Add test for race condition when starting workers

Author: mpkorstanje

platform-tests/src/test/java/org/junit/platform/engine/support/hierarchical/ConcurrentHierarchicalTestExecutorServiceTests.java

@@ -32,6 +32,7 @@
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.locks.ReentrantLock;
+import java.util.stream.Stream;
 
 import org.jspecify.annotations.NullMarked;
 import org.jspecify.annotations.Nullable;
@@ -275,6 +276,45 @@ void prioritizesTestsOverContainers() throws Exception {
 		assertThat(child2.startTime).isBeforeOrEqualTo(child1.startTime);
 	}
 
+	@Test
+	void limitsWorkerThreadsToMaxPoolSize() throws Exception {
+		service = new ConcurrentHierarchicalTestExecutorService(configuration(3, 3));
+
+		CountDownLatch latch = new CountDownLatch(3);
+		Executable behavior = () -> {
+			latch.countDown();
+			latch.await();
+		};
+		var leaf1a = new TestTaskStub(ExecutionMode.CONCURRENT, behavior) //
+				.withName("leaf1a").withLevel(3);
+		var leaf1b = new TestTaskStub(ExecutionMode.CONCURRENT, behavior) //
+				.withName("leaf1b").withLevel(3);
+		var leaf2a = new TestTaskStub(ExecutionMode.CONCURRENT, behavior) //
+				.withName("leaf2a").withLevel(3);
+		var leaf2b = new TestTaskStub(ExecutionMode.CONCURRENT, behavior) //
+				.withName("leaf2b").withLevel(3);
+
+		// When executed, there are 2 worker threads active and 1 available.
+		// Both invokeAlls race each other trying to start 1 more.
+		var child1 = new TestTaskStub(ExecutionMode.CONCURRENT,
+			() -> requiredService().invokeAll(List.of(leaf1a, leaf1b))) //
+					.withName("child1").withLevel(2);
+		var child2 = new TestTaskStub(ExecutionMode.CONCURRENT,
+			() -> requiredService().invokeAll(List.of(leaf2a, leaf2b))) //
+					.withName("child2").withLevel(2);
+
+		var root = new TestTaskStub(ExecutionMode.SAME_THREAD,
+			() -> requiredService().invokeAll(List.of(child1, child2))) //
+					.withName("root").withLevel(1);
+
+		service.submit(root).get();
+
+		assertThat(List.of(root, child1, child2, leaf1a, leaf1b, leaf2a, leaf2b)) //
+				.allSatisfy(TestTaskStub::assertExecutedSuccessfully);
+		assertThat(Stream.of(leaf1a, leaf1b, leaf2a, leaf2b).map(TestTaskStub::executionThread).distinct()) //
+				.hasSize(3);
+	}
+
 	private static ExclusiveResource exclusiveResource() {
 		return new ExclusiveResource("key", ExclusiveResource.LockMode.READ_WRITE);
 	}
@@ -284,7 +324,11 @@ private ConcurrentHierarchicalTestExecutorService requiredService() {
 	}
 
 	private static ParallelExecutionConfiguration configuration(int parallelism) {
-		return new DefaultParallelExecutionConfiguration(parallelism, parallelism, 256 + parallelism, parallelism, 0,
+		return configuration(parallelism, 256 + parallelism);
+	}
+
+	private static ParallelExecutionConfiguration configuration(int parallelism, int maxPoolSize) {
+		return new DefaultParallelExecutionConfiguration(parallelism, parallelism, maxPoolSize, parallelism, 0,
 			__ -> true);
 	}