Spring MockMvc's MVC_RESULT_ATTRIBUTE lost with HandlerMappingIntrospector and RouterFunctions in use

Related to https://github.com/spring-projects/spring-security-samples/issues/9

A contributor shared the following sample application: https://github.com/hantsy/spring-webmvc-auth0-sample

The tests result in a NullPointerException because the MockMvc.MVC_RESULT_ATTRIBUTE is missing.

HandlerExecutionChain chain = super.getHandler(request);
if (chain != null) {
    DefaultMvcResult mvcResult = getMvcResult(request); // returns null
    mvcResult.setHandler(chain.getHandler());
    mvcResult.setInterceptors(chain.getInterceptors());
}
return chain;

It gets removed due to the following arrangement:

Spring Security's CorsFilter by default uses HandlerMappingIntrospector
HandlerMappingIntrospector uses RequestAttributeChangeIgnoringWrapper which ignores all but PATH_ATTRIBUTE
RequestPredicates#restoreAttributes attempts to restore the attributes to a previous state by clearing the attribute set and then re-adding each attribute one by one

Before CorsFilter runs, MVC_RESULT_ATTRIBUTE is present in the request. When RequestPredicates#restoreAttributes is run, it removes all attributes. Then, when it tries to add the original set back in, RequestAttributeChangeIgnoringWrapper only adds PATH_ATTRIBUTE back in.

For the specific sample, the tests can be repaired by removing the CorsFilter or by exposing a custom CorsConfigurationSource bean since either of those will prevent RequestAttributeChangeIgnoringWrapper from wrapping the request.

I was also able to fix the tests by adding the following to RequestAttributeChangeIgnoringWrapper:

@Override
public void removeAttribute(String name) {
    if (name.equals(ServletRequestPathUtils.PATH_ATTRIBUTE) || name.equals(UrlPathHelper.PATH_ATTRIBUTE)) {
        super.removeAttribute(name);
    }
}

At least in this isolated case, it seems reasonable that if an attribute cannot be set, it also should not be able to be removed.

Comment From: sbrannen