Fall back to all media types if encountering invalid Accept header

A warn log message is printed, and if log level is set to debug, the
exception stacktrace is logged, too.

Closes gh-37455

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

@@ -29,6 +29,7 @@
 import org.springframework.core.io.Resource;
 import org.springframework.core.log.LogMessage;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.InvalidMediaTypeException;
 import org.springframework.http.MediaType;
 import org.springframework.util.StringUtils;
 import org.springframework.web.servlet.handler.AbstractUrlHandlerMapping;
@@ -41,6 +42,7 @@
  *
  * @author Andy Wilkinson
  * @author Bruce Brouwer
+ * @author Moritz Halbritter
  * @see WelcomePageNotAcceptableHandlerMapping
  */
 final class WelcomePageHandlerMapping extends AbstractUrlHandlerMapping {
@@ -80,7 +82,13 @@ private boolean isHtmlTextAccepted(HttpServletRequest request) {
 	private List<MediaType> getAcceptedMediaTypes(HttpServletRequest request) {
 		String acceptHeader = request.getHeader(HttpHeaders.ACCEPT);
 		if (StringUtils.hasText(acceptHeader)) {
-			return MediaType.parseMediaTypes(acceptHeader);
+			try {
+				return MediaType.parseMediaTypes(acceptHeader);
+			}
+			catch (InvalidMediaTypeException ex) {
+				logger.warn("Received invalid Accept header. Assuming all media types are accepted",
+						logger.isDebugEnabled() ? ex : null);
+			}
 		}
 		return MEDIA_TYPES_ALL;
 	}

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

@@ -23,6 +23,7 @@
 import javax.servlet.http.HttpServletResponse;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
 
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.factory.annotation.Value;
@@ -31,6 +32,8 @@
 import org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider;
 import org.springframework.boot.autoconfigure.template.TemplateAvailabilityProviders;
 import org.springframework.boot.test.context.runner.WebApplicationContextRunner;
+import org.springframework.boot.test.system.CapturedOutput;
+import org.springframework.boot.test.system.OutputCaptureExtension;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -54,7 +57,9 @@
  * Tests for {@link WelcomePageHandlerMapping}.
  *
  * @author Andy Wilkinson
+ * @author Moritz Halbritter
  */
+@ExtendWith(OutputCaptureExtension.class)
 class WelcomePageHandlerMappingTests {
 
 	private final WebApplicationContextRunner contextRunner = new WebApplicationContextRunner()
@@ -164,6 +169,17 @@ void prefersAStaticResourceToATemplate() {
 			});
 	}
 
+	@Test
+	void logsInvalidAcceptHeader(CapturedOutput output) {
+		this.contextRunner.withUserConfiguration(TemplateConfiguration.class).run((context) -> {
+			MockMvc mockMvc = MockMvcBuilders.webAppContextSetup(context).build();
+			mockMvc.perform(get("/").accept("*/*q=0.8"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("index template"));
+		});
+		assertThat(output).contains("Received invalid Accept header. Assuming all media types are accepted");
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class HandlerMappingConfiguration {