Ensure that the task is not in the tasks queue when it is rejected fr…

…om the executor

Closes #4900

src/main/java/io/vertx/core/impl/TaskQueue.java

@@ -16,6 +16,7 @@
 
 import java.util.LinkedList;
 import java.util.concurrent.Executor;
+import java.util.concurrent.RejectedExecutionException;
 
 /**
  * A task queue that always run all tasks in order. The executor to run the tasks is passed
@@ -87,11 +88,27 @@ private void run() {
    */
   public void execute(Runnable task, Executor executor) {
     synchronized (tasks) {
-      tasks.add(new Task(task, executor));
       if (current == null) {
-        current = executor;
-        executor.execute(runner);
+        try {
+          current = executor;
+          executor.execute(runner);
+        } catch (RejectedExecutionException e) {
+          current = null;
+          throw e;
+        }
       }
+      // Add the task after the runner has been accepted to the executor
+      // to cover the case of a rejected execution exception.
+      tasks.add(new Task(task, executor));
+    }
+  }
+
+  /**
+   * Test if the task queue is empty and no current executor is running anymore.
+   */
+  public boolean isEmpty() {
+    synchronized (tasks) {
+      return tasks.isEmpty() && current == null;
     }
   }
 }

src/test/java/io/vertx/core/impl/TaskQueueTest.java

@@ -0,0 +1,34 @@
+package io.vertx.core.impl;
+
+import org.assertj.core.api.Assertions;
+import org.junit.Test;
+
+import java.util.concurrent.Executor;
+import java.util.concurrent.RejectedExecutionException;
+
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+/**
+ * @author Alexander Schwartz
+ */
+public class TaskQueueTest {
+
+  Executor executorThatAlwaysThrowsRejectedExceptions = new Executor() {
+    @Override
+    public void execute(Runnable command) {
+      throw new RejectedExecutionException();
+    }
+  };
+
+  TaskQueue taskQueue = new TaskQueue();
+
+  @Test
+  public void shouldNotHaveTaskInQueueWhenTaskHasBeenRejected() {
+    assertThatThrownBy(
+      () -> taskQueue.execute(new Thread(), executorThatAlwaysThrowsRejectedExceptions)
+    ).isInstanceOf(RejectedExecutionException.class);
+
+    Assertions.assertThat(taskQueue.isEmpty()).isTrue();
+  }
+
+}