Synchronize request init in AbstractHttpSockJsSession

Although unlikely in practice (but not impossible), the SockJS
integration tests write a message while the request is initializing.
This change adds synchronization around request intiailization
for the SockJS HTTP sesion.

Issue: SPR-11916

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/transport/session/AbstractHttpSockJsSession.java

@@ -201,18 +201,20 @@ public void handleInitialRequest(ServerHttpRequest request, ServerHttpResponse r
 		this.frameFormat = frameFormat;
 		this.asyncRequestControl = request.getAsyncRequestControl(response);
 
-		try {
-			// Let "our" handler know before sending the open frame to the remote handler
-			delegateConnectionEstablished();
-			writePrelude(request, response);
-			writeFrame(SockJsFrame.openFrame());
-			if (isStreaming() && !isClosed()) {
-				startAsyncRequest();
+		synchronized (this.responseLock) {
+			try {
+				// Let "our" handler know before sending the open frame to the remote handler
+				delegateConnectionEstablished();
+				writePrelude(request, response);
+				writeFrame(SockJsFrame.openFrame());
+				if (isStreaming() && !isClosed()) {
+					startAsyncRequest();
+				}
+			}
+			catch (Throwable ex) {
+				tryCloseWithSockJsTransportError(ex, CloseStatus.SERVER_ERROR);
+				throw new SockJsTransportFailureException("Failed to open session", getId(), ex);
 			}
-		}
-		catch (Throwable ex) {
-			tryCloseWithSockJsTransportError(ex, CloseStatus.SERVER_ERROR);
-			throw new SockJsTransportFailureException("Failed to open session", getId(), ex);
 		}
 	}
 
@@ -244,8 +246,8 @@ private void startAsyncRequest() {
 	 * @param response the current response
 	 * @param frameFormat the transport-specific SocksJS frame format to use
 	 */
-	public void handleSuccessiveRequest(ServerHttpRequest request,
-			ServerHttpResponse response, SockJsFrameFormat frameFormat) throws SockJsException {
+	public void handleSuccessiveRequest(ServerHttpRequest request, ServerHttpResponse response,
+			SockJsFrameFormat frameFormat) throws SockJsException {
 
 		synchronized (this.responseLock) {
 			try {