Fix empty body writing in EncoderHttpMessageWriter

Prior to this commit, an bug introduced in SPR-16949 prevented
`Mono.empty` bodies from being written to the response.

This commit ensures that empty bodies still trigger the writing to the
response and does not hang the processing of the exchange.

Issue: SPR-17220

Author: bclozel

spring-web/src/main/java/org/springframework/http/codec/EncoderHttpMessageWriter.java

@@ -48,6 +48,7 @@
  * @author Arjen Poutsma
  * @author Sebastien Deleuze
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  * @since 5.0
  * @param <T> the type of objects in the input stream
  */
@@ -119,9 +120,10 @@ public Mono<Void> write(Publisher<? extends T> inputStream, ResolvableType eleme
 			HttpHeaders headers = message.getHeaders();
 			if (headers.getContentLength() < 0 && !headers.containsKey(HttpHeaders.TRANSFER_ENCODING)) {
 				return Mono.from(body)
-						.flatMap(dataBuffer -> {
-							headers.setContentLength(dataBuffer.readableByteCount());
-							return message.writeWith(Mono.just(dataBuffer));
+						.defaultIfEmpty(message.bufferFactory().wrap(new byte[0]))
+						.flatMap(buffer -> {
+							headers.setContentLength(buffer.readableByteCount());
+							return message.writeWith(Mono.just(buffer));
 						});
 			}
 		}

spring-web/src/test/java/org/springframework/http/codec/EncoderHttpMessageWriterTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.http.codec;
 
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
@@ -28,8 +29,12 @@
 import org.mockito.MockitoAnnotations;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
+import reactor.test.StepVerifier;
 
 import org.springframework.core.codec.Encoder;
+import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DefaultDataBufferFactory;
+import org.springframework.core.io.buffer.support.DataBufferTestUtils;
 import org.springframework.http.MediaType;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpResponse;
 import org.springframework.util.MimeType;
@@ -50,6 +55,7 @@
 /**
  * Unit tests for {@link EncoderHttpMessageWriter}.
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  */
 public class EncoderHttpMessageWriterTests {
 
@@ -75,13 +81,13 @@ public void setUp() throws Exception {
 
 
 	@Test
-	public void getWritableMediaTypes() throws Exception {
+	public void getWritableMediaTypes() {
 		HttpMessageWriter<?> writer = getWriter(MimeTypeUtils.TEXT_HTML, MimeTypeUtils.TEXT_XML);
 		assertEquals(Arrays.asList(TEXT_HTML, TEXT_XML), writer.getWritableMediaTypes());
 	}
 
 	@Test
-	public void canWrite() throws Exception {
+	public void canWrite() {
 		HttpMessageWriter<?> writer = getWriter(MimeTypeUtils.TEXT_HTML);
 		when(this.encoder.canEncode(forClass(String.class), TEXT_HTML)).thenReturn(true);
 
@@ -90,7 +96,7 @@ public void canWrite() throws Exception {
 	}
 
 	@Test
-	public void useNegotiatedMediaType() throws Exception {
+	public void useNegotiatedMediaType() {
 		HttpMessageWriter<String> writer = getWriter(MimeTypeUtils.ALL);
 		writer.write(Mono.just("body"), forClass(String.class), TEXT_PLAIN, this.response, NO_HINTS);
 
@@ -99,7 +105,7 @@ public void useNegotiatedMediaType() throws Exception {
 	}
 
 	@Test
-	public void useDefaultMediaType() throws Exception {
+	public void useDefaultMediaType() {
 		testDefaultMediaType(null);
 		testDefaultMediaType(new MediaType("text", "*"));
 		testDefaultMediaType(new MediaType("*", "*"));
@@ -108,7 +114,6 @@ public void useDefaultMediaType() throws Exception {
 
 	private void testDefaultMediaType(MediaType negotiatedMediaType) {
 
-		this.response = new MockServerHttpResponse();
 		this.mediaTypeCaptor = ArgumentCaptor.forClass(MediaType.class);
 
 		MimeType defaultContentType = MimeTypeUtils.TEXT_XML;
@@ -120,7 +125,7 @@ private void testDefaultMediaType(MediaType negotiatedMediaType) {
 	}
 
 	@Test
-	public void useDefaultMediaTypeCharset() throws Exception {
+	public void useDefaultMediaTypeCharset() {
 		HttpMessageWriter<String> writer = getWriter(TEXT_PLAIN_UTF_8, TEXT_HTML);
 		writer.write(Mono.just("body"), forClass(String.class), TEXT_HTML, response, NO_HINTS);
 
@@ -129,7 +134,7 @@ public void useDefaultMediaTypeCharset() throws Exception {
 	}
 
 	@Test
-	public void useNegotiatedMediaTypeCharset() throws Exception {
+	public void useNegotiatedMediaTypeCharset() {
 
 		MediaType negotiatedMediaType = new MediaType("text", "html", ISO_8859_1);
 
@@ -141,7 +146,7 @@ public void useNegotiatedMediaTypeCharset() throws Exception {
 	}
 
 	@Test
-	public void useHttpOutputMessageMediaType() throws Exception {
+	public void useHttpOutputMessageMediaType() {
 
 		MediaType outputMessageMediaType = MediaType.TEXT_HTML;
 		this.response.getHeaders().setContentType(outputMessageMediaType);
@@ -153,11 +158,35 @@ public void useHttpOutputMessageMediaType() throws Exception {
 		assertEquals(outputMessageMediaType, this.mediaTypeCaptor.getValue());
 	}
 
+	@Test
+	public void setContentLengthForMonoBody() {
+
+		DefaultDataBufferFactory factory = new DefaultDataBufferFactory();
+		DataBuffer buffer = factory.wrap("body".getBytes(StandardCharsets.UTF_8));
+		HttpMessageWriter<String> writer = getWriter(Flux.just(buffer), MimeTypeUtils.TEXT_PLAIN);
+
+		writer.write(Mono.just("body"), forClass(String.class), TEXT_PLAIN, this.response, NO_HINTS).block();
+
+		assertEquals(4, this.response.getHeaders().getContentLength());
+	}
+
+	@Test // SPR-17220
+	public void emptyBodyWritten() {
+		HttpMessageWriter<String> writer = getWriter(MimeTypeUtils.TEXT_PLAIN);
+		writer.write(Mono.empty(), forClass(String.class), TEXT_PLAIN, this.response, NO_HINTS).block();
+		StepVerifier.create(this.response.getBody()).expectNextCount(1).verifyComplete();
+		assertEquals(0, this.response.getHeaders().getContentLength());
+	}
+
 
 	private HttpMessageWriter<String> getWriter(MimeType... mimeTypes) {
+		return getWriter(Flux.empty(), mimeTypes);
+	}
+
+	private HttpMessageWriter<String> getWriter(Flux<DataBuffer> encodedStream, MimeType... mimeTypes) {
 		List<MimeType> typeList = Arrays.asList(mimeTypes);
 		when(this.encoder.getEncodableMimeTypes()).thenReturn(typeList);
-		when(this.encoder.encode(any(), any(), any(), this.mediaTypeCaptor.capture(), any())).thenReturn(Flux.empty());
+		when(this.encoder.encode(any(), any(), any(), this.mediaTypeCaptor.capture(), any())).thenReturn(encodedStream);
 		return new EncoderHttpMessageWriter<>(this.encoder);
 	}