如何在Go服务器中设置HTTP预告片？

Question

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?

Answer 1

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
")            // eof
t.Write(buf)
buf.WriteString("
") // end of trailers
buf.Flush()
conn.Close()