Fix DataBufferUtils::write AsynchronousFileChannel race condition

See gh-29943

Author: poutsma

spring-core/src/main/java/org/springframework/core/io/buffer/DataBufferUtils.java

@@ -1159,23 +1159,22 @@ protected void hookOnComplete() {
 
 		@Override
 		public void completed(Integer written, Attachment attachment) {
-			this.writing.set(false);
-			attachment.iterator().close();
+			DataBuffer.ByteBufferIterator iterator = attachment.iterator();
+			iterator.close();
 
 			long pos = this.position.addAndGet(written);
 			ByteBuffer byteBuffer = attachment.byteBuffer();
-			DataBuffer.ByteBufferIterator iterator = attachment.iterator();
+
 			if (byteBuffer.hasRemaining()) {
-				this.writing.set(true);
 				this.channel.write(byteBuffer, pos, attachment, this);
 			}
 			else if (iterator.hasNext()) {
 				ByteBuffer next = iterator.next();
-				this.writing.set(true);
 				this.channel.write(next, pos, attachment, this);
 			}
 			else {
-				sinkDataBuffer(attachment.dataBuffer());
+				this.sink.next(attachment.dataBuffer());
+				this.writing.set(false);
 
 				Throwable throwable = this.error.get();
 				if (throwable != null) {
@@ -1192,15 +1191,12 @@ else if (this.completed.get()) {
 
 		@Override
 		public void failed(Throwable exc, Attachment attachment) {
-			this.writing.set(false);
 			attachment.iterator().close();
 
-			sinkDataBuffer(attachment.dataBuffer());
-			this.sink.error(exc);
-		}
+			this.sink.next(attachment.dataBuffer());
+			this.writing.set(false);
 
-		private void sinkDataBuffer(DataBuffer dataBuffer) {
-			this.sink.next(dataBuffer);
+			this.sink.error(exc);
 		}
 
 		@Override