Introduce OutputStream BodyInserter

This commit introduces a BodyInserter that inssert any bytes written to
an output stream to the body of an output message.

Closes gh-31184

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

@@ -18,6 +18,7 @@ package org.springframework.core.io.buffer;
 
 import java.io.IOException;
 import java.io.OutputStream;
+import java.io.OutputStreamWriter;
 import java.net.URI;
 import java.nio.ByteBuffer;
 import java.nio.channels.AsynchronousFileChannel;
@@ -34,11 +35,13 @@ import java.nio.file.StandardOpenOption;
 import java.time.Duration;
 import java.util.List;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executors;
 
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.PooledByteBufAllocator;
 import org.junit.jupiter.api.Test;
 import org.mockito.stubbing.Answer;
+import org.reactivestreams.Publisher;
 import org.reactivestreams.Subscription;
 import reactor.core.publisher.BaseSubscriber;
 import reactor.core.publisher.Flux;
@@ -52,6 +55,8 @@ import org.springframework.core.io.Resource;
 import org.springframework.core.testfixture.io.buffer.AbstractDataBufferAllocatingTests;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIOException;
+import static org.assertj.core.api.Assertions.fail;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyLong;
 import static org.mockito.BDDMockito.given;
@@ -543,6 +548,147 @@ class DataBufferUtilsTests extends AbstractDataBufferAllocatingTests {
 		assertThat(written).contains("foobar");
 	}
 
+	@ParameterizedDataBufferAllocatingTest
+	void outputStreamPublisher(DataBufferFactory bufferFactory) {
+		super.bufferFactory = bufferFactory;
+
+		byte[] foo = "foo".getBytes(StandardCharsets.UTF_8);
+		byte[] bar = "bar".getBytes(StandardCharsets.UTF_8);
+		byte[] baz = "baz".getBytes(StandardCharsets.UTF_8);
+
+		Publisher<DataBuffer> publisher = DataBufferUtils.outputStreamPublisher(outputStream -> {
+			try {
+				outputStream.write(foo);
+				outputStream.write(bar);
+				outputStream.write(baz);
+			}
+			catch (IOException ex) {
+				fail(ex.getMessage(), ex);
+			}
+		}, super.bufferFactory, Executors.newSingleThreadExecutor());
+
+		StepVerifier.create(publisher)
+				.consumeNextWith(stringConsumer("foobarbaz"))
+				.verifyComplete();
+	}
+
+	@ParameterizedDataBufferAllocatingTest
+	void outputStreamPublisherFlush(DataBufferFactory bufferFactory) {
+		super.bufferFactory = bufferFactory;
+
+		byte[] foo = "foo".getBytes(StandardCharsets.UTF_8);
+		byte[] bar = "bar".getBytes(StandardCharsets.UTF_8);
+		byte[] baz = "baz".getBytes(StandardCharsets.UTF_8);
+
+		Publisher<DataBuffer> publisher = DataBufferUtils.outputStreamPublisher(outputStream -> {
+			try {
+				outputStream.write(foo);
+				outputStream.flush();
+				outputStream.write(bar);
+				outputStream.flush();
+				outputStream.write(baz);
+				outputStream.flush();
+			}
+			catch (IOException ex) {
+				fail(ex.getMessage(), ex);
+			}
+		}, super.bufferFactory, Executors.newSingleThreadExecutor());
+
+		StepVerifier.create(publisher)
+				.consumeNextWith(stringConsumer("foo"))
+				.consumeNextWith(stringConsumer("bar"))
+				.consumeNextWith(stringConsumer("baz"))
+				.verifyComplete();
+	}
+
+	@ParameterizedDataBufferAllocatingTest
+	void outputStreamPublisherChunkSize(DataBufferFactory bufferFactory) {
+		super.bufferFactory = bufferFactory;
+
+		byte[] foo = "foo".getBytes(StandardCharsets.UTF_8);
+		byte[] bar = "bar".getBytes(StandardCharsets.UTF_8);
+		byte[] baz = "baz".getBytes(StandardCharsets.UTF_8);
+
+		Publisher<DataBuffer> publisher = DataBufferUtils.outputStreamPublisher(outputStream -> {
+			try {
+				outputStream.write(foo);
+				outputStream.write(bar);
+				outputStream.write(baz);
+			}
+			catch (IOException ex) {
+				fail(ex.getMessage(), ex);
+			}
+		}, super.bufferFactory, Executors.newSingleThreadExecutor(), 3);
+
+		StepVerifier.create(publisher)
+				.consumeNextWith(stringConsumer("foo"))
+				.consumeNextWith(stringConsumer("bar"))
+				.consumeNextWith(stringConsumer("baz"))
+				.verifyComplete();
+	}
+
+	@ParameterizedDataBufferAllocatingTest
+	void outputStreamPublisherCancel(DataBufferFactory bufferFactory) throws InterruptedException {
+		super.bufferFactory = bufferFactory;
+
+		byte[] foo = "foo".getBytes(StandardCharsets.UTF_8);
+		byte[] bar = "bar".getBytes(StandardCharsets.UTF_8);
+
+		CountDownLatch latch = new CountDownLatch(1);
+
+		Publisher<DataBuffer> publisher = DataBufferUtils.outputStreamPublisher(outputStream -> {
+			try {
+				assertThatIOException()
+						.isThrownBy(() -> {
+							outputStream.write(foo);
+							outputStream.flush();
+							outputStream.write(bar);
+							outputStream.flush();
+						})
+						.withMessage("Subscription has been terminated");
+			}
+			finally {
+				latch.countDown();
+			}
+		}, super.bufferFactory, Executors.newSingleThreadExecutor());
+
+		StepVerifier.create(publisher, 1)
+				.consumeNextWith(stringConsumer("foo"))
+				.thenCancel()
+				.verify();
+
+		latch.await();
+	}
+
+	@ParameterizedDataBufferAllocatingTest
+	void outputStreamPublisherClosed(DataBufferFactory bufferFactory) throws InterruptedException {
+		super.bufferFactory = bufferFactory;
+
+		CountDownLatch latch = new CountDownLatch(1);
+
+		Publisher<DataBuffer> publisher = DataBufferUtils.outputStreamPublisher(outputStream -> {
+			try {
+				OutputStreamWriter writer = new OutputStreamWriter(outputStream, StandardCharsets.UTF_8);
+				writer.write("foo");
+				writer.close();
+				assertThatIOException().isThrownBy(() -> writer.write("bar"))
+						.withMessage("Stream closed");
+			}
+			catch (IOException ex) {
+				fail(ex.getMessage(), ex);
+			}
+			finally {
+				latch.countDown();
+			}
+		}, super.bufferFactory, Executors.newSingleThreadExecutor());
+
+		StepVerifier.create(publisher)
+				.consumeNextWith(stringConsumer("foo"))
+				.verifyComplete();
+
+		latch.await();
+	}
+
 	@ParameterizedDataBufferAllocatingTest
 	void readAndWriteByteChannel(DataBufferFactory bufferFactory) throws Exception {
 		super.bufferFactory = bufferFactory;