Prevent ErrorReportValve from squashing HEAD error response

FlushHeadErrorResponseFilter.groovy

package com.example.servlet.filters

import javax.servlet.Filter
import javax.servlet.FilterChain
import javax.servlet.FilterConfig
import javax.servlet.ServletException
import javax.servlet.ServletRequest
import javax.servlet.ServletResponse

public class FlushHeadErrorResponseFilter implements Filter {

    public void destroy() {
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        chain.doFilter(request, response);
        
        if (request.method == null || 
          !request.method.equals("HEAD") ||
          response.status < 400) return
        
        // flush the buffer to commit the response
        // ErrorReportValve will not touch committed responses
        response.flushBuffer()
    }

    public void init(FilterConfig filterConfig) throws ServletException {
    }
}