Use updated message in HttpEntityMethodProcessor

Prior to this commit, the `HttpEntityMethodProcessor` would create a new
`ServletServerHttpRequest` input message to parse the native Servlet
request, but would not reuse it for reading the request body using the
message converters.

In gh-32471, we applied a change that updates HTTP headers accordingly
when request parameters are read. But not reusing the input message
means that we are losing this update when instantiating the resulting
`HttpEntity`.

This commit ensures that `HttpEntityMethodProcessor` uses the input
message it just created when decoding the request body.

Fixes gh-36298

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

@@ -154,7 +154,7 @@ public class HttpEntityMethodProcessor extends AbstractMessageConverterMethodPro
 					"' in method " + parameter.getMethod() + " is not parameterized");
 		}
 
-		Object body = readWithMessageConverters(webRequest, parameter, paramType);
+		Object body = readWithMessageConverters(inputMessage, parameter, paramType);
 		if (RequestEntity.class == parameter.getParameterType()) {
 			return new RequestEntity<>(body, inputMessage.getHeaders(),
 					inputMessage.getMethod(), inputMessage.getURI());

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

@@ -36,6 +36,7 @@ import org.springframework.core.MethodParameter;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
+import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.converter.ByteArrayHttpMessageConverter;
 import org.springframework.http.converter.HttpMessageConverter;
@@ -131,6 +132,26 @@ public class HttpEntityMethodProcessorTests {
 		assertThat(result.getBody()).isNull();
 	}
 
+	@Test // gh-36298
+	@SuppressWarnings("unchecked")
+	void shouldResolveEntityWithMutatedHeaders() throws Exception {
+		servletRequest.setMethod("POST");
+		servletRequest.addParameter("project", "spring%20framework");
+		servletRequest.addHeader("Content-Length", "project=spring%20framework".length());
+		servletRequest.setContentType(MediaType.APPLICATION_FORM_URLENCODED_VALUE);
+
+		List<HttpMessageConverter<?>> converters = new ArrayList<>();
+		converters.add(new ByteArrayHttpMessageConverter());
+		HttpEntityMethodProcessor processor = new HttpEntityMethodProcessor(converters);
+
+		Method method = getClass().getDeclaredMethod("resolveAsByteArray", RequestEntity.class);
+		MethodParameter requestEntity = new MethodParameter(method, 0);
+		HttpEntity<byte[]> result = (HttpEntity<byte[]>) processor.resolveArgument(requestEntity,
+				this.mavContainer, this.webRequest, this.binderFactory);
+
+		assertThat(result.getBody().length).isEqualTo(result.getHeaders().getContentLength());
+	}
+
 	@Test
 	void resolveGenericArgument() throws Exception {
 		String content = "[{\"name\" : \"Jad\"}, {\"name\" : \"Robert\"}]";
@@ -271,6 +292,9 @@ public class HttpEntityMethodProcessorTests {
 		return null;
 	}
 
+	private void resolveAsByteArray(RequestEntity<byte[]> request) {
+	}
+
 
 	@SuppressWarnings("unused")
 	private abstract static class MyParameterizedController<DTO extends Identifiable> {