Querying the database for basic auth using go-http-auth with martini-go

I am attempting to use go-http-auth with martini-go. In the example given here - https://github.com/abbot/go-http-auth

package main

import (
        auth "github.com/abbot/go-http-auth"
        "fmt"
        "net/http"
)

func Secret(user, realm string) string {
        if user == "john" {
                // password is "hello"
                return "$1$dlPL2MqE$oQmn16q49SqdmhenQuNgs1"
        }
        return ""
}

func handle(w http.ResponseWriter, r *auth.AuthenticatedRequest) {
        fmt.Fprintf(w, "<html><body><h1>Hello, %s!</h1></body></html>", r.Username)
}


func main() {
    db, err := sql.Open("postgres", "postgres://blabla:blabla@localhost/my_db")
    authenticator := auth.NewBasicAuthenticator("example.com", Secret)
    m := martini.Classic()
    m.Map(db)
    m.Get("/users", authenticator.Wrap(MyUserHandler))
    m.Run()

}

The Secret function is using a hardcoded user "john".

The authentication is successful when I execute

curl --user john:hello localhost:3000/users

Obviously, this is a trivial example with hardcoded username and password.

I am now changing the Secret function to this

func Secret(user, realm string) string {

    fmt.Println("Executing Secret")

    var db *sql.DB

    var (
        username string
        password string
    )

    err := db.QueryRow("select username, password from users where username = ?", user).Scan(&username, &password)

    if err == sql.ErrNoRows {
        return ""
    }

    if err != nil {
        log.Fatal(err)
    }
    //if user == "john" {
        //// password is "hello"
        //return "$1$dlPL2MqE$oQmn16q49SqdmhenQuNgs1"
    //}
    //return ""
    return ""

}

But it fails with PANIC: runtime error: invalid memory address or nil pointer dereference. Which is obviously because I am attempting to instantiate var db *sql.DB in the Secret function. I cannot pass db *sql.DB into the Secret function either because auth.BasicNewAuthentication is expecting a Secret argument that conforms to type func (string, string) string.

How can I implement my database query correctly and return the password for comparison?

Solution

You can use a simple closure to pass in a reference to your DB to the authenticator function:

authenticator := auth.NewBasicAuthenticator("example.com", func(user, realm string) string {
    return Secret(db, user, realm)
})

…and then change your Secret to accept the database as the first argument:

func Secret(db *sql.DB, user, realm string) string {
    // do your db lookup here…
}