fix the validator for anyOf

Author: zhongkechen

sdk/src/main/java/software/amazon/lambda/durable/execution/ExecutionManager.java

@@ -14,8 +14,6 @@
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ThreadPoolExecutor;
-import java.util.concurrent.TimeUnit;
-import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.stream.Collectors;
 import org.slf4j.Logger;
@@ -282,35 +280,38 @@ public CompletableFuture<Operation> pollForOperationUpdates(String operationId,
     /** Shutdown the checkpoint batcher. */
     @Override
     public void close() {
-        checkpointManager.shutdown();
-
         validateRunningThreads();
+
+        checkpointManager.shutdown();
     }
 
     private void validateRunningThreads() {
         // This will detect stuck user thread and thread leaks in the thread pool
         for (BaseDurableOperation op : registeredOperations.values()) {
             var userHandlerFuture = op.getRunningUserHandler();
             if (userHandlerFuture != null && !userHandlerFuture.isDone()) {
-                // We wait a few more milliseconds for the last user thread to complete because
-                // it may have deregistered itself but haven't released the thread yet.
+                // Some user threads can still be running because
+                // the operations that run them have never been waiting for and the execution has completed.
                 logger.info("Waiting for operation to complete before shutting down: {}", op.getOperationId());
                 try {
                     userHandlerFuture.get();
                 } catch (InterruptedException | CancellationException e) {
                     // if the user handler is stuck
-                    throw new IllegalStateException("Stuck running user handler when shutting down: " + op.getOperationId());
+                    throw new IllegalStateException(
+                            "Stuck running user handler when shutting down: " + op.getOperationId());
                 } catch (Exception e) {
                     // ok if the future completed exceptionally
                 }
             }
         }
 
+        // double check if the thread pool is empty
         if (durableConfig.getExecutorService() instanceof ThreadPoolExecutor threadPoolExecutor) {
             var threadCount = threadPoolExecutor.getActiveCount();
             if (threadCount > 0) {
-                // this may or may not be a problem because getActiveCount doesn't return an accurate number
                 logger.warn("{} active threads in user executor pool when shutting down", threadCount);
+                throw new IllegalStateException(
+                        "Threads still running on user executor pool when shutting down: " + threadCount);
             }
         }
     }