Avoid thread pinning in SseEmitter write operations

This commit changes the `synchronized` usage into a `ReentrantLock`, in
order to guard write operations with a construct that does not pin
virtual threads to the current platform thread on JDK21.

Closes gh-30996

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/SseEmitter.java

@@ -21,6 +21,8 @@ import java.nio.charset.StandardCharsets;
 import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.Set;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
@@ -36,12 +38,18 @@ import org.springframework.util.StringUtils;
  * @author Rossen Stoyanchev
  * @author Juergen Hoeller
  * @author Sam Brannen
+ * @author Brian Clozel
  * @since 4.2
  */
 public class SseEmitter extends ResponseBodyEmitter {
 
 	private static final MediaType TEXT_PLAIN = new MediaType("text", "plain", StandardCharsets.UTF_8);
 
+	/**
+	 * Guards access to write operations on the response.
+	 */
+	private final Lock writeLock = new ReentrantLock();
+
 	/**
 	 * Create a new SseEmitter instance.
 	 */
@@ -122,9 +130,13 @@ public class SseEmitter extends ResponseBodyEmitter {
 	 */
 	public void send(SseEventBuilder builder) throws IOException {
 		Set<DataWithMediaType> dataToSend = builder.build();
-		synchronized (this) {
+		this.writeLock.lock();
+		try {
 			super.send(dataToSend);
 		}
+		finally {
+			this.writeLock.unlock();
+		}
 	}
 
 	@Override