Restore host header initialization in StompBrokerRelayMessageHandler

Closes gh-36907

Author: rstoyanchev

spring-messaging/src/main/java/org/springframework/messaging/simp/stomp/StompBrokerRelayMessageHandler.java

@@ -456,7 +456,7 @@ protected void startInternal() {
 		accessor.setLogin(this.systemLogin);
 		accessor.setPasscode(this.systemPasscode);
 		accessor.setHeartbeat(this.systemHeartbeatSendInterval, this.systemHeartbeatReceiveInterval);
-		accessor.setHost(getVirtualHost() != null ? getVirtualHost() : getRelayHost());
+		accessor.setHost(getVirtualHost() != null ? getVirtualHost() : null);
 		accessor.setSessionId(SYSTEM_SESSION_ID);
 		if (logger.isDebugEnabled()) {
 			logger.debug("Forwarding " + accessor.getShortLogMessage(EMPTY_PAYLOAD));
@@ -579,7 +579,7 @@ else if (accessor instanceof SimpMessageHeaderAccessor) {
 			stompHeaderAccessor = (stompHeaderAccessor.isMutable() ? stompHeaderAccessor : StompHeaderAccessor.wrap(message));
 			stompHeaderAccessor.setLogin(this.clientLogin);
 			stompHeaderAccessor.setPasscode(this.clientPasscode);
-			stompHeaderAccessor.setHost(getVirtualHost() != null ? getVirtualHost() : getRelayHost());
+			stompHeaderAccessor.setHost(getVirtualHost() != null ? getVirtualHost() : null);
 			RelayConnectionHandler handler = new RelayConnectionHandler(sessionId, stompHeaderAccessor);
 			this.connectionHandlers.put(sessionId, handler);
 			this.stats.incrementConnectCount();

spring-messaging/src/test/java/org/springframework/messaging/simp/stomp/StompBrokerRelayMessageHandlerTests.java

@@ -98,6 +98,27 @@ void virtualHost() {
 		assertThat(headers2.getHost()).isEqualTo("ABC");
 	}
 
+	@Test
+	void virtualHostDefault() {
+		Message<byte[]> connectMessage = connectMessage("sess1", "joe");
+		MessageHeaderAccessor.getAccessor(connectMessage, StompHeaderAccessor.class).setHost("ABC");
+
+		this.brokerRelay.start();
+		this.brokerRelay.handleMessage(connectMessage("sess1", "joe"));
+
+		assertThat(this.tcpClient.getSentMessages()).hasSize(2);
+
+		StompHeaderAccessor headers1 = this.tcpClient.getSentHeaders(0);
+		assertThat(headers1.getCommand()).isEqualTo(StompCommand.CONNECT);
+		assertThat(headers1.getSessionId()).isEqualTo(StompBrokerRelayMessageHandler.SYSTEM_SESSION_ID);
+		assertThat(headers1.getHost()).isNull();
+
+		StompHeaderAccessor headers2 = this.tcpClient.getSentHeaders(1);
+		assertThat(headers2.getCommand()).isEqualTo(StompCommand.CONNECT);
+		assertThat(headers2.getSessionId()).isEqualTo("sess1");
+		assertThat(headers2.getHost()).isNull();
+	}
+
 	@Test
 	void loginAndPasscode() {
 		this.brokerRelay.setSystemLogin("syslogin");