Shutting down HTTP server after returning response

Question

I'm in the process of building a little command line based Go bot that interacts with the Instagram API.

The Instagram API is OAuth based, and so not overly great for command line based apps.

To get around this, I am opening the appropriate authorization URL in the browser and using a local server I spin up for the redirect URI - this way I can capture and gracefully show the access token as opposed to the user needing to get this from the URL manually.

So far so good, the application can successfully open the browser to the authorisation URL, you authorise it and it redirects you to the local HTTP server.

Now, I have no need for the HTTP server after the access token has been displayed to the user and so I am wanting to manually shut the server down after doing this.

To do this, I drew inspiration from this answer and drummed up the below:

package main

import (
    "fmt"
    "io"
    "log"
    "net/http"
    "os/exec"
    "runtime"
    "time"
)

var client_id = "my_client_id"
var client_secret = "my_client_secret"
var redirect_url = "http://localhost:8000/instagram/callback"

func main() {

    srv := startHttpServer()

    openbrowser(fmt.Sprintf("https://api.instagram.com/oauth/authorize/?client_id=%v&redirect_uri=%v&response_type=code", client_id, redirect_url))

    // Backup to gracefully shutdown the server
    time.Sleep(20 * time.Second)
    if err := srv.Shutdown(nil); err != nil {
        panic(err) // failure/timeout shutting down the server gracefully
    }
}

func showTokenToUser(w http.ResponseWriter, r *http.Request, srv *http.Server) {
    io.WriteString(w, fmt.Sprintf("Your access token is: %v", r.URL.Query().Get("code")))
    if err := srv.Shutdown(nil); err != nil {
        log.Fatal(err) // failure/timeout shutting down the server gracefully
    }
}

func startHttpServer() *http.Server {
    srv := &http.Server{Addr: ":8000"}

    http.HandleFunc("/instagram/callback", func(w http.ResponseWriter, r *http.Request) {
        showTokenToUser(w, r, srv)
    })

    go func() {
        if err := srv.ListenAndServe(); err != nil {
            // cannot panic, because this probably is an intentional close
            log.Printf("Httpserver: ListenAndServe() error: %s", err)
        }
    }()

    // returning reference so caller can call Shutdown()
    return srv
}

func openbrowser(url string) {
    var err error

    switch runtime.GOOS {
    case "linux":
        err = exec.Command("xdg-open", url).Start()
    case "windows":
        err = exec.Command("rundll32", "url.dll,FileProtocolHandler", url).Start()
    case "darwin":
        err = exec.Command("open", url).Start()
    default:
        err = fmt.Errorf("unsupported platform")
    }
    if err != nil {
        log.Fatal(err)
    }
}

However, the above causes this error:

2017/11/23 16:02:03 Httpserver: ListenAndServe() error: http: Server closed

2017/11/23 16:02:03 http: panic serving [::1]:61793: runtime error: invalid memory address or nil pointer dereference

If I comment out these lines in the handler then it works flawlessly, albeit without shutting down the server when I hit the callback route:

if err := srv.Shutdown(nil); err != nil {
    log.Fatal(err) // failure/timeout shutting down the server gracefully
}

Where am I going wrong? What do I need to change so that I can shut the server down when I hit the callback route, after displaying the text to the user.

Reflexive

Answer 1

1. You may use context.WithCancel:

package main

import (
    "context"
    "io"
    "log"
    "net/http"
)

func main() {
    ctx, cancel := context.WithCancel(context.Background())

    http.HandleFunc("/quit", func(w http.ResponseWriter, r *http.Request) {
        io.WriteString(w, "Bye\n")
        cancel()
    })
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        io.WriteString(w, "Hi\n")
    })

    srv := &http.Server{Addr: ":8080"}
    go func() {
        err := srv.ListenAndServe()
        if err != http.ErrServerClosed {
            log.Println(err)
        }
    }()

    <-ctx.Done() // wait for the signal to gracefully shutdown the server

    // gracefully shutdown the server:
    // waiting indefinitely for connections to return to idle and then shut down.
    err := srv.Shutdown(context.Background())
    if err != nil {
        log.Println(err)
    }

    log.Println("done.")
}

---

2. The same Context may be passed to functions running in different goroutines:

"Contexts are safe for simultaneous use by multiple goroutines."

You may use the same context - if you don't want to wait extera:

package main

import (
    "context"
    "io"
    "log"
    "net/http"
)

func main() {
    ctx, cancel := context.WithCancel(context.Background())
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        io.WriteString(w, "Hi\n")
    })
    http.HandleFunc("/quit", func(w http.ResponseWriter, r *http.Request) {
        io.WriteString(w, "Bye\n")
        cancel()
    })
    srv := &http.Server{Addr: ":8080"}
    go func() {
        if err := srv.ListenAndServe(); err != nil {
            log.Printf("Httpserver: ListenAndServe() error: %s", err)
        }
    }()
    <-ctx.Done()
    // if err := srv.Shutdown(ctx); err != nil && err != context.Canceled {
    //  log.Println(err)
    // }
    log.Println("done.")
}

Server.Shutdown:
Shutdown gracefully shuts down the server without interrupting any active connections. Shutdown works by first closing all open listeners, then closing all idle connections, and then waiting indefinitely for connections to return to idle and then shut down. If the provided context expires before the shutdown is complete, Shutdown returns the context's error, otherwise it returns any error returned from closing the Server's underlying Listener(s).

When Shutdown is called, Serve, ListenAndServe, and ListenAndServeTLS immediately return ErrServerClosed. Make sure the program doesn't exit and waits instead for Shutdown to return.

Shutdown does not attempt to close nor wait for hijacked connections such as WebSockets. The caller of Shutdown should separately notify such long-lived connections of shutdown and wait for them to close, if desired. See RegisterOnShutdown for a way to register shutdown notification functions.

Once Shutdown has been called on a server, it may not be reused; future calls to methods such as Serve will return ErrServerClosed.

Cheerless

Works perfectly. – Reflexive

<-ctx.Done returns after it's been cancelled (done). You shouldn't pass the same context to the Shutdown method because the ctx will always be in a cancelled state. – Laburnum

Thanks for the comment, I think there are some options here: waiting indefinitely for connections to return to idle and then shut down - or not. Also we may or may not want to use context - or use some other synchronization method. The possibilities are limited only by your imagination. – Cheerless

Answer 2

Shutdown function accepts parameter ctx context.Context. Try to pass it an empty context.

ctx := context.Background()

Also:

When Shutdown is called, Serve, ListenAndServe, and ListenAndServeTLS immediately return ErrServerClosed. Make sure the program doesn't exit and waits instead for Shutdown to return.

Streaming