ioutil.ReadAll leads to goroutine leak

Question

Why do I have more than one goroutine before termination, even though I closed resp.body, while I only used blocking calls? If I do not consume resp.Body it terminates with only one goroutine.

package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
    "runtime"
    "time"
)

func fetch() {
    client := http.Client{Timeout: time.Second * 10}
    url := "http://example.com"
    req, err := http.NewRequest("POST", url, nil)
    resp, err := client.Do(req)
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close()
    _, err = ioutil.ReadAll(resp.Body)
    if err != nil {
        panic(err)
    }
}

func main() {
    fmt.Println("#Goroutines:", runtime.NumGoroutine())
    fetch()
    // runtime.GC()
    time.Sleep(time.Second * 5)
    fmt.Println("#Goroutines:", runtime.NumGoroutine())
}

Outputs:

#Goroutines: 1
#Goroutines: 3

Answer 1

The default http transport maintains a connection pool.

DefaultTransport is the default implementation of Transport and is used by DefaultClient. It establishes network connections as needed and caches them for reuse by subsequent calls.

Each connection is managed by at least one goroutine. This is not a leak though, you are just impatient. If you wait long enough, you will see that the connection is closed eventually and the goroutine goes away. The default idle timeout is 90 seconds.

If you want to close connections asap, set either of http.Request.Close or http.Transport.DisableKeepAlives to true.