openssl req -newkey rsa:2048 -nodes -keyout server.key -x509 -days 365 -out server.crt

package main

import (
	"log"
	"net/http"
)

func main() {
	http.HandleFunc("/header", func(w http.ResponseWriter, r *http.Request) {
		w.Header().Add("X-custom-header", "custom header")
		w.WriteHeader(http.StatusNoContent)

		if f, ok := w.(http.Flusher); ok {
			f.Flush()
		}
		select {}
	})

	log.Println("start listen on 8080...")
	log.Fatal(http.ListenAndServeTLS(":8080", "server.crt", "server.key", nil))
}

curl  "https://localhost:8080/header"  -k -i --http2

// A Frame is the base interface implemented by all frame types.
// Callers will generally type-assert the specific frame type:
// *HeadersFrame, *SettingsFrame, *WindowUpdateFrame, etc.
//
// Frames are only valid until the next call to Framer.ReadFrame.
type http2Frame interface {
	Header() http2FrameHeader

	// invalidate is called by Framer.ReadFrame to make this
	// frame's buffers as being invalid, since the subsequent
	// frame will reuse them.
	invalidate()
}

// A FrameHeader is the 9 byte header of all HTTP/2 frames.
//
// See http://http2.github.io/http2-spec/#FrameHeader
type http2FrameHeader struct {
	valid bool // caller can access []byte fields in the Frame

	// Type is the 1 byte frame type. There are ten standard frame
	// types, but extension frame types may be written by WriteRawFrame
	// and will be returned by ReadFrame (as UnknownFrame).
	Type http2FrameType

	// Flags are the 1 byte of 8 potential bit flags per frame.
	// They are specific to the frame type.
	Flags http2Flags

	// Length is the length of the frame, not including the 9 byte header.
	// The maximum size is one byte less than 16MB (uint24), but only
	// frames up to 16KB are allowed without peer agreement.
	Length uint32

	// StreamID is which stream this frame is for. Certain frames
	// are not stream-specific, in which case this field is 0.
	StreamID uint32
}

// A FrameType is a registered frame type as defined in
// http://http2.github.io/http2-spec/#rfc.section.11.2
type http2FrameType uint8

const (
	http2FrameData         http2FrameType = 0x0
	http2FrameHeaders      http2FrameType = 0x1
	http2FramePriority     http2FrameType = 0x2
	http2FrameRSTStream    http2FrameType = 0x3
	http2FrameSettings     http2FrameType = 0x4
	http2FramePushPromise  http2FrameType = 0x5
	http2FramePing         http2FrameType = 0x6
	http2FrameGoAway       http2FrameType = 0x7
	http2FrameWindowUpdate http2FrameType = 0x8
	http2FrameContinuation http2FrameType = 0x9
)

// A HeadersFrame is used to open a stream and additionally carries a
// header block fragment.
type http2HeadersFrame struct {
   http2FrameHeader

   // Priority is set if FlagHeadersPriority is set in the FrameHeader.
   Priority http2PriorityParam

   headerFragBuf []byte // not owned
}

// PriorityParam are the stream prioritzation parameters.
type http2PriorityParam struct {
   // StreamDep is a 31-bit stream identifier for the
   // stream that this stream depends on. Zero means no
   // dependency.
   StreamDep uint32

   // Exclusive is whether the dependency is exclusive.
   Exclusive bool

   // Weight is the stream's zero-indexed weight. It should be
   // set together with StreamDep, or neither should be set. Per
   // the spec, "Add one to the value to obtain a weight between
   // 1 and 256."
   Weight uint8
}

// Frame-specific FrameHeader flag bits.
const (
	//  ...

	// Headers Frame
	http2FlagHeadersEndStream  http2Flags = 0x1
	http2FlagHeadersEndHeaders http2Flags = 0x4
	http2FlagHeadersPadded     http2Flags = 0x8
    http2FlagHeadersPriority   http2Flags = 0x20
    
    // ...
)

    // 计算Pad的长度
    var padLength uint8
	if fh.Flags.Has(http2FlagHeadersPadded) {
		if p, padLength, err = http2readByte(p); err != nil {
			return
		}
    }

    // ...

    // 取出 Header Block Fragment
    hf.headerFragBuf = p[:len(p)-int(padLength)]

// A Framer reads and writes Frames.
type http2Framer struct {
	r         io.Reader
	lastFrame http2Frame
	errDetail error

	lastHeaderStream uint32

	maxReadSize uint32
	headerBuf   [http2frameHeaderLen]byte

	getReadBuf func(size uint32) []byte
	readBuf    []byte // cache for default getReadBuf

	maxWriteSize uint32 // zero means unlimited; TODO: implement

	w    io.Writer
	wbuf []byte

    // ....
}

// http2Framer的操作方法
type http2Framer
    func http2NewFramer(w io.Writer, r io.Reader) *http2Framer
    func (fr *http2Framer) ErrorDetail() error
    func (fr *http2Framer) ReadFrame() (http2Frame, error)
    // ...
    func (f *http2Framer) WriteData(streamID uint32, endStream bool, data []byte) error
    // ...
    func (f *http2Framer) WriteHeaders(p http2HeadersFrameParam) error
    // ...
    func (f *http2Framer) WritePushPromise(p http2PushPromiseParam) error
    func (f *http2Framer) WriteRSTStream(streamID uint32, code http2ErrCode) error
    // ...

func (s *http2Server) ServeConn(c net.Conn, opts *http2ServeConnOpts) {
	baseCtx, cancel := http2serverConnBaseContext(c, opts)
	defer cancel()

	sc := &http2serverConn{
		srv:                         s,
		hs:                          opts.baseConfig(),
		conn:                        c,
		baseCtx:                     baseCtx,
		remoteAddrStr:               c.RemoteAddr().String(),
        bw:                          http2newBufferedWriter(c),
        // ...
    }

    // ...

    // 将conn交接给http2Framer进行最小粒度的Frame读写.
    fr := http2NewFramer(sc.bw, c)
	fr.ReadMetaHeaders = hpack.NewDecoder(http2initialHeaderTableSize, nil)
	fr.MaxHeaderListSize = sc.maxHeaderListSize()
	fr.SetMaxReadFrameSize(s.maxReadFrameSize())
    sc.framer = fr
}

func (sc *http2serverConn) serve() {
    // ...
    go sc.readFrames()  // 读取Frame
    // ...
    select {
    case wr := <-sc.wantWriteFrameCh:
        sc.writeFrame(wr) // 写Frame
    // ...
    }
    // ...
}

// Optional http.ResponseWriter interfaces implemented.
var (
	_ CloseNotifier     = (*http2responseWriter)(nil)
	_ Flusher           = (*http2responseWriter)(nil)
	_ http2stringWriter = (*http2responseWriter)(nil)
)

// ...

func (w *http2responseWriter) Flush() {
	rws := w.rws
	if rws == nil {
		panic("Header called after Handler finished")
	}
	if rws.bw.Buffered() > 0 {
		if err := rws.bw.Flush(); err != nil {
			// Ignore the error. The frame writer already knows.
			return
		}
	} else {
		// The bufio.Writer won't call chunkWriter.Write
		// (writeChunk with zero bytes, so we have to do it
		// ourselves to force the HTTP response header and/or
		// final DATA frame (with END_STREAM) to be sent.
		rws.writeChunk(nil)
	}
}

// ...

func (rws *http2responseWriterState) writeChunk(p []byte) (n int, err error) {
	if !rws.wroteHeader {
		rws.writeHeader(200)
	}

	isHeadResp := rws.req.Method == "HEAD"
	if !rws.sentHeader {
        // ...
        err = rws.conn.writeHeaders(rws.stream, &http2writeResHeaders{
			streamID:      rws.stream.id,
			httpResCode:   rws.status,
			h:             rws.snapHeader,
			endStream:     endStream,
			contentType:   ctype,
			contentLength: clen,
			date:          date,
        })
    }
    // ...
}