test: document SerialExecutor expected behavior

Classes that depend on the SerialExecutor assume that the tasks are
executed and processed in the order of insertion. This test verifies
this.

Signed-off-by: Marc Nuri <[email protected]>

kubernetes-client/src/main/java/io/fabric8/kubernetes/client/utils/internal/SerialExecutor.java

@@ -25,7 +25,8 @@
  * See {@link Executor} docs
  *
  * <br>
- * Effectively creates a derived single thread executor
+ * Effectively creates a derived single thread executor, runnable tasks are executed in the order they are added.
+ * This is a replacement for Executors.newSingleThreadExecutor() that uses threads in a non-dedicated way
  * <br>
  * Added shutdown support
  */

kubernetes-client/src/test/java/io/fabric8/kubernetes/client/utils/internal/SerialExecutorTest.java

@@ -19,9 +19,12 @@
 import org.junit.jupiter.api.Test;
 
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 
 class SerialExecutorTest {
@@ -53,4 +56,31 @@ void clearInterrupt() {
     assertFalse(interrupted.join()); // make sure the second is not interrrupted
   }
 
+  @Test
+  void taskExecutedInOrderOfInsertion() throws InterruptedException {
+    final ExecutorService es = Executors.newSingleThreadExecutor();
+    try {
+      final StringBuffer sb = new StringBuffer();
+      final SerialExecutor se = new SerialExecutor(es);
+      final CountDownLatch latch = new CountDownLatch(1);
+      se.execute(() -> {
+        sb.append("1");
+        try {
+          Thread.sleep(100L);
+        } catch (InterruptedException e) {
+          throw new RuntimeException(e);
+        }
+      });
+      se.execute(() -> sb.append("2"));
+      se.execute(() -> sb.append("3"));
+      se.execute(() -> sb.append("4"));
+      se.execute(() -> sb.append("5"));
+      se.execute(latch::countDown);
+      assertThat(latch.await(500L, TimeUnit.MILLISECONDS)).isTrue();
+      assertThat(sb).hasToString("12345");
+    } finally {
+      es.shutdownNow();
+    }
+  }
+
 }