Fix concurrency issues in SockJS session impls

This change ensures the server "WebSocketHandler" is notified of the
opening of a session before writing the open frame to the remote
handler. Any messages sent by the server "WebSocketHandler" while
getting notified of the opening get cached and flushed after the open
frame has been written.

This change introduces locking in AbtractHttpSockJsSession to guard
access to the HTTP response. The goal is to prevent contention between
client requests to receive messages (i.e. long polling) and
the application trying to write.

Issue: SPR-11916

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/client/JettySockJsIntegrationTests.java

@@ -27,9 +27,6 @@ import org.springframework.web.socket.client.jetty.JettyWebSocketClient;
 import org.springframework.web.socket.server.RequestUpgradeStrategy;
 import org.springframework.web.socket.server.jetty.JettyRequestUpgradeStrategy;
 
-import java.util.ArrayList;
-import java.util.List;
-
 /**
  * SockJS integration tests using Jetty for client and server.
  *

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/transport/session/HttpSockJsSessionTests.java

@@ -94,10 +94,8 @@ public class HttpSockJsSessionTests extends AbstractSockJsSessionTests<TestAbstr
 		this.session.handleSuccessiveRequest(this.request, this.response, this.frameFormat);
 
 		assertTrue(this.servletRequest.isAsyncStarted());
-
 		assertTrue(this.session.wasHeartbeatScheduled());
 		assertTrue(this.session.wasCacheFlushed());
-
 		assertEquals("hhh\n", this.servletResponse.getContentAsString());
 
 		verifyNoMoreInteractions(this.webSocketHandler);
@@ -119,6 +117,11 @@ public class HttpSockJsSessionTests extends AbstractSockJsSessionTests<TestAbstr
 			super("1", config, handler, attributes);
 		}
 
+		@Override
+		protected boolean isStreaming() {
+			return false;
+		}
+
 		@Override
 		protected void writePrelude(ServerHttpRequest request, ServerHttpResponse response) throws IOException {
 			response.getBody().write("hhh\n".getBytes());
@@ -139,6 +142,7 @@ public class HttpSockJsSessionTests extends AbstractSockJsSessionTests<TestAbstr
 		@Override
 		protected void flushCache() {
 			this.cacheFlushed = true;
+			scheduleHeartbeat();
 		}
 
 		@Override

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/transport/session/TestHttpSockJsSession.java

@@ -53,6 +53,11 @@ public class TestHttpSockJsSession extends AbstractHttpSockJsSession {
 		super(sessionId, config, wsHandler, attributes);
 	}
 
+	@Override
+	protected boolean isStreaming() {
+		return true;
+	}
+
 	@Override
 	public String getAcceptedProtocol() {
 		return this.subProtocol;

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/transport/session/WebSocketServerSockJsSessionTests.java

@@ -28,6 +28,8 @@ import org.junit.Test;
 import org.springframework.web.socket.CloseStatus;
 import org.springframework.web.socket.TextMessage;
 import org.springframework.web.socket.WebSocketHandler;
+import org.springframework.web.socket.WebSocketSession;
+import org.springframework.web.socket.handler.TextWebSocketHandler;
 import org.springframework.web.socket.sockjs.transport.SockJsServiceConfig;
 import org.springframework.web.socket.sockjs.transport.session.WebSocketServerSockJsSessionTests.TestWebSocketServerSockJsSession;
 import org.springframework.web.socket.handler.TestWebSocketSession;
@@ -71,17 +73,27 @@ public class WebSocketServerSockJsSessionTests extends AbstractSockJsSessionTest
 
 	@Test
 	public void afterSessionInitialized() throws Exception {
-
 		this.session.initializeDelegateSession(this.webSocketSession);
-
-		assertEquals("Open frame not sent",
-				Collections.singletonList(new TextMessage("o")), this.webSocketSession.getSentMessages());
-
+		assertEquals(Collections.singletonList(new TextMessage("o")), this.webSocketSession.getSentMessages());
 		assertEquals(Arrays.asList("schedule"), this.session.heartbeatSchedulingEvents);
 		verify(this.webSocketHandler).afterConnectionEstablished(this.session);
 		verifyNoMoreInteractions(this.taskScheduler, this.webSocketHandler);
 	}
 
+	@Test
+	public void afterSessionInitializedOpenFrameFirst() throws Exception {
+		TextWebSocketHandler handler = new TextWebSocketHandler() {
+			@Override
+			public void afterConnectionEstablished(WebSocketSession session) throws Exception {
+				session.sendMessage(new TextMessage("go go"));
+			}
+		};
+		TestWebSocketServerSockJsSession session = new TestWebSocketServerSockJsSession(this.sockJsConfig, handler, null);
+		session.initializeDelegateSession(this.webSocketSession);
+		List<TextMessage> expected = Arrays.asList(new TextMessage("o"), new TextMessage("a[\"go go\"]"));
+		assertEquals(expected, this.webSocketSession.getSentMessages());
+	}
+
 	@Test
 	public void handleMessageEmptyPayload() throws Exception {
 		this.session.handleMessage(new TextMessage(""), this.webSocketSession);