Avoid locking in WebSocket session "close" callback

When processing a "close" notification from the server make an effort
to cancel any outstanding heartbeat but avoid going as far as acquiring
the responseLock since the server itself may already hold a lock of its
own leading to a potential deadlock.

The heartbeat task is now also further protected with an isClosed()
check in case the heartbeat does not get cancelled in a concurrent
scenario.

Issue: SPR-14917

Author: rstoyanchev

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

@@ -396,7 +396,12 @@ public final void delegateConnectionClosed(CloseStatus status) throws Exception
 		if (!isClosed()) {
 			try {
 				updateLastActiveTime();
-				cancelHeartbeat();
+				// Avoid cancelHeartbeat() and responseLock within server "close" callback
+				ScheduledFuture<?> future = this.heartbeatFuture;
+				if (future != null) {
+					this.heartbeatFuture = null;
+					future.cancel(false);
+				}
 			}
 			finally {
 				this.state = State.CLOSED;
@@ -446,7 +451,7 @@ private class HeartbeatTask implements Runnable {
 		@Override
 		public void run() {
 			synchronized (responseLock) {
-				if (!this.expired) {
+				if (!this.expired && !isClosed()) {
 					try {
 						sendHeartbeat();
 					}

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

@@ -31,8 +31,17 @@
 import org.springframework.web.socket.sockjs.SockJsTransportFailureException;
 import org.springframework.web.socket.sockjs.frame.SockJsFrame;
 
-import static org.junit.Assert.*;
-import static org.mockito.BDDMockito.*;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+import static org.mockito.BDDMockito.any;
+import static org.mockito.BDDMockito.given;
+import static org.mockito.BDDMockito.mock;
+import static org.mockito.BDDMockito.verify;
+import static org.mockito.BDDMockito.verifyNoMoreInteractions;
+import static org.mockito.BDDMockito.willReturn;
+import static org.mockito.BDDMockito.willThrow;
 
 /**
  * Test fixture for {@link AbstractSockJsSession}.
@@ -130,7 +139,6 @@ public void delegateConnectionClosed() throws Exception {
 
 		assertClosed();
 		assertEquals(1, this.session.getNumberOfLastActiveTimeUpdates());
-		assertTrue(this.session.didCancelHeartbeat());
 		verify(this.webSocketHandler).afterConnectionClosed(this.session, CloseStatus.GOING_AWAY);
 	}