Fix multi-buffer data loss in DataBufferUtils.write

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

@@ -1138,9 +1138,11 @@ protected void hookOnSubscribe(Subscription subscription) {
 		protected void hookOnNext(DataBuffer dataBuffer) {
 			try {
 				try (DataBuffer.ByteBufferIterator iterator = dataBuffer.readableByteBuffers()) {
-					ByteBuffer byteBuffer = iterator.next();
-					while (byteBuffer.hasRemaining()) {
-						this.channel.write(byteBuffer);
+					while (iterator.hasNext()) {
+						ByteBuffer byteBuffer = iterator.next();
+						while (byteBuffer.hasRemaining()) {
+							this.channel.write(byteBuffer);
+						}
 					}
 				}
 				this.sink.next(dataBuffer);

spring-core/src/test/java/org/springframework/core/io/buffer/DataBufferUtilsTests.java

@@ -338,6 +338,27 @@ void writeWritableByteChannel(DataBufferFactory bufferFactory) throws Exception
 		channel.close();
 	}
 
+	@ParameterizedDataBufferAllocatingTest
+	void writeWritableByteChannelWithJoinedBuffer(DataBufferFactory bufferFactory) throws Exception {
+		super.bufferFactory = bufferFactory;
+
+		DataBuffer foo = stringBuffer("foo");
+		DataBuffer bar = stringBuffer("bar");
+		DataBuffer joined = bufferFactory.join(List.of(foo, bar));
+
+		WritableByteChannel channel = Files.newByteChannel(tempFile, StandardOpenOption.WRITE);
+
+		Flux<DataBuffer> writeResult = DataBufferUtils.write(Flux.just(joined), channel);
+		StepVerifier.create(writeResult)
+				.consumeNextWith(stringConsumer("foobar"))
+				.verifyComplete();
+
+		String result = String.join("", Files.readAllLines(tempFile));
+
+		assertThat(result).isEqualTo("foobar");
+		channel.close();
+	}
+
 	@ParameterizedDataBufferAllocatingTest
 	void writeWritableByteChannelErrorInFlux(DataBufferFactory bufferFactory) throws Exception {
 		super.bufferFactory = bufferFactory;