How to set HTTP trailers in Go server?

I want to compute the entity tag for a response by hashing the response body as it's written out. By the time I compute the entity tag, it's too late to add the entity tag to the response header. I'd like to add the entity tag to the trailer. I see the the net/http package has support for writing trailers, but I cannot figure out how to use them.

The trailer code is in https://golang.org/src/pkg/net/http/transfer.go. How do I set a trailer from my application?

2021 answer (or really go1.5+)

You need to pre-set the trailer header names before the first Write then you can add the headers later.

for example (copied from https://pkg.go.dev/net/http#example-ResponseWriter-Trailers):

// Before any call to WriteHeader or Write, declare
		// the trailers you will set during the HTTP
		// response. These three headers are actually sent in
		// the trailer.
		w.Header().Set("Trailer", "AtEnd1, AtEnd2")
		w.Header().Add("Trailer", "AtEnd3")

		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.\n")
		w.Header().Set("AtEnd2", "value 2")
		w.Header().Set("AtEnd3", "value 3") // These will appear as trailers.

original answer (< go1.5)

use a bytes.Buffer, and wrap it to hash in the same time, something like:

type HashedBuffer struct {
	h hash.Hash
	b bytes.Buffer
}

func NewHashedBuffer(h hash.Hash) *HashedBuffer {
	return &HashedBuffer{h: h}
}

func (h *HashedBuffer) Write(p []byte) (n int, err error) {
	n, err = h.b.Write(p)
	h.h.Write(p)
	return
}

func (h *HashedBuffer) Output(w http.ResponseWriter) {
	w.Header().Set("ETag", hex.EncodeToString(h.h.Sum(nil)))
	h.b.WriteTo(w)
}

//handler
func Handler(w http.ResponseWriter, r *http.Request) {
	hb := NewHashedBuffer(sha256.New())
	hb.Write([]byte("stuff"))
	hb.Output(w)
}

As of right now, you can't set trailer headers, there's an open issue about it.

There's a workaround, hijack the connection (from the above issue) :

// TODO: There's no way yet for the server to set trailers
// without hijacking, so do that for now, just to test the client.
// Later, in Go 1.4, it should be be implicit that any mutations
// to w.Header() after the initial write are the trailers to be
// sent, if and only if they were previously declared with
// w.Header().Set("Trailer", ..keys..)
w.(Flusher).Flush()
conn, buf, _ := w.(Hijacker).Hijack()
t := Header{}
t.Set("Server-Trailer-A", "valuea")
t.Set("Server-Trailer-C", "valuec") // skipping B
buf.WriteString("0\r\n")            // eof
t.Write(buf)
buf.WriteString("\r\n") // end of trailers
buf.Flush()
conn.Close()