Add regression test for writer invocation lock scope

Signed-off-by: l2yuPa <jeungwon28@gmail.com>

spring-web/src/test/java/org/springframework/http/server/reactive/ChannelSendOperatorTests.java

@@ -21,6 +21,7 @@ import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 
@@ -216,6 +217,44 @@ class ChannelSendOperatorTests {
 				.verify(Duration.ofMillis(5000));
 	}
 
+	@Test
+	void writeFunctionIsNotInvokedUnderMonitorLock() {
+		ChannelSendOperator<String> operator = new ChannelSendOperator<>(
+				Mono.just("one"),
+				publisher -> {
+					CountDownLatch acquired = new CountDownLatch(1);
+					Thread t = new Thread(() -> {
+						synchronized (publisher) {
+							acquired.countDown();
+						}
+					});
+					t.start();
+
+					try {
+						if (!acquired.await(1, TimeUnit.SECONDS)) {
+							throw new IllegalStateException("writeFunction appears to be invoked under monitor lock");
+						}
+					}
+					catch (InterruptedException ex) {
+						Thread.currentThread().interrupt();
+						throw new IllegalStateException(ex);
+					}
+					finally {
+						try {
+							t.join(1_000);
+						}
+						catch (InterruptedException ex) {
+							Thread.currentThread().interrupt();
+						}
+					}
+
+					return Mono.empty();
+				});
+
+		StepVerifier.create(operator)
+				.expectComplete()
+				.verify(Duration.ofSeconds(2));
+	}
 
 	private Mono<Void> sendOperator(Publisher<String> source){
 		return new ChannelSendOperator<>(source, writer::send);