HTTP Frame {
  Length (24),             // 00 00 0C; Frame length: 12
  Type (8),                // 01; Frame type: HEADERS

  Flags (8),               // 04; Flags: END_HEADERS

  Reserved (1),
  Stream Identifier (31),  // 00 00 00 01; Stream Identifier: 1

  Frame Payload (..),      // 87 01 84 8D 4E 3D 6F C8; Binary data for request header information
}

type FrameType uint8
type Flags uint8

type FrameHeader struct {
	valid bool // caller can access []byte fields in the Frame

	// Type is the 1 byte frame type. There are ten standard frame types
	Type FrameType

	// Flags are the 1 byte of 8 potential bit flags per frame.
	Flags Flags

	// 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
}

const (
	FrameData         FrameType = 0x0
	FrameHeaders      FrameType = 0x1
	FramePriority     FrameType = 0x2
	FrameRSTStream    FrameType = 0x3
	FrameSettings     FrameType = 0x4
	FramePushPromise  FrameType = 0x5
	FramePing         FrameType = 0x6
	FrameGoAway       FrameType = 0x7
	FrameWindowUpdate FrameType = 0x8
	FrameContinuation FrameType = 0x9
)

var frameName = map[FrameType]string{
	FrameData:         "DATA",
	FrameHeaders:      "HEADERS",
	FramePriority:     "PRIORITY",
	FrameRSTStream:    "RST_STREAM",
	FrameSettings:     "SETTINGS",
	FramePushPromise:  "PUSH_PROMISE",
	FramePing:         "PING",
	FrameGoAway:       "GOAWAY",
	FrameWindowUpdate: "WINDOW_UPDATE",
	FrameContinuation: "CONTINUATION",
}

func (sc *serverConn) processData(f *DataFrame) error {
	...
	
	if st == nil || state != stateOpen || st.gotTrailerHeader || st.resetQueued {
		...
		// 暂时从server端的流控窗口减去 DATA 帧中的内容长度
		// 然后告诉 client 端可以继续处理 length 长度的内容
		// 最后恢复 server 端流控窗口的长度
		// 之所以先take后add，是为了防止在给client发送 WINDOW_UPDATE 期间，读取了额外内容
		sc.inflow.take(int32(f.Length))
		sc.sendWindowUpdate(nil, int(f.Length))
	}
	
	if f.Length > 0 {
		...
		
		if pad := int32(f.Length) - int32(len(data)); pad > 0 {
			sc.sendWindowUpdate32(nil, pad)
			sc.sendWindowUpdate32(st, pad)
		}
	}
	...
}

func (sc *serverConn) sendWindowUpdate(st *stream, n int) {
	sc.serveG.check()
	const maxUint31 = 1<<31 - 1
	for n >= maxUint31 {
		sc.sendWindowUpdate32(st, maxUint31)
		n -= maxUint31
	}
	sc.sendWindowUpdate32(st, int32(n))
}

func (sc *serverConn) sendWindowUpdate32(st *stream, n int32) {
	sc.serveG.check()
	if n == 0 {
		return
	}
	if n < 0 {
		panic("negative update")
	}
	var streamID uint32
	if st != nil {
		streamID = st.id
	}
	sc.writeFrame(FrameWriteRequest{
		write:  writeWindowUpdate{streamID: streamID, n: uint32(n)},
		stream: st,
	})
	var ok bool
	if st == nil {
		// 恢复 server 端conn的流控窗口
		ok = sc.inflow.add(n)
	} else {
		// 恢复 server 端stream的流控窗口
		ok = st.inflow.add(n)
	}
	if !ok {
		panic("internal error; sent too many window updates without decrements?")
	}
}

type ContinuationFrame struct {
	http2.FrameHeader
	headerFragBuf []byte
}

func (f *ContinuationFrame) HeaderBlockFragment() []byte {
	return f.headerFragBuf
}

func (fr *Framer) readMetaFrame(hf *HeadersFrame) (*MetaHeadersFrame, error) {
	...
	// MAX_HEADER_LIST_SIZE
	var remainSize = fr.maxHeaderListSize()
	hdec := fr.ReadMetaHeaders
	hdec.SetEmitFunc(func(hf hpack.HeaderField) {
		...
		if !httpguts.ValidHeaderFieldValue(hf.Value) {
			invalid = headerFieldValueError(hf.Value)
		}
		isPseudo := strings.HasPrefix(hf.Name, ":")
		if isPseudo {
			if sawRegular {
				invalid = errPseudoAfterRegular
			}
		} else {
			sawRegular = true
			if !validWireHeaderFieldName(hf.Name) {
				invalid = headerFieldNameError(hf.Name)
			}
		}

		if invalid != nil {
			hdec.SetEmitEnabled(false)
			return
		}
		
		// 限制头部大小
		size := hf.Size()
		if size > remainSize {
			hdec.SetEmitEnabled(false)
			mh.Truncated = true
			return
		}
		remainSize -= size

		mh.Fields = append(mh.Fields, hf)
	})
	
	...
	var hc headersOrContinuation = hf
	for {
		frag := hc.HeaderBlockFragment()
		// 解码器写入
		if _, err := hdec.Write(frag); err != nil {
			return nil, ConnectionError(ErrCodeCompression)
		}

		// END_HEADERS 标识
		if hc.HeadersEnded() {
			break
		}
		
		if f, err := fr.ReadFrame(); err != nil {
			return nil, err
		} else {
			hc = f.(*ContinuationFrame) // guaranteed by checkFrameOrder
		}
	}
	...
}

func (fr *Framer) readMetaFrame(hf *HeadersFrame) (*MetaHeadersFrame, error) {
	...
	var remainSize = fr.maxHeaderListSize()
	var invalid error // pseudo header field errors
	hdec := fr.ReadMetaHeaders
	hdec.SetEmitEnabled(true)
	hdec.SetMaxStringLength(fr.maxHeaderStringLen())
	hdec.SetEmitFunc(func(hf hpack.HeaderField) {
		if VerboseLogs && fr.logReads {
			fr.debugReadLoggerf("http2: decoded hpack field %+v", hf)
		}
		if !httpguts.ValidHeaderFieldValue(hf.Value) {
			// Don't include the value in the error, because it may be sensitive.
			invalid = headerFieldValueError(hf.Name)
		}
		isPseudo := strings.HasPrefix(hf.Name, ":")
		if isPseudo {
			if sawRegular {
				invalid = errPseudoAfterRegular
			}
		} else {
			sawRegular = true
			if !validWireHeaderFieldName(hf.Name) {
				invalid = headerFieldNameError(hf.Name)
			}
		}

		if invalid != nil {
			hdec.SetEmitEnabled(false)
			return
		}

		size := hf.Size()
		if size > remainSize {
			hdec.SetEmitEnabled(false)
			mh.Truncated = true
			remainSize = 0
			return
		}
		remainSize -= size

		mh.Fields = append(mh.Fields, hf)
	})
	// Lose reference to MetaHeadersFrame:
	defer hdec.SetEmitFunc(func(hf hpack.HeaderField) {})

	var hc headersOrContinuation = hf
	for {
		frag := hc.HeaderBlockFragment()

		// 打断条件添加了 remainSize 的判断
		// 在头部超出 **MAX_HEADER_LIST_SIZE** 限制后，remainSize 会变成 0
		if int64(len(frag)) > int64(2*remainSize) {
			if VerboseLogs {
				log.Printf("http2: header list too large")
			}
			return nil, ConnectionError(ErrCodeProtocol)
		}

		// 添加了 emit 回调方法中其他异常对for循环的打断
		if invalid != nil {
			if VerboseLogs {
				log.Printf("http2: invalid header: %v", invalid)
			}
			return nil, ConnectionError(ErrCodeProtocol)
		}
		
		if _, err := hdec.Write(frag); err != nil {
			return nil, ConnectionError(ErrCodeCompression)
		}

		if hc.HeadersEnded() {
			break
		}
		if f, err := fr.ReadFrame(); err != nil {
			return nil, err
		} else {
			hc = f.(*ContinuationFrame) // guaranteed by checkFrameOrder
		}
	}

}