Resume Undertow writes only when data is available

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

@@ -150,6 +150,18 @@ public abstract class AbstractListenerWriteProcessor<T> implements Processor<T,
 	 */
 	protected abstract boolean write(T data) throws IOException;
 
+	/**
+	 * Suspend writing. Defaults to no-op.
+	 */
+	protected void suspendWriting() {
+	}
+
+	/**
+	 * Invoked when writing is complete. Defaults to no-op.
+	 */
+	protected void writingComplete() {
+	}
+
 
 	private boolean changeState(State oldState, State newState) {
 		return this.state.compareAndSet(oldState, newState);
@@ -223,6 +235,7 @@ public abstract class AbstractListenerWriteProcessor<T> implements Processor<T,
 			@Override
 			public <T> void onComplete(AbstractListenerWriteProcessor<T> processor) {
 				if (processor.changeState(this, COMPLETED)) {
+					processor.writingComplete();
 					processor.resultPublisher.publishComplete();
 				}
 			}
@@ -248,10 +261,12 @@ public abstract class AbstractListenerWriteProcessor<T> implements Processor<T,
 							processor.releaseData();
 							if (!processor.subscriberCompleted) {
 								processor.changeState(WRITING, REQUESTED);
+								processor.suspendWriting();
 								processor.subscription.request(1);
 							}
 							else {
 								processor.changeState(WRITING, COMPLETED);
+								processor.writingComplete();
 								processor.resultPublisher.publishComplete();
 							}
 						}
@@ -311,6 +326,7 @@ public abstract class AbstractListenerWriteProcessor<T> implements Processor<T,
 
 		public <T> void onError(AbstractListenerWriteProcessor<T> processor, Throwable ex) {
 			if (processor.changeState(this, COMPLETED)) {
+				processor.writingComplete();
 				processor.resultPublisher.publishError(ex);
 			}
 		}

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

@@ -306,6 +306,11 @@ public class ServletServerHttpResponse extends AbstractListenerServerHttpRespons
 				return false;
 			}
 		}
+
+		@Override
+		protected void writingComplete() {
+			bodyProcessor = null;
+		}
 	}
 
 }

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

@@ -138,9 +138,7 @@ public class UndertowServerHttpResponse extends AbstractListenerServerHttpRespon
 		if (this.responseChannel == null) {
 			this.responseChannel = this.exchange.getResponseChannel();
 		}
-		ResponseBodyProcessor bodyProcessor = new ResponseBodyProcessor( this.responseChannel);
-		bodyProcessor.registerListener();
-		return bodyProcessor;
+		return new ResponseBodyProcessor(this.responseChannel);
 	}
 
 
@@ -153,16 +151,18 @@ public class UndertowServerHttpResponse extends AbstractListenerServerHttpRespon
 		public ResponseBodyProcessor(StreamSinkChannel channel) {
 			Assert.notNull(channel, "StreamSinkChannel must not be null");
 			this.channel = channel;
-		}
-
-		public void registerListener() {
 			this.channel.getWriteSetter().set(c -> onWritePossible());
-			this.channel.resumeWrites();
+			this.channel.suspendWrites();
 		}
 
 		@Override
 		protected boolean isWritePossible() {
-			return false;
+			if (this.channel.isWriteResumed()) {
+				return true;
+			} else {
+				this.channel.resumeWrites();
+				return false;
+			}
 		}
 
 		@Override
@@ -214,6 +214,17 @@ public class UndertowServerHttpResponse extends AbstractListenerServerHttpRespon
 		protected boolean isDataEmpty(DataBuffer dataBuffer) {
 			return (dataBuffer.readableByteCount() == 0);
 		}
+
+		@Override
+		protected void suspendWriting() {
+			this.channel.suspendWrites();
+		}
+
+		@Override
+		protected void writingComplete() {
+			this.channel.getWriteSetter().set(null);
+			this.channel.resumeWrites();
+		}
 	}