Support for HTTP Trailers

[shashwathi]

Hi,

We were trying to implement a reverse proxy using fasthttp library and we noticed that the client does not support reading HTTP trailers from the response.

Sample backend application

package main

import (
       	"fmt"
       	"io"
       	"net/http"
       	"os"
)

func main() {
       	http.HandleFunc("/", hello)
       	fmt.Println("listening...")
       	err := http.ListenAndServe(":"+os.Getenv("PORT"), nil)
       	if err != nil {
       		panic(err)
       	}
}

func hello(w http.ResponseWriter, req *http.Request) {
       	w.Header().Set("Trailer", "AtEnd1, AtEnd2")

       	w.Header().Set("Content-Type", "text/plain; charset=utf-8") // normal header
       	w.WriteHeader(http.StatusOK)

       	w.Header().Set("AtEnd1", "value 1")
       	io.WriteString(w, "This HTTP response has both headers before this text and trailers at the end.\r\n")
       	w.Header().Set("AtEnd2", "value 2")
}

fasthttp client

package main

import (
       	"fmt"
       	"github.com/valyala/fasthttp"
)

func main() {
       	req := fasthttp.AcquireRequest()
       	resp := fasthttp.AcquireResponse()
       	defer fasthttp.ReleaseRequest(req)
       	defer fasthttp.ReleaseResponse(resp)
       	req.SetHost("localhost:9090")
       	req.Header.SetMethod("GET")
       	err := fasthttp.Do(req, resp)
       	if err != nil {
       		fmt.Printf("Error making HTTP request: %s\n", err.Error())
       	}
       	fmt.Printf("Response %#v", resp)
}

Is this feature anywhere in the roadmap for the project?

Regards
Shash && Edwin (@flawedmatrix)

[shalako]

Hello @valyala
Would you take a PR to add support for HTTP trailers?

[erikdubbelboer]

Yes we would take PR to add support for trailers.

[rof20004]

@shashwathi @erikdubbelboer

I saw somewhere that is not possible to write to response more than once.

You wrote the response status, than you can not write to response anymore.

I will search where I saw this.

[rof20004]

I founded: https://stackoverflow.com/questions/39427544/golang-http-response-headers-being-removed

Spec: https://golang.org/pkg/net/http/#ResponseWriter

[rof20004]

Here is the part of spec:

type ResponseWriter interface {
        // Header returns the header map that will be sent by
        // WriteHeader. The Header map also is the mechanism with which
        // Handlers can set HTTP trailers.
        //
        // Changing the header map after a call to WriteHeader (or
        // Write) has no effect unless the modified headers are
        // trailers.
        //
        // There are two ways to set Trailers. The preferred way is to
        // predeclare in the headers which trailers you will later
        // send by setting the "Trailer" header to the names of the
        // trailer keys which will come later. In this case, those
        // keys of the Header map are treated as if they were
        // trailers. See the example. The second way, for trailer
        // keys not known to the Handler until after the first Write,
        // is to prefix the Header map keys with the TrailerPrefix
        // constant value. See TrailerPrefix.
        //
        // To suppress automatic response headers (such as "Date"), set
        // their value to nil.
        Header() Header

I think this issue can be closed, because this is like golang works and was designed.

[kirillDanshin]

@rof20004 your links are all about net/http. fasthttp is an alternative ground-up implementation incompatible with net/http. we're interested in HTTP RFCs and can do things in ways that differ from net/http

[erikdubbelboer]

We could also very easily implement this with a Trailer header like net/http. If anyone needs this please let me know and I'll fix it.

[dmarkhas]

@erikdubbelboer we would appreciate having trailer headers to indicate errors in a streaming response