Allow ScheduledTask.cancel() without actually being started

Throws a NPE without this change.

Author: rob-bygrave

metrics/src/main/java/io/avaje/metrics/DScheduledTask.java

@@ -65,6 +65,9 @@ public void start() {
 
   @Override
   public boolean cancel(boolean mayInterruptIfRunning) {
+    if (backgroundTask == null) {
+      return false;
+    }
     return this.backgroundTask.cancel(mayInterruptIfRunning);
   }
 

metrics/src/main/java/io/avaje/metrics/ScheduledTask.java

@@ -25,6 +25,7 @@ static Builder builder() {
 
   /**
    * Cancel the scheduled task.
+   * @return true if the task was cancelled otherwise false
    */
   boolean cancel(boolean mayInterruptIfRunning);
 

metrics/src/test/java/io/avaje/metrics/ScheduledTaskTest.java

@@ -9,6 +9,26 @@
 
 class ScheduledTaskTest {
 
+  @Test
+  void stopWithoutStart() {
+    ScheduledTask task = ScheduledTask.builder()
+      .schedule(1, 1, TimeUnit.MILLISECONDS)
+      .task(ScheduledTaskTest::hello)
+      .build();
+
+    task.cancel(true);
+  }
+
+  @Test
+  void waitWithoutStart() {
+    ScheduledTask task = ScheduledTask.builder()
+      .schedule(1, 1, TimeUnit.MILLISECONDS)
+      .task(ScheduledTaskTest::hello)
+      .build();
+
+    task.waitIfRunning(10, TimeUnit.SECONDS);
+  }
+
   @Test
   void runIt() throws InterruptedException {