWhenFailFromOtherThreadComplexChain() throws InterruptedException {
+    WitnessContinuation continuation = new WitnessContinuation();
+    CountDownLatch latch = new CountDownLatch(1);
+    continuation.onComplete = (ignored) -> latch.countDown();
+    CompletableFuture<Void> async = CompletableFuture.supplyAsync(() -> null)
+            .thenAccept((v) -> {
+              throw new RuntimeException();
+            })
+            .thenCompose((v) -> CompletableFuture.completedFuture(null));
+    EventTask.resumeWhenComplete(async).execute(continuation);
+    latch.await();
+
+    assertTrue(continuation.completedWithError(), "Failed future completed successfully");
+  }
+
+  /**
+   * An extremely simplified implementation of {@link Continuation} for verifying the completion
+   * of an operation.
+   */
+  private static class WitnessContinuation implements Continuation {
+
+    private static final AtomicIntegerFieldUpdater<WitnessContinuation> STATUS_UPDATER =
+        AtomicIntegerFieldUpdater.newUpdater(WitnessContinuation.class, "status");
+
+    private static final int UNCOMPLETED = 0;
+    private static final int COMPLETED = 1;
+    private static final int COMPLETED_WITH_EXCEPTION = 2;
+
+    private volatile int status = UNCOMPLETED;
+    private Consumer<Throwable> onComplete;
+
+    @Override
+    public void resume() {
+      if (!STATUS_UPDATER.compareAndSet(this, UNCOMPLETED, COMPLETED)) {
+        throw new IllegalStateException("Continuation is already completed");
+      }
+
+      this.onComplete.accept(null);
+    }
+
+    @Override
+    public void resumeWithException(Throwable exception) {
+      if (!STATUS_UPDATER.compareAndSet(this, UNCOMPLETED, COMPLETED_WITH_EXCEPTION)) {
+        throw new IllegalStateException("Continuation is already completed");
+      }
+
+      this.onComplete.accept(exception);
+    }
+
+    public boolean completedSuccessfully() {
+      return STATUS_UPDATER.get(this) == COMPLETED;
+    }
+
+    public boolean completedWithError() {
+      return STATUS_UPDATER.get(this) == COMPLETED_WITH_EXCEPTION;
+    }
+  }
+
+}