http.Flusher compatiability

[mooijtech]

Hello,

I want to implement Server Sent Events and am using this library: https://github.com/r3labs/sse The library expects the http.ResponseWriter to be able to cast to a http.Flusher for flushing.

The http.Flusher documentation states:

// The default HTTP/1.x and HTTP/2 ResponseWriter implementations
// support Flusher, *but ResponseWriter wrappers may not*. Handlers
// should always test for this ability at runtime.

Your library is wrapping http.ResponseWriter and is breaking the library. Would it be possible for you to make the bufferedResponseWriter implement the http.Flusher?

References: https://github.com/r3labs/sse/issues/130

[mooijtech]

Ended up switching to Gorilla Sessions which works.

[gandaldf]

Could you please post your code? A simple example (even if not working) would be helpfull to debug and serve as a common reference.

[mooijtech]

Sure, here is the code to test it:

package main

import (
    "fmt"
    "github.com/alexedwards/scs/v2"
    "github.com/gorilla/mux"
    "github.com/r3labs/sse/v2"
    "github.com/rs/cors"
    "net/http"
    "time"
)

func main() {
    server := Server{
        Router:           mux.NewRouter(),
        ServerSentEvents: sse.New(),
        SessionManager:   scs.New(),
    }

    server.Start()
}

type Server struct {
    Router           *mux.Router
    ServerSentEvents *sse.Server
    SessionManager   *scs.SessionManager
}

func (server *Server) Start() {
    server.Router.Handle("/outlook/loading", server.handleOutlookLoading())

    corsHandler := cors.New(cors.Options{
        AllowedOrigins:   []string{"http://localhost:3000", "http://127.0.0.1:3000"},
        AllowedMethods:   []string{http.MethodGet, http.MethodPost, http.MethodDelete},
        AllowCredentials: true,
    }).Handler(server.Router)

    fmt.Println("Starting the server...")
    // TODO - The handleOutlookLoading will not work with the session manager.
    fmt.Println(http.ListenAndServe(":1337", server.SessionManager.LoadAndSave(corsHandler)))

    // The following works:
    //fmt.Println(http.ListenAndServe(":1337", corsHandler))
}

func (server *Server) handleOutlookLoading() http.HandlerFunc {
    return func(responseWriter http.ResponseWriter, request *http.Request) {
        server.ServerSentEvents.CreateStream("test")

        go func() {
            for {
                server.ServerSentEvents.Publish("test", &sse.Event{Data: []byte("hello")})
                time.Sleep(time.Second)
            }
        }()

        server.ServerSentEvents.ServeHTTP(responseWriter, request)
    }
}

On the front-end I am using React and have this:

 const source = new EventSource("http://localhost:1337/outlook/loading?stream=" + "test")
 source.onmessage = e => alert(e.data)

[gandaldf]

Mm... I've tried to keep it simpler, removing Gorilla Mux and the CORS handler:

package main

import (
    "fmt"
    "github.com/alexedwards/scs/v2"
    "github.com/r3labs/sse/v2"
    "net/http"
    "time"
)

func main() {
    server := Server{
        Router:           http.NewServeMux(),
        ServerSentEvents: sse.New(),
        SessionManager:   scs.New(),
    }

    server.Start()
}

type Server struct {
    Router           *http.ServeMux
    ServerSentEvents *sse.Server
    SessionManager   *scs.SessionManager
}

func (server *Server) Start() {
    server.Router.Handle("/outlook/loading", server.handleOutlookLoading())

    fmt.Println("Starting the server...")
    // TODO - The handleOutlookLoading will not work with the session manager.
    fmt.Println(http.ListenAndServe(":1337", server.SessionManager.LoadAndSave(server.Router)))

    // The following works:
    //fmt.Println(http.ListenAndServe(":1337", server.Router))
}

func (server *Server) handleOutlookLoading() http.HandlerFunc {
    return func(responseWriter http.ResponseWriter, request *http.Request) {
        server.ServerSentEvents.CreateStream("test")

        go func() {
            for {
                server.ServerSentEvents.Publish("test", &sse.Event{Data: []byte("hello")})
                time.Sleep(time.Second)
            }
        }()

        server.ServerSentEvents.ServeHTTP(responseWriter, request)
    }
}

But nothing really changed, so I've tried a basic implementation of the Flush interface in session.go:

func (bw *bufferedResponseWriter) Flush() {
    if flusher, ok := bw.ResponseWriter.(http.Flusher); ok {
        flusher.Flush()
    }
}

The server seems responding now, but I can't see data coming. I think @alexedwards can find a proper solution!

[mooijtech]

I couldn't see any data either, thanks for trying!

[arianvp]

I think the problem is that it's buffering the response. This means there is nothing to flush

[arianvp]

Because it waits for the underlying response to finish to decide if the session is modified, this can never work with never-ending responses like SSE.

Never-ending responses like SSE usually also do not modify the session in the first place (How would they?) So I think the solution is to just Get the session in SSE endpoints and call Save manually before the SSE streaming starts instead of using the GetAndSave middleware

I actually think this is function as designed.

[alexedwards]

I've just pushed commit https://github.com/alexedwards/scs/commit/62e546ce9d2d95a370da0629bd877d9e0c4ed9a0 which changes the way that LoadAndSave() works so that the response is no longer buffered.

Combined with the new Go 1.20 http.ResponseController, this means that you should now be able to use the LoadAndSave() middleware in conjuction with http.Flusher and everything should Just Work :slightly_smiling_face:

I'll leave this issue open until the commit has made it in to a new major release.

[arianvp]

Amazing! Thanks so much.

[ctian1]

Seems that even after https://github.com/alexedwards/scs/commit/62e546ce9d2d95a370da0629bd877d9e0c4ed9a0, http.Flusher is not implemented by sessionResponseWriter, so https://github.com/r3labs/sse is still incompatible.. for compatibility, it'd be nice if it implemented Flusher directly

[theadell]

Is there any update on this? It seems that it is still not compatible with http.Flusher

fmt.Printf("Type of ResponseWriter: %T\n", w)
flusher, ok := w.(http.Flusher)
if !ok {
    http.Error(w, "Streaming unsupported!", http.StatusInternalServerError)
    return
}

this prints out

Type of ResponseWriter: *scs.bufferedResponseWriter

and returns an error as scs.bufferedResponseWriter doesn't implement http.Flusher I am using the LoadAndSave middleware, which when removed everything works fine again

r.Use(app.sessionManager.LoadAndSave)

[ctian1]

@theadell My workaround was to add this middleware:

type middleware struct {
    store *scs.SessionManager
}

func Middleware(store *scs.SessionManager) func(next http.Handler) http.Handler {
    middleware := &middleware{
        store: store,
    }
    return middleware.Handler
}

// flusherResponseWriter wraps http.ResponseController and http.ResponseWriter so that we pass an
// http.Flusher compatible ResponseWriter to the next handler. This is done because scs's middleware
// wraps ResponseWriter and we need to re-expose http.Flusher for the sse library.
type flusherResponseWriter struct {
    *http.ResponseController
    http.ResponseWriter
}

func (w flusherResponseWriter) Flush() {
    err := w.ResponseController.Flush()
    if err != nil {
        panic(err)
    }
}

func (w flusherResponseWriter) Unwrap() http.ResponseWriter {
    return w.ResponseWriter
}

var _ http.Flusher = flusherResponseWriter{}

func (m *middleware) Handler(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        ctx := r.Context()

        rc := http.NewResponseController(w)
        flusher := &flusherResponseWriter{
            ResponseController: rc,
            ResponseWriter:     w,
        }
        next.ServeHTTP(flusher, r.WithContext(ctx))
    })
}

[theadell]

@ctian1 Thanks for your solution but unless I misunderstood your approach it is not working for me, as you suggested I created a middleware that passes an http.Flusher compatible ResponseWriter to the next handler

type flusherResponseWriter struct {
    *http.ResponseController
    http.ResponseWriter
}

func (w *flusherResponseWriter) Flush() {
    if err := w.ResponseController.Flush(); err != nil {
        panic(err)
    }
}

func (w *flusherResponseWriter) Unwrap() http.ResponseWriter {
    return w.ResponseWriter
}

var _ http.Flusher = &flusherResponseWriter{}

func WrapWithFlusher(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        rc := http.NewResponseController(w)
        flusher := &flusherResponseWriter{
            ResponseController: rc,
            ResponseWriter:     w,
        }
        next.ServeHTTP(flusher, r)
    })
}

Then I used this middleware directly after SCS LoadAndSave

r := chi.NewRouter()
r.Use(middleware.RealIP)
r.Use(middleware.Recoverer)
r.Use(app.sessionManager.LoadAndSave)
r.Use(WrapWithFlusher)

Now the type of the ResponseWriter is

Type of ResponseWriter: *main.flusherResponseWriter

But this panics with

 panic: feature not supported

Here is a simplified version of my SSE handler

func handleStatusUpdates(w http.ResponseWriter, r *http.Request) {

    w.Header().Set("Content-Type", "text/event-stream")
    w.Header().Set("Cache-Control", "no-cache")
    w.Header().Set("Connection", "keep-alive")
    w.Header().Set("Access-Control-Allow-Origin", "*")
        fmt.Printf("Type of ResponseWriter: %T\n", w)
    flusher, ok := w.(http.Flusher)
        if !ok {
            http.Error(w, "Streaming unsupported!", http.StatusInternalServerError)
            return
        }
    for {
        // logic .... 
        // ... 
        flusher.Flush() // panics
        time.Sleep(StatusUpdateInterval)
    }
}

[alexedwards]

@ctian1 @theadell Flushing responses now seems to work great with SCS when you use the Go 1.20 http.ResponseController type to manage the flushing. Any session cookie is written with the first Write operation, and subsequent writes are flushed nicely.

package main

import (
    "fmt"
    "io"
    "log"
    "net/http"
    "time"

    "github.com/alexedwards/scs/v2"
)

var sessionManager = scs.New()

func main() {
    mux := http.NewServeMux()
    mux.HandleFunc("/flush", flushHandler)
    mux.HandleFunc("/get", getHandler)

    http.ListenAndServe(":4000", sessionManager.LoadAndSave(mux))
}

func flushHandler(w http.ResponseWriter, r *http.Request) {
    sessionManager.Put(r.Context(), "message", "Hello from a flushing handler!")

    rc := http.NewResponseController(w)

    for i := 0; i < 5; i++ {
        fmt.Fprintf(w, "Write %d\n", i)

        err := rc.Flush()
        if err != nil {
            log.Println(err)
            return
        }

        time.Sleep(time.Second)
    }
}

func getHandler(w http.ResponseWriter, r *http.Request) {
    msg := sessionManager.GetString(r.Context(), "message")
    io.WriteString(w, msg)
}

Now that http.ResponseController exists, casting w.(http.Flusher) should really be something from the past and only necessary if you are using Go < 1.20.

At this point, I think that the right move here is for the github.com/r3labs/sse package to start supporting a http.ResponseController-compatible flushing pattern. If it doesn't, I expect there will be increasing compatibility problems between sse and other packages in the Go ecosystem, as other packages start using custom middleware with a http.ResponseController-compatible Unwrap method instead of manually implementing the http.Flusher interface.

I think it was right to open this issue against scs originally, but now that 62e546ce9d2d95a370da0629bd877d9e0c4ed9a0 is implemented, I think the remainder of the issue now lies with sse and the need to better support http.ResponseController.

[theadell]

@alexedwards

Thanks for your follow-up. I've followed your suggestion and tried using the http.ResponseController, but unfortunately, it doesn't seem to work in my case. I'm still encountering the "feature not supported" error during the flush. I tried the exact example you provided and the error still persists. My env

go version go1.21.0 darwin/arm64
GOARCH='arm64'
GOOS='darwin'

SCS version

require github.com/alexedwards/scs/v2 v2.5.1

For the sake of clarity, here is a complete Go Program that demonstrates the problem

package main

import (
    "fmt"
    "log"
    "net/http"
    "time"

    "github.com/alexedwards/scs/v2"
)

var sessionManager *scs.SessionManager

func main() {

    sessionManager = scs.New()
    sessionManager.Lifetime = 24 * time.Hour

    mux := http.NewServeMux()
    mux.HandleFunc("/sse", sseHandler)
    mux.HandleFunc("/", indexHandler)

    log.Println("Server started on localhost:4000")
    http.ListenAndServe("localhost:4000", sessionManager.LoadAndSave(mux))
}

func sseHandler(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Content-Type", "text/event-stream")
    w.Header().Set("Cache-Control", "no-cache")
    w.Header().Set("Connection", "keep-alive")
    w.Header().Set("Access-Control-Allow-Origin", "*")

    // Create a new ResponseController for flushing
    rc := http.NewResponseController(w)

    for {
        currentTime := time.Now().Format(time.RFC1123)
        data := fmt.Sprintf("data: %s\n\n", currentTime)
        _, writeErr := w.Write([]byte(data))
        if writeErr != nil {
            log.Printf("Error writing to client: %v", writeErr)
            return
        }

        // Use the ResponseController to flush the data
        flushErr := rc.Flush()
        if flushErr != nil {
            log.Printf("Error flushing data to client: %v", flushErr)
            return
        }

        time.Sleep(1 * time.Second)
    }
}

func indexHandler(w http.ResponseWriter, r *http.Request) {
    html := `
        <!DOCTYPE html>
        <html lang="en">
        <head>
            <meta charset="UTF-8">
            <title>SSE with Go</title>
        </head>
        <body>
            <h1>Server Sent Events with Go</h1>
            <div id="sse-data"></div>
            <script>
                const eventSource = new EventSource("/sse");
                eventSource.onmessage = function(event) {
                    document.getElementById("sse-data").innerHTML = event.data;
                };
            </script>
        </body>
        </html>
    `
    w.Write([]byte(html))
}

This results in the error

 Error flushing data to client: feature not supported

[alexedwards]

@theadell Does it work if you use the tip version of SCS?

$ go get github.com/alexedwards/scs/v2@a803960

[theadell]

@alexedwards,

You're right! Switching to the tip version sorted everything out.

I mistakenly assumed 62e546c was part of v2.5.1. Thanks for pointing me in the right direction and for your patience.

Much appreciated!

[alexedwards]

I've released a new major version v2.6.0 which includes https://github.com/alexedwards/scs/commit/62e546ce9d2d95a370da0629bd877d9e0c4ed9a0 now :+1:

I believe that this issue can now be closed, but if there continue to be any problems please comment and I'll reopen it.

[verygoodsoftwarenotvirus]

Hi, I don't mean to ruffle any feathers, but while this works for the manual implementation of SSE handling implemented in the example code in this post, it still doesn't work with the proper SSE library (https://github.com/r3labs/sse).

For one, while the r3labs library obviously expects Flush() to be implemented, it also expects CloseNotifier to be implemented.

I think the easiest solution might be to make sessionResponseWriter public, since I could then check for it in my handler, and call the Unwrap() method to get the root ResponseWriter (which I imagine would properly support these interfaces).

[alexedwards]

@verygoodsoftwarenotvirus I think my comment in https://github.com/alexedwards/scs/issues/141#issuecomment-1774050802 applies here. I think the onus is really on the sse package to start supporting http.ResponseController. That it doesn't is a wider issue which will increasingly affect it's compatibility with many other packages and programs, not just SCS.

Is it possible for you to not use the LoadAndSave middleware on your flushing handlers, and load/write the session data yourself instead? See https://gist.github.com/alexedwards/0570e5a59677e278e13acb8ea53a3b30