Correct use of go context.Context

Question

I just read the article: Build You Own Web Framework In Go and for sharing values among handlers I picked the context.Context and I'm using it in the following way to share values across handlers and middlewares:

type appContext struct {
    db     *sql.DB
    ctx    context.Context
    cancel context.CancelFunc
 }


func (c *appContext)authHandler(next http.Handler) http.Handler {
    fn := func(w http.ResponseWriter, r *http.Request {
        defer c.cancel() //this feels weird
        authToken := r.Header.Get("Authorization") // this fakes a form
        c.ctx = getUser(c.ctx, c.db, authToken) // this also feels weird
        next.ServeHTTP(w, r)
    }

    return http.HandlerFunc(fn)
}

func (c *appContext)adminHandler(w http.ResponseWriter, r *http.Request) {
    defer c.cancel()
    user := c.ctx.Value(0).(user)
    json.NewEncoder(w).Encode(user)
}

func getUser(ctx context.Context, db *sql.DB, token string) context.Context{
    //this function mimics a database access
    return context.WithValue(ctx, 0, user{Nome:"Default user"})
}

func main() {
    db, err := sql.Open("my-driver", "my.db")
    if err != nil {
        panic(err)
    }
    ctx, cancel := context.WithCancel(context.Background())
    appC := appContext{db, ctx, cancel}
    //....
}

Everything is working and handlers are loading faster than using gorilla/context So my questions are:

1. Is this approach safe?
2. Is it really necessary to defer the c.cancel() function the way I'm doing it?
3. Can I use it to implement a custom web framework by using controllers like struct to share values with models?

Answer 1

You have a problem with your code because you are storing the user into the app context. With multiple users at the same time, it doesn't work. The context must be related to the request to not be overwrote by other requests. The user must be stored in a request context. In my articles I use the following gorilla function: context.Set(r, "user", user). r is the request.

If you want to use context.Context in your app, you should use their gorilla wrapper (you can find it at the end of this article: https://blog.golang.org/context).

Also, you don't need a context with cancel. context.Background() is okay for the root context.