Fix race condition in sendBlockingMessage

Previously, tests in ConcurrentWebSocketSessionDecoratorTests that use
the BlockingSession would fail intermittently. This appears to have
been due to a race condition in sendBlockingMessage where the call
to getSentMessageLatch() that stores a latch in nextMessageLatch on
the main thread may happen after the call to sendMessage that counts
down the latch if it is non-null occurs on the executor's thread.

This commit updates sendBlockingMessage to call getSentMessageLatch()
(and therefore store the latch) before it sumbmits the task to the
executor. This ensures that the latch will be available when the
exeuctor's thread attempts to retrieve and decrement it.
BlockingSession's AtomicReference fields have also been made final to
eliminate the possibility of any visibility problems across threads.

Closes gh-23642

spring-websocket/src/test/java/org/springframework/web/socket/handler/ConcurrentWebSocketSessionDecoratorTests.java

@@ -203,6 +203,8 @@ public class ConcurrentWebSocketSessionDecoratorTests {
 	}
 
 	private void sendBlockingMessage(ConcurrentWebSocketSessionDecorator session) throws InterruptedException {
+		BlockingSession delegate = (BlockingSession) session.getDelegate();
+		CountDownLatch sentMessageLatch = delegate.getSentMessageLatch();
 		Executors.newSingleThreadExecutor().submit(() -> {
 			TextMessage message = new TextMessage("slow message");
 			try {
@@ -212,17 +214,16 @@ public class ConcurrentWebSocketSessionDecoratorTests {
 				e.printStackTrace();
 			}
 		});
-		BlockingSession delegate = (BlockingSession) session.getDelegate();
-		assertThat(delegate.getSentMessageLatch().await(5, TimeUnit.SECONDS)).isTrue();
+		assertThat(sentMessageLatch.await(5, TimeUnit.SECONDS)).isTrue();
 	}
 
 
 
 	private static class BlockingSession extends TestWebSocketSession {
 
-		private AtomicReference<CountDownLatch> nextMessageLatch = new AtomicReference<>();
+		private final AtomicReference<CountDownLatch> nextMessageLatch = new AtomicReference<>();
 
-		private AtomicReference<CountDownLatch> releaseLatch = new AtomicReference<>();
+		private final AtomicReference<CountDownLatch> releaseLatch = new AtomicReference<>();
 
 
 		public CountDownLatch getSentMessageLatch() {