Handle very early completion notification

On a Servlet container a completion notification may come at any time
even in the UNSUBSCRIBED state, i.e. before the write Publisher has
called onSubscribe.

See: gh-23553

Author: rstoyanchev

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

@@ -246,6 +246,17 @@ public <T> void onSubscribe(AbstractListenerWriteFlushProcessor<T> processor, Su
 					super.onSubscribe(processor, subscription);
 				}
 			}
+
+			@Override
+			public <T> void onComplete(AbstractListenerWriteFlushProcessor<T> processor) {
+				// This can happen on (very early) completion notification from container..
+				if (processor.changeState(this, COMPLETED)) {
+					processor.resultPublisher.publishComplete();
+				}
+				else {
+					processor.state.get().onComplete(processor);
+				}
+			}
 		},
 
 		REQUESTED {

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

@@ -322,6 +322,12 @@ public <T> void onSubscribe(AbstractListenerWriteProcessor<T> processor, Subscri
 					super.onSubscribe(processor, subscription);
 				}
 			}
+
+			@Override
+			public <T> void onComplete(AbstractListenerWriteProcessor<T> processor) {
+				// This can happen on (very early) completion notification from container..
+				processor.changeStateToComplete(this);
+			}
 		},
 
 		REQUESTED {