In my HTTP application written in golang I have a few routes that rely on 3rd party services (and vendored code) to do some work before I am actually able to register the route. This might fail or need to be retried, but I still want the application to respond to other requests while this is process is potentially ongoing.
This means I am registering handlers on http.DefaultServeMux in goroutines that I spawn from my main func. This works as expected, but I will find my tests complaining about data races now.
A minimal case to repro looks like this:
package main
import (
"log"
"net/http"
)
func main() {
go func() {
http.HandleFunc("/hello", func(w http.ResponseWriter, r *http.Request) {
w.Write([]byte("hello"))
})
}()
srv := http.Server{
Addr: ":3000",
}
log.Fatal(srv.ListenAndServe())
}
With a test like:
package main
import (
"io/ioutil"
"net/http"
"os"
"testing"
"time"
)
func TestMain(m *testing.M) {
go main()
time.Sleep(time.Second)
os.Exit(m.Run())
}
func TestHello(t *testing.T) {
t.Run("default", func(t *testing.T) {
res, err := http.DefaultClient.Get("http://0.0.0.0:3000/hello")
if err != nil {
t.Fatalf("Calling /hello returned %v", err)
}
if res.StatusCode != http.StatusOK {
b, _ := ioutil.ReadAll(res.Body)
defer res.Body.Close()
t.Errorf("Expected /hello to return 200 response, got %v with body %v", res.StatusCode, string(b))
}
})
}
will show me the following output:
==================
WARNING: DATA RACE
Read at 0x000000a337d8 by goroutine 14:
net/http.(*ServeMux).shouldRedirect()
/usr/local/go/src/net/http/server.go:2239 +0x162
net/http.(*ServeMux).redirectToPathSlash()
/usr/local/go/src/net/http/server.go:2224 +0x64
net/http.(*ServeMux).Handler()
/usr/local/go/src/net/http/server.go:2293 +0x184
net/http.(*ServeMux).ServeHTTP()
/usr/local/go/src/net/http/server.go:2336 +0x6d
net/http.serverHandler.ServeHTTP()
/usr/local/go/src/net/http/server.go:2694 +0xb9
net/http.(*conn).serve()
/usr/local/go/src/net/http/server.go:1830 +0x7dc
Previous write at 0x000000a337d8 by goroutine 8:
net/http.(*ServeMux).Handle()
/usr/local/go/src/net/http/server.go:2357 +0x216
net/http.(*ServeMux).HandleFunc()
/usr/local/go/src/net/http/server.go:2368 +0x62
net/http.HandleFunc()
/usr/local/go/src/net/http/server.go:2380 +0x68
github.com/m90/test.main.func1()
/home/frederik/projects/go/src/github.com/m90/test/main.go:10 +0x4f
Goroutine 14 (running) created at:
net/http.(*Server).Serve()
/usr/local/go/src/net/http/server.go:2795 +0x364
net/http.(*Server).ListenAndServe()
/usr/local/go/src/net/http/server.go:2711 +0xc4
github.com/m90/test.main()
/home/frederik/projects/go/src/github.com/m90/test/main.go:17 +0xb6
Goroutine 8 (finished) created at:
github.com/m90/test.main()
/home/frederik/projects/go/src/github.com/m90/test/main.go:9 +0x46
==================
From what I understand reading the code of package http net/http.(*ServeMux).Handler() in the stacktrace does not lock the mutex that protects the handler map, as it expects this to be done by net/http.(*ServeMux).handler() which in my scenario does not get called.
Am I doing something that is not supposed to be done? Is this an issue with the standard library? Am I doing something wrong in the way I attach the handlers in a goroutine?
