Avoid 406 Not Acceptable for error pages

Prior to this commit, the `ErrorController` would override the original
error response status if the error map cannot be written due to content
negotiation with the HTTP client. In that case, the error handling
infrastructure returns a `406 Not Acceptable` response.

This commit improves the `ErrorController` so that
`HttpMediaTypeNotAcceptableException` instances thrown by that
controller are not returned as is but instead we write the error
response with an empty body and the original HTTP error status.

Fixes gh-19522

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/error/BasicErrorController.java

@@ -32,6 +32,8 @@ import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.stereotype.Controller;
 import org.springframework.util.Assert;
+import org.springframework.web.HttpMediaTypeNotAcceptableException;
+import org.springframework.web.bind.annotation.ExceptionHandler;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.servlet.ModelAndView;
 
@@ -102,6 +104,12 @@ public class BasicErrorController extends AbstractErrorController {
 		return new ResponseEntity<>(body, status);
 	}
 
+	@ExceptionHandler(HttpMediaTypeNotAcceptableException.class)
+	public ResponseEntity<String> mediaTypeNotAcceptable(HttpServletRequest request) {
+		HttpStatus status = getStatus(request);
+		return ResponseEntity.status(status).build();
+	}
+
 	/**
 	 * Determine if the stacktrace attribute should be included.
 	 * @param request the source request

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/error/BasicErrorControllerIntegrationTests.java

@@ -200,6 +200,17 @@ class BasicErrorControllerIntegrationTests {
 		assertThat(resp).contains("We are out of storage");
 	}
 
+	@Test
+	void testIncompatibleMediaType() {
+		load();
+		RequestEntity<?> request = RequestEntity.get(URI.create(createUrl("/incompatibleType")))
+				.accept(MediaType.TEXT_PLAIN).build();
+		ResponseEntity<String> entity = new TestRestTemplate().exchange(request, String.class);
+		assertThat(entity.getStatusCode()).isEqualTo(HttpStatus.BAD_REQUEST);
+		assertThat(entity.getHeaders().getContentType()).isNull();
+		assertThat(entity.getBody()).isNull();
+	}
+
 	private void assertErrorAttributes(Map<?, ?> content, String status, String error, Class<?> exception,
 			String message, String path) {
 		assertThat(content.get("status")).as("Wrong status").isEqualTo(status);
@@ -298,6 +309,11 @@ class BasicErrorControllerIntegrationTests {
 				throw new InsufficientStorageException();
 			}
 
+			@RequestMapping(path = "/incompatibleType", produces = "text/plain")
+			String incompatibleType() {
+				throw new ExpectedException();
+			}
+
 			@ResponseStatus(value = HttpStatus.BAD_REQUEST, reason = "Expected!")
 			@SuppressWarnings("serial")
 			static class ExpectedException extends RuntimeException {