Return 500 (not 406) if content-type was preset

If content-type is preset in the returned ResponseEntity, then any
failure to find a matching converter is a server error.

Closes gh-23205

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/AbstractMessageWriterResultHandler.java

@@ -161,10 +161,17 @@ protected Mono<Void> writeBody(@Nullable Object body, MethodParameter bodyParame
 			}
 		}
 
+		MediaType contentType = exchange.getResponse().getHeaders().getContentType();
+		if (contentType != null && contentType.equals(bestMediaType)) {
+			return Mono.error(new IllegalStateException(
+					"No Encoder for [" + elementType + "] with preset Content-Type '" + contentType + "'"));
+		}
+
 		List<MediaType> mediaTypes = getMediaTypesFor(elementType);
 		if (bestMediaType == null && mediaTypes.isEmpty()) {
 			return Mono.error(new IllegalStateException("No HttpMessageWriter for " + elementType));
 		}
+
 		return Mono.error(new NotAcceptableStatusException(mediaTypes));
 	}
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ResponseEntityResultHandlerTests.java

@@ -337,7 +337,7 @@ public void handleMonoWithWildcardBodyTypeAndNullBody() throws Exception {
 	}
 
 	@Test // SPR-17082
-	public void handleResponseEntityWithExistingResponseHeaders() throws Exception {
+	public void handleWithPresetContentType() {
 		ResponseEntity<Void> value = ResponseEntity.ok().contentType(MediaType.APPLICATION_JSON).build();
 		MethodParameter returnType = on(TestController.class).resolveReturnType(entity(Void.class));
 		HandlerResult result = handlerResult(value, returnType);
@@ -351,6 +351,24 @@ public void handleResponseEntityWithExistingResponseHeaders() throws Exception {
 		assertResponseBodyIsEmpty(exchange);
 	}
 
+	@Test // gh-23205
+	public void handleWithPresetContentTypeShouldFailWithServerError() {
+		ResponseEntity<String> value = ResponseEntity.ok().contentType(MediaType.APPLICATION_XML).body("<foo/>");
+		MethodParameter returnType = on(TestController.class).resolveReturnType(entity(String.class));
+		HandlerResult result = handlerResult(value, returnType);
+
+		MockServerWebExchange exchange = MockServerWebExchange.from(get("/path"));
+		ResponseEntityResultHandler resultHandler = new ResponseEntityResultHandler(
+				Collections.singletonList(new EncoderHttpMessageWriter<>(CharSequenceEncoder.textPlainOnly())),
+				new RequestedContentTypeResolverBuilder().build()
+		);
+
+		StepVerifier.create(resultHandler.handleResult(exchange, result))
+				.consumeErrorWith(ex -> assertThat(ex)
+						.isInstanceOf(IllegalStateException.class)
+						.hasMessageContaining("with preset Content-Type"))
+				.verify();
+	}
 
 
 	private void testHandle(Object returnValue, MethodParameter returnType) {

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/AbstractMessageConverterMethodProcessor.java

@@ -218,7 +218,8 @@ protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter
 
 		MediaType selectedMediaType = null;
 		MediaType contentType = outputMessage.getHeaders().getContentType();
-		if (contentType != null && contentType.isConcrete()) {
+		boolean isContentTypePreset = contentType != null && contentType.isConcrete();
+		if (isContentTypePreset) {
 			if (logger.isDebugEnabled()) {
 				logger.debug("Found 'Content-Type:" + contentType + "' in response");
 			}
@@ -304,6 +305,10 @@ else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
 		}
 
 		if (body != null) {
+			if (isContentTypePreset) {
+				throw new IllegalStateException(
+						"No converter for [" + valueType + "] with preset Content-Type '" + contentType + "'");
+			}
 			throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
 		}
 	}

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/HttpEntityMethodProcessorMockTests.java

@@ -59,6 +59,7 @@
 import static java.time.format.DateTimeFormatter.RFC_1123_DATE_TIME;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyCollection;
 import static org.mockito.ArgumentMatchers.argThat;
@@ -315,6 +316,21 @@ public void shouldFailHandlingWhenContentTypeNotSupported() throws Exception {
 				processor.handleReturnValue(returnValue, returnTypeResponseEntity, mavContainer, webRequest));
 	}
 
+	@Test // gh-23205
+	public void shouldFailWithServerErrorIfContentTypeFromResponseEntity() {
+		ResponseEntity<String> returnValue = ResponseEntity.ok()
+				.contentType(MediaType.APPLICATION_XML)
+				.body("<foo/>");
+
+		given(stringHttpMessageConverter.canWrite(String.class, null)).willReturn(true);
+		given(stringHttpMessageConverter.getSupportedMediaTypes()).willReturn(Collections.singletonList(TEXT_PLAIN));
+
+		assertThatIllegalStateException()
+				.isThrownBy(() ->
+						processor.handleReturnValue(returnValue, returnTypeResponseEntity, mavContainer, webRequest))
+				.withMessageContaining("with preset Content-Type");
+	}
+
 	@Test
 	public void shouldFailHandlingWhenConverterCannotWrite() throws Exception {
 		String body = "Foo";