related https://github.com/golang/go/issues/51914

Comment From: pscheid92

I've got curious and checked how Go 1.19 now allows HTTP 104 Early Hint.

Research: How to HTTP 103 in Go

In this article I found a snippet:

func main() {
    helloHandler := func(w http.ResponseWriter, req *http.Request) {
        w.Header().Add("Link", "</style.css>; rel=preload; as=style")
        w.Header().Add("Link", "</script.js>; rel=preload; as=script")

        w.WriteHeader(103)

        // do your heavy tasks such as DB or remote APIs calls here

        w.WriteHeader(200)

        io.WriteString(w, "<!doctype html>\n[... snip snip ...]")
    }

    http.HandleFunc("/hello", helloHandler)
    log.Fatal(http.ListenAndServeTLS(":443", "cert.pem", "key.pem", nil))
}

This boils down to three steps:

  1. Attach the Link headers to the response.
  2. Write the status line HTTP 103 Early Hint
  3. Enforce the writing the early hint with w.WriteHeaderNow()

Reproduction with Gin

I then was able to reproduce this with Gin:

func main() {
    r := gin.Default()

    r.Static("/assets", "./assets")

    r.GET("/testing", func(c *gin.Context) {
        c.Writer.Header().Add("Link", "</assets/file1.js>; rel=preload; as=script")
        c.Writer.Header().Add("Link", "<https://gphrase.de/r/>; rel=preconnect")
        c.Writer.WriteHeader(http.StatusEarlyHints)
        c.Writer.WriteHeaderNow()

                 // server thinking time
        time.Sleep(5 * time.Second)

        c.String(200, "success")
    })

    err := r.RunTLS(":443", "cert.pem", "key.pem")
    log.Err(err).Msg("server ended")
}

$ curl -v https://localhost/testing

# ... snip snip ...

> 
* Connection state changed (MAX_CONCURRENT_STREAMS == 250)!
< HTTP/2 103 
< link: </assets/file1.js>; rel=preload; as=script
< link: <https://gphrase.de/r/>; rel=preconnect
< HTTP/2 200 
< content-type: text/plain; charset=utf-8
< link: </assets/file1.js>; rel=preload; as=script
< link: <https://gphrase.de/r/>; rel=preconnect
< content-length: 7
< date: Sat, 08 Oct 2022 17:32:16 GMT
< 
* Connection #0 to host localhost left intact
success* Closing connection 0

Proposal

We could encapsule the above attaching of Link headers, the writing of HTTP status 103, and forcing the write to the client.

func (c *Context) EarlyHint(links ...string) {
    if len(links) == 0 {
        return
    }

    for _, l := range links {
        c.Writer.Header().Add("Link", l)
    }

    c.Writer.WriteHeader(http.StatusEarlyHints)
    c.Writer.WriteHeaderNow()
}

With that the above example would become:

func main() {
    r := gin.Default()

    r.Static("/assets", "./assets")

    r.GET("/testing", func(c *gin.Context) {
        c.EarlyHint("</assets/file1.js>; rel=preload; as=script", "<https://someserver.com/r/>; rel=preconnect")

                 // server thinking time
        time.Sleep(5 * time.Second)

        c.String(200, "success")
    })

    err := r.RunTLS(":443", "cert.pem", "key.pem")
    log.Err(err).Msg("server ended")
}

Comment From: allesan

`func main() { r := gin.Default()

r.Static("/assets", "./assets")

r.GET("/testing", func(c *gin.Context) {
    c.Writer.Header().Add("Link", "</assets/file1.js>; rel=preload; as=script")
    c.Writer.Header().Add("Link", "<https://gphrase.de/r/>; rel=preconnect")
    c.Writer.WriteHeader(http.StatusEarlyHints)
    c.Writer.WriteHeaderNow()

             // server thinking time
    time.Sleep(5 * time.Second)

    c.String(200, "success")
})

err := r.RunTLS(":443", "cert.pem", "key.pem")
log.Err(err).Msg("server ended")

}`

If I try this I get warning that headers are already sent?

Comment From: pscheid92

Hello @allesan, I think I have bad news for you:

This warning was just information in the past, so the workaround was functioning.

BUT since commit https://github.com/gin-gonic/gin/commit/41f2669ebcc24dbbef7dcca705a4cd75b7c43f28, the new status code is ignored. So, unfortunately, this workaround for early hints isn't working anymore.

Theoretically, we could update the WriteHeader method to be early hint aware. What do you think @thinkerou? 

func (w *responseWriter) WriteHeader(code int) {
    if code > 0 && w.status != code {
        if w.Written() {
            debugPrint("[WARNING] Headers were already written. Wanted to override status code %d with %d", w.status, code)
            return
        }
        w.status = code
    }
}

Proposal: Updated WriteHeader method

We accept a new status code if the previous one was EarlyHints

func (w *responseWriter) WriteHeader(code int) {
    // Ignore invalid or redundant status codes
    if code <= 0 || code == w.status {
        return
    }

    // Warn if attempting to change headers after they are written, except for EarlyHints
    if w.Written() && w.status != http.StatusEarlyHints {
        debugPrint("[WARNING] Headers were already written. Wanted to override status code %d with %d", w.status, code)
        return
    }

    // Update the status code
    w.status = code
}