Double-checked lock in ChannelSendOperator#request

Closes gh-31865

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/reactive/ChannelSendOperator.java

@@ -281,6 +281,10 @@ class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				return;
 			}
 			synchronized (this) {
+				if (this.state == State.READY_TO_WRITE) {
+					s.request(n);
+					return;
+				}
 				if (this.writeSubscriber != null) {
 					if (this.state == State.EMITTING_CACHED_SIGNALS) {
 						this.demandBeforeReadyToWrite = n;

spring-web/src/main/java/org/springframework/http/server/reactive/ChannelSendOperator.java

@@ -273,6 +273,10 @@ public class ChannelSendOperator<T> extends Mono<Void> implements Scannable {
 				return;
 			}
 			synchronized (this) {
+				if (this.state == State.READY_TO_WRITE) {
+					s.request(n);
+					return;
+				}
 				if (this.writeSubscriber != null) {
 					if (this.state == State.EMITTING_CACHED_SIGNALS) {
 						this.demandBeforeReadyToWrite = n;