Return 500 if producible attribute present

When a request is mapped through a producible condition on an
@RequestMapping, then a failure to find a converter/decoder should be
a 500 because the return type + media type pair were declared by the
controller and that should be possible to render.

Closes gh-23287

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@
 import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Set;
 
 import kotlin.reflect.KFunction;
 import kotlin.reflect.jvm.ReflectJvmMapping;
@@ -36,6 +37,8 @@
 import org.springframework.http.converter.HttpMessageNotWritableException;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.CollectionUtils;
+import org.springframework.web.reactive.HandlerMapping;
 import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.reactive.result.HandlerResultHandlerSupport;
 import org.springframework.web.server.NotAcceptableStatusException;
@@ -163,7 +166,9 @@ protected Mono<Void> writeBody(@Nullable Object body, MethodParameter bodyParame
 		}
 
 		MediaType contentType = exchange.getResponse().getHeaders().getContentType();
-		if (contentType != null && contentType.equals(bestMediaType)) {
+		boolean isPresentMediaType = (contentType != null && contentType.equals(bestMediaType));
+		Set<MediaType> producibleTypes = exchange.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
+		if (isPresentMediaType || !CollectionUtils.isEmpty(producibleTypes)) {
 			return Mono.error(new HttpMessageNotWritableException(
 					"No Encoder for [" + elementType + "] with preset Content-Type '" + contentType + "'"));
 		}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,6 +26,7 @@
 import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 
 import org.junit.jupiter.api.BeforeEach;
@@ -371,6 +372,28 @@ public void handleWithPresetContentTypeShouldFailWithServerError() {
 				.verify();
 	}
 
+	@Test // gh-23287
+	public void handleWithProducibleContentTypeShouldFailWithServerError() {
+		ResponseEntity<String> value = ResponseEntity.ok().body("<foo/>");
+		MethodParameter returnType = on(TestController.class).resolveReturnType(entity(String.class));
+		HandlerResult result = handlerResult(value, returnType);
+
+		MockServerWebExchange exchange = MockServerWebExchange.from(get("/path"));
+		Set<MediaType> mediaTypes = Collections.singleton(MediaType.APPLICATION_XML);
+				exchange.getAttributes().put(PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE, mediaTypes);
+
+		ResponseEntityResultHandler resultHandler = new ResponseEntityResultHandler(
+				Collections.singletonList(new EncoderHttpMessageWriter<>(CharSequenceEncoder.textPlainOnly())),
+				new RequestedContentTypeResolverBuilder().build()
+		);
+
+		StepVerifier.create(resultHandler.handleResult(exchange, result))
+				.consumeErrorWith(ex -> assertThat(ex)
+						.isInstanceOf(HttpMessageNotWritableException.class)
+						.hasMessageContaining("with preset Content-Type"))
+				.verify();
+	}
+
 
 	private void testHandle(Object returnValue, MethodParameter returnType) {
 		MockServerWebExchange exchange = MockServerWebExchange.from(get("/path"));

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -309,7 +309,11 @@ else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
 		}
 
 		if (body != null) {
-			if (isContentTypePreset) {
+			Set<MediaType> producibleMediaTypes =
+					(Set<MediaType>) inputMessage.getServletRequest()
+							.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
+
+			if (isContentTypePreset || !CollectionUtils.isEmpty(producibleMediaTypes)) {
 				throw new HttpMessageNotWritableException(
 						"No converter for [" + valueType + "] with preset Content-Type '" + contentType + "'");
 			}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,6 +28,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.Date;
+import java.util.Set;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -323,7 +324,24 @@ public void shouldFailWithServerErrorIfContentTypeFromResponseEntity() {
 				.contentType(MediaType.APPLICATION_XML)
 				.body("<foo/>");
 
-		given(stringHttpMessageConverter.canWrite(String.class, null)).willReturn(true);
+		given(stringHttpMessageConverter.canWrite(String.class, TEXT_PLAIN)).willReturn(true);
+		given(stringHttpMessageConverter.getSupportedMediaTypes()).willReturn(Collections.singletonList(TEXT_PLAIN));
+
+		assertThatThrownBy(() ->
+				processor.handleReturnValue(
+						returnValue, returnTypeResponseEntity, mavContainer, webRequest))
+				.isInstanceOf(HttpMessageNotWritableException.class)
+				.hasMessageContaining("with preset Content-Type");
+	}
+
+	@Test // gh-23287
+	public void shouldFailWithServerErrorIfContentTypeFromProducibleAttribute() {
+		Set<MediaType> mediaTypes = Collections.singleton(MediaType.APPLICATION_XML);
+		servletRequest.setAttribute(PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE, mediaTypes);
+
+		ResponseEntity<String> returnValue = ResponseEntity.ok().body("<foo/>");
+
+		given(stringHttpMessageConverter.canWrite(String.class, TEXT_PLAIN)).willReturn(true);
 		given(stringHttpMessageConverter.getSupportedMediaTypes()).willReturn(Collections.singletonList(TEXT_PLAIN));
 
 		assertThatThrownBy(() ->