Modifying dynamically MaxIdleConnsPerHost on http.Transport creates a data race

maxidle_test.go

package main

import (
	"fmt"
	"io/ioutil"
	"math/rand"
	"net/http"
	"net/http/httptest"
	"sync"
	"testing"
	"time"
)

var server = httptest.NewServer(mux)
var mux = http.NewServeMux()
var wg sync.WaitGroup

var t *http.Transport
var c *http.Client

func init() {
	mux.HandleFunc("/", HelloServer)

	t = &http.Transport{}
	c = &http.Client{Transport: t}
}

func HelloServer(w http.ResponseWriter, req *http.Request) {
	time.Sleep(1 * time.Second)
	fmt.Fprintf(w, ".")
}

func TestMain(t *testing.T) {

	for i := 0; i < 100; i++ {
		wg.Add(1)
		go client()
	}

	wg.Wait()

}

func client() {

	defer wg.Done()

	//t.MaxIdleConnsMutex.Lock()
	t.MaxIdleConnsPerHost = rand.Intn(10) + 1
	//t.MaxIdleConnsMutex.Unlock()

	r, err := c.Get(server.URL)
	if err != nil {
		fmt.Println(err.Error())
	}

	defer r.Body.Close()
	body, _ := ioutil.ReadAll(r.Body)

	fmt.Print(string(body))
}

Running this gist with go test -race creates a data race.

$ go test -race

==================
WARNING: DATA RACE
Write by goroutine 9:
  github.com/go-loco.client()
      /Users/hernan/apps/go/src/github.com/go-loco/s_test.go:49 +0xa6

Previous write by goroutine 8:
  github.com/go-loco.client()
      /Users/hernan/apps/go/src/github.com/go-loco/s_test.go:49 +0xa6

Goroutine 9 (running) created at:
  github.com/go-loco.TestMain()
      /Users/hernan/apps/go/src/github.com/go-loco/s_test.go:37 +0x62
  testing.tRunner()
      /Users/hernan/apps/go/go-src/go/src/testing/testing.go:547 +0xbf

Goroutine 8 (running) created at:
  github.com/go-loco.TestMain()
      /Users/hernan/apps/go/src/github.com/go-loco/s_test.go:37 +0x62
  testing.tRunner()
      /Users/hernan/apps/go/go-src/go/src/testing/testing.go:547 +0xbf
==================
....................................................................................................PASS
Found 1 data race(s)
exit status 66
FAIL    github.com/go-loco  2.157s