How can I terminate my handler early if some permission check fails?

Question

I am finding a way to implement the permission check functionality using http

The idea is there are APIs that should be used only by login sessions.

func CheckPermissionFilter(w http.ResponseWriter, r *http.Response){
    sid, err := r.Cookie("sid")
    // check the permission with sid, if permission is granted then just let the 
    // process go on, otherwise, just break the filter chain and return Http Error Code.

}

func SomeHttpHandler(w http.ResponseWriter, r *http.Response){
     CheckPermissionFilter(w, r)
     // if not breaked by above filter function, process the request...
   
}

I have no problem with the permission checking, but I can't find a way to break the HTTP Request processing.

Answer 1

The call to CheckPermissionFilter within your SomeHttpHandler handler cannot terminate the latter early. Instead, you should define CheckPermissionFilter as a middleware (see also decorator pattern):

package main

import (
    "net/http"
)

func main() {
    http.Handle("/foo", CheckPermissionFilter(SomeHttpHandler))
    // ...
}

func CheckPermissionFilter(h http.HandlerFunc) http.HandlerFunc {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        sid, err := r.Cookie("sid")
        // handle err
        if !Validate(sid) {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        h(w, r)
    })
}

func SomeHttpHandler(w http.ResponseWriter, r *http.Request) {
    // ...
}

func Validate(sid string) bool {
    return true // simplistic implementation for this example
}