Async tasks cancellation fix (#907)

src/main/java/net/minestom/server/timer/SchedulerImpl.java

@@ -79,7 +79,12 @@ final class SchedulerImpl implements Scheduler {
         // By either adding the task to the execution queue or submitting it to the pool
         switch (task.executionType()) {
             case SYNC -> taskQueue.offer(task);
-            case ASYNC -> EXECUTOR.submit(() -> handleTask(task));
+            case ASYNC -> EXECUTOR.submit(() -> {
+                if (!task.isAlive()) {
+                    return;
+                }
+                handleTask(task);
+            });
         }
     }
 

src/test/java/net/minestom/server/timer/TestScheduler.java

@@ -2,6 +2,7 @@ package net.minestom.server.timer;
 
 import org.junit.jupiter.api.Test;
 
+import java.time.Duration;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.atomic.AtomicBoolean;
 
@@ -66,6 +67,22 @@ public class TestScheduler {
         assertFalse(result.get(), "Task should be cancelled");
     }
 
+    @Test
+    public void cancelAsyncDelayedTask() throws InterruptedException {
+        Scheduler scheduler = Scheduler.newScheduler();
+        AtomicBoolean result = new AtomicBoolean(false);
+        var task = scheduler.buildTask(() -> result.set(true))
+                .delay(Duration.ofMillis(1))
+                .executionType(ExecutionType.ASYNC)
+                .schedule();
+        assertTrue(task.isAlive(), "Task should still be alive");
+        task.cancel();
+        assertFalse(task.isAlive(), "Task should not be alive anymore");
+        scheduler.process();
+        Thread.sleep(10L);
+        assertFalse(result.get(), "Task should be cancelled");
+    }
+
     @Test
     public void parkTask() {
         Scheduler scheduler = Scheduler.newScheduler();