Solving data race in benchmark function that simulates stream - go

Question

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")
}
}

Answer 1

You are sharing buf between two goroutines.

Answer 2

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")
}
}