methods for interface mocking in go - testing

Question

If I have a handlerfunc like the one below. What is the best way to "mock" or inject a interface that wraps some object for testing?
func GetOrCreateUser(w http.ResponseWriter, r *http.Request) {
user := GetUserFromContext(r.Context())
if created :=user.GetOrCreate(); created {
smtp.SendEmail(...)
return
} else {
w.Write([]byte("Hello User!"))
}
}
The only way that I have come by seems to be to do this:
type Mailer interface { SendMail() }
func HandlerWithMailer(m Mailer) http.handlerFunc {
return func(w http.ResponseWriter, r *http.Request) {
user := GetUserFromContext(r.Context())
if created := user.GetOrCreate(); created {
m.SendEmail(...)
return
} else {
w.Write([]byte("Hello User!"))
}
}
}
Then calling the mailer like this in the router (using httprouter):
m := mailer.New() // assuming this returns a new mailer
r := httprouter.New()
r.GET("/mailer", HandlerWithMailer(m))
Which could then be tested by making a struct that implements the interface and returns whatever I want, which can be called in the tests. I know this works, but I am wondering if this is the preferred method of testing in Go and if there is any other way to go about accomplishing this.

Answer 1

I would call my handlers from a struct like so:
type Handlers struct {
mailer *Mailer
}
func(h *Handlers) GetOrCreateUser(w http.ResponseWriter, r *http.Request) {
user := GetUserFromContext(r.Context())
if created :=user.GetOrCreate(); created {
h.mailer.SendEmail(...)
return
} else {
w.Write([]byte("Hello User!"))
}
}
This way you can instansiate the struct with the implementation of Mailer you want to use.
m := mailer.New() // assuming this returns a new mailer
h := Handlers{&m}
r := httprouter.New()
r.GET("/mailer", h.HandlerWithMailer)
and in your tests
m := mockMailer.New() // assuming this returns a new mailer
h := Handlers{&m}
r := httprouter.New()
r.GET("/mailer", h.HandlerWithMailer)