Golang setCookie() failed after template Execute() - templates

Question

As a beginner of GO, I've got a situation as following:
t, err := template.ParseFiles("/template/login.tpl")
err = t.Execute(w, nil) //if executed before SetCookie
http.SetCookie（...） //failed, browser received nothing
If the sequence is changed, to SetCookie first, it'll work.
My plan was to ParseForm() username and password in login.tpl, if success, a sessionID would be sent by SetCookie. But now SetCookie() must be placed before login.tpl is Executed, which also makes ParseForm() run before login.tpl is executed :
r.ParseForm( ) //ParseForm() works even before template is executed
email := r.FormValue("email")
pass := r.FormValue("pass")
var loginPass = checkLogin(email, pass) //verify username and password
if loginPass == true { //if correct
cookie1 := http.Cookie{Name: "test", Value: "testvalue", Path: "/", MaxAge: 86400}
http.SetCookie(w, &cookie1) //setCookie works
fmt.Println("Login success: ", email)
} else { //if incorrect username or pass
fmt.Println("Please login: ", email)
}
t, err := template.ParseFiles("/template/login.tpl")
err = t.Execute(w, nil) //now template execution is here, also works
But why it should be written like this? Please anyone give me some advice! Thanks a lot.

Answer 1

This is a common error tied to the HTTP protocol working: the cookie is sent in a header of the HTTP respo7nse, which cannot be changed after the body start being sent.
So, when you do the t.Execute(w, nil) call, you start writing the body of the response and thus cannot add cookies afterwards.
This is the exact same reason why in PHP the session_start() call must be the very first instruction of the page, even before any whitespace.