Solving data race in benchmark function that simulates stream

I'm trying to write a function that basically benchmarks streamed CSV over to a HTTP endpoint.

To do this I want to generate data and POST that data.

However, gos data race detector says that there is a data race and the benchmark finishes faster than I would believe is reasonable, so I guess the HTTP request is not properly processed.

How should I structure my test code to avoid this?

Is there a way to wait until the HTTP client call has been processed?

func BenchmarkStream(b *testing.B) {
	header := "header\n"
	buf := bytes.NewBufferString(header)

	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
		for i := 0; i < b.N; i++ {
			buf.WriteString(fmt.Sprintf("%d\n", i+1))
		}
		wg.Done()
	}()                 <-- this line is mentioned in the data race detector

	w := httptest.NewRecorder()
	r, _ := http.NewRequest("POST", "/", buf)
    h := &MyHandler{}
	h.ServeHTTP(w, r)  

	wg.Wait()

	if w.Code != 200 {
		b.Errorf("test failed")
	}
}

EDIT: @Grzegorz Żurs comment made me question my approach to begin with, I refactored it with an io.Pipe:

func BenchmarkStream(b *testing.B) {
    pr, pw := io.Pipe()
	go func() {
	    pw.Write([]byte("header\n"))

		for i := 0; i < b.N; i++ {
			pw.Write([]byte(fmt.Sprintf("%d\n", i+1)))
		}
	}()

	w := httptest.NewRecorder()
	r, _ := http.NewRequest("POST", "/", pr)
    h := &MyHandler{}
	h.ServeHTTP(w, r)  
    
	if w.Code != 200 {
		b.Errorf("test failed")
	}
}

You are sharing buf between two goroutines.

You're not going to get useful benchmark results if you only invoke the handler once. Build the request body once and then invoke your handler over and over again.

buf := &bytes.Buffer{}
buf.WriteString("header\n")
buf.WriteString(strings.Repeat("1\n", 1000)
body := buf.Bytes()

b.ResetTimer()

for i := 0; i < b.N; i++ {
    w := httptest.NewRecorder()
    r, err := http.NewRequest("POST", "/", bytes.NewReader(body))
    if err != nil {
        b.Fatal(err)
    }

    h := &MyHandler{}
    h.ServeHTTP(w, r)  

    if w.Code != 200 {
        b.Errorf("test failed")
    }
}