go - 如何在 Go 服务器中设置 HTTP 尾部？

Question

我想通过对写出的响应主体进行哈希处理来计算响应的实体标签。当我计算实体标签时，将实体标签添加到响应 header 已经太晚了。我想将实体标签添加到预告片中。我看到 net/http 包支持编写预告片，但我不知道如何使用它们。

预告片代码在https://golang.org/src/pkg/net/http/transfer.go中.如何从我的应用程序设置预告片？

Answer 1

2021 answer (or really go1.5+)

您需要在第一次 Write 之前预先设置尾部标题名称，然后您可以稍后添加标题。

例如(复制自 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.

---

原始答案(

使用 bytes.Buffer，同时将其包装到哈希中，例如:

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)
}

截至目前，您无法设置预告片标题，有一个开放的 issue关于它。

有一个解决方法，劫持连接(来自上述问题):

// 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()