config.rb

threads 1, 48
workers 1
persistent_timeout 9999999
preload_app!

config.ru

require 'rack'
require 'rack-ssl-enforcer'

use Rack::SslEnforcer, except: '/heartbeat'

run Proc.new { |env|
  path = Rack::Request.new(env).path
  case path
  when '/api'
    ['200', {'Content-Type' => 'text/plain'}, ['foo']]
  when '/heartbeat'
    ['200', {'Content-Type' => 'text/plain'}, ['ok']]
  else
    ['404', {'Content-Type' => 'text/plain'}, ['not found']]
  end
}

Gemfile

source 'https://rubygems.org'
gem 'puma', '~> 3.10.0'
gem 'rack-ssl-enforcer', path: '../rack-ssl-enforcer'
gem 'rack'

Gemfile.lock

PATH
  remote: ../rack-ssl-enforcer
  specs:
    rack-ssl-enforcer (0.2.9)

GEM
  remote: https://rubygems.org/
  specs:
    puma (3.10.0)
    rack (2.0.7)

PLATFORMS
  ruby

DEPENDENCIES
  puma (~> 3.10.0)
  rack
  rack-ssl-enforcer!

BUNDLED WITH
   1.13.6

main.go

package main

import (
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"os"
)

func main() {
	url := os.Args[1]
	launch(1, url+"/heartbeat", "ok", 200)
	launch(10, url+"/api", "foo", 200)
	<-make(chan struct{})
}

func launch(n int, url, expectedResult string, expectedStatus int) {
	for i := 0; i < n; i++ {
		go func() {
			log.Fatal(request(url, expectedResult, expectedStatus))
		}()
	}
}

var httpClient = http.Client{
	CheckRedirect: func(*http.Request, []*http.Request) error {
		return http.ErrUseLastResponse
	},
}

func request(url, expectedResult string, expectedStatus int) error {
	fmt.Println("start", url)
	for {
		req, _ := http.NewRequest("GET", url, nil)
		if expectedResult == "foo" {
			req.Header.Set("X-Forwarded-Proto", "https")
		}
		res, err := httpClient.Do(req)
		if err != nil {
			return err
		}
		body, err := ioutil.ReadAll(res.Body)
		res.Body.Close()
		if err != nil {
			return err
		}
		if res.StatusCode != expectedStatus {
			return fmt.Errorf("%s unexpected status %d, wanted %d", url, res.StatusCode, expectedStatus)
		}
		if res.StatusCode == 301 {
			if res.Header.Get("Location") != expectedResult {
				return fmt.Errorf("%s unexpected location %q, wanted %q", url, res.Header.Get("Location"), expectedResult)
			}
		} else {
			if string(body) != expectedResult {
				return fmt.Errorf("%s unexpected body %q, wanted %q", url, string(body), expectedResult)
			}
		}
	}
}