Empty response when using ContentCachingResponseWrapper, virtual threads and GraphQl all together

[comshepherd]

Hi there!

Recently faced an issue when using ContentCachingResponseWrapper and virtual thread all together

Symptoms:
While performing any (except graphql schema) graphql request you'll get 200 HTTP, empty response body and 'Content-Length:0' as a header

Prerequisites:

• using graphql, spring boot (3.4.4 in my case)
• having ContentCachingResponseWrapper in the Filter
• virtual thread turned on (threads.virtual.enabled: true) in your properties or yaml file

Code:

Filter

import jakarta.servlet.*;
import jakarta.servlet.annotation.WebFilter;
import jakarta.servlet.http.HttpServletResponse;
import java.io.IOException;
import org.springframework.stereotype.Component;
import org.springframework.web.util.ContentCachingResponseWrapper;

@Component
@WebFilter("/graphql")
public class GraphQlContentLengthFilter implements Filter {

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {

        var responseWrapper = new ContentCachingResponseWrapper((HttpServletResponse) response);
        chain.doFilter(request, responseWrapper);
        responseWrapper.copyBodyToResponse();
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        Filter.super.init(filterConfig);
    }

    @Override
    public void destroy() {
        Filter.super.destroy();
    }
}

application.yaml

threads:
  virtual:
    enabled: true

Reasons:
It seems that spring graphql-lib builds response body in async way and with virtual threads on it happens in 2 different threads in incorrect sequence.
First it will add a header with a filter and then write a response body. Therefore when the filter tries to add the header (copyBodyToResponse method) there is no body yet.

Fix:
For now, I don't know how to fix it, further investigation needed. In my case I've just disabled the virtual threads

[rstoyanchev]

The Servlet API is blocking by default. You start and finish request handling in one call stack. To fetch data asynchronously (e.g. through virtual threads), you need Servlet API asynchornous request handling, which means exiting the initial REQUEST dispatch, and completing later, which typically involves an additional ASYNC dispatch.

In short, your filter is not suitable for async handling because it assumes a single pass through the filter chain and that the response is fully written on the way out, which is not the case.

You need to declare your filter as being able to handle ASYNC dispatches, and in the implementation you need to differentiate between the initial REQUEST dispatch vs an ASYNC dispatch. Take a look at built-in filter implementations we have as an example, like ShallowEtagHeaderFilter, and pay attention to the shouldNotFilterAsyncDispatch method override as well as the !isAsyncStarted(request) check in doFilterInternal.

[comshepherd]

Got it, appreciate for the help!