Go initiates HTTP2.0 request process analysis (Part 2) - Data frame & flow control

func http2readFrameHeader(buf []byte, r io.Reader) (http2FrameHeader, error) {
	_, err := io.ReadFull(r, buf[:http2frameHeaderLen])
if err != nil {
return http2FrameHeader{}, err
	}
return http2FrameHeader{
		Length:   (uint32(buf[0])<<16 | uint32(buf[1])<<8 | uint32(buf[2])),
		Type:     http2FrameType(buf[3]),
		Flags:    http2Flags(buf[4]),
		StreamID: binary.BigEndian.Uint32(buf[5:]) & (1<<31 - 1),
		valid:    true,
	}, nil
}

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
)

const (
// Data Frame
	http2FlagDataEndStream http2Flags = 0x1

// Headers Frame
	http2FlagHeadersEndStream  http2Flags = 0x1

// Settings Frame
	http2FlagSettingsAck http2Flags = 0x1
// 此处省略定义其他数据帧标识符的代码
)

type http2flow struct {
// n is the number of DATA bytes we're allowed to send.
// A flow is kept both on a conn and a per-stream.
	n int32

// conn points to the shared connection-level flow that is
// shared by all streams on that conn. It is nil for the flow
// that's on the conn directly.
	conn *http2flow
}

func (f *http2flow) available() int32 {
	n := f.n
if f.conn != nil && f.conn.n < n {
		n = f.conn.n
	}
return n
}

func (f *http2flow) take(n int32) {
if n > f.available() {
panic("internal error: took too much")
	}
	f.n -= n
if f.conn != nil {
		f.conn.n -= n
	}
}

func (f *http2flow) add(n int32) bool {
	sum := f.n + n
if (sum > n) == (f.n > 0) {
		f.n = sum
return true
	}
return false
}

func (cc *http2ClientConn) readLoop() {
	rl := &http2clientConnReadLoop{cc: cc}
defer rl.cleanup()
	cc.readerErr = rl.run()
if ce, ok := cc.readerErr.(http2ConnectionError); ok {
		cc.wmu.Lock()
		cc.fr.WriteGoAway(0, http2ErrCode(ce), nil)
		cc.wmu.Unlock()
	}
}

func (rl *http2clientConnReadLoop) run() error {
	cc := rl.cc
	rl.closeWhenIdle = cc.t.disableKeepAlives() || cc.singleUse
	gotReply := false // ever saw a HEADERS reply
	gotSettings := false
for {
		f, err := cc.fr.ReadFrame()
// 此处省略代码
		maybeIdle := false // whether frame might transition us to idle

switch f := f.(type) {
case *http2MetaHeadersFrame:
			err = rl.processHeaders(f)
			maybeIdle = true
			gotReply = true
case *http2DataFrame:
			err = rl.processData(f)
			maybeIdle = true
case *http2GoAwayFrame:
			err = rl.processGoAway(f)
			maybeIdle = true
case *http2RSTStreamFrame:
			err = rl.processResetStream(f)
			maybeIdle = true
case *http2SettingsFrame:
			err = rl.processSettings(f)
case *http2PushPromiseFrame:
			err = rl.processPushPromise(f)
case *http2WindowUpdateFrame:
			err = rl.processWindowUpdate(f)
case *http2PingFrame:
			err = rl.processPing(f)
default:
			cc.logf("Transport: unhandled response frame type %T", f)
		}
if err != nil {
if http2VerboseLogs {
				cc.vlogf("http2: Transport conn %p received error from processing frame %v: %v", cc, http2summarizeFrame(f), err)
			}
return err
		}
if rl.closeWhenIdle && gotReply && maybeIdle {
			cc.closeIfIdle()
		}
	}
}

if f.IsAck() {
if cc.wantSettingsAck {
    cc.wantSettingsAck = false
return nil
  }
return http2ConnectionError(http2ErrCodeProtocol)
}

err := f.ForeachSetting(func(s http2Setting) error {
switch s.ID {
case http2SettingMaxFrameSize:
    cc.maxFrameSize = s.Val
case http2SettingMaxConcurrentStreams:
    cc.maxConcurrentStreams = s.Val
case http2SettingMaxHeaderListSize:
    cc.peerMaxHeaderListSize = uint64(s.Val)
case http2SettingInitialWindowSize:
if s.Val > math.MaxInt32 {
return http2ConnectionError(http2ErrCodeFlowControl)
    }
    delta := int32(s.Val) - int32(cc.initialWindowSize)
for _, cs := range cc.streams {
      cs.flow.add(delta)
    }
    cc.cond.Broadcast()
    cc.initialWindowSize = s.Val
default:
// TODO(bradfitz): handle more settings? SETTINGS_HEADER_TABLE_SIZE probably.
    cc.vlogf("Unhandled Setting: %v", s)
  }
return nil
})

	cc.wmu.Lock()
defer cc.wmu.Unlock()

	cc.fr.WriteSettingsAck()
	cc.bw.Flush()
return cc.werr

func (rl *http2clientConnReadLoop) processWindowUpdate(f *http2WindowUpdateFrame) error {
	cc := rl.cc
	cs := cc.streamByID(f.StreamID, false)
if f.StreamID != 0 && cs == nil {
return nil
	}

	cc.mu.Lock()
defer cc.mu.Unlock()

	fl := &cc.flow
if cs != nil {
		fl = &cs.flow
	}
if !fl.add(int32(f.Increment)) {
return http2ConnectionError(http2ErrCodeFlowControl)
	}
	cc.cond.Broadcast()
return nil
}

func (rl *http2clientConnReadLoop) processHeaders(f *http2MetaHeadersFrame) error {
	cc := rl.cc
	cs := cc.streamByID(f.StreamID, false)
// 此处省略代码
	res, err := rl.handleResponse(cs, f)
if err != nil {
// 此处省略代码
		cs.resc <- http2resAndError{err: err}
return nil // return nil from process* funcs to keep conn alive
	}
if res == nil {
// (nil, nil) special case. See handleResponse docs.
return nil
	}
	cs.resTrailer = &res.Trailer
	cs.resc <- http2resAndError{res: res}
return nil
}

header := make(Header)
res := &Response{
  Proto:      "HTTP/2.0",
  ProtoMajor: 2,
  Header:     header,
  StatusCode: statusCode,
  Status:     status + " " + StatusText(statusCode),
}

for _, hf := range f.RegularFields() {
  key := CanonicalHeaderKey(hf.Name)
if key == "Trailer" {
    t := res.Trailer
if t == nil {
      t = make(Header)
      res.Trailer = t
    }
    http2foreachHeaderElement(hf.Value, func(v string) {
      t[CanonicalHeaderKey(v)] = nil
    })
  } else {
    header[key] = append(header[key], hf.Value)
  }
}

streamEnded := f.StreamEnded()
isHead := cs.req.Method == "HEAD"
if !streamEnded || isHead {
  res.ContentLength = -1
if clens := res.Header["Content-Length"]; len(clens) == 1 {
if clen64, err := strconv.ParseInt(clens[0], 10, 64); err == nil {
      res.ContentLength = clen64
    } else {
// TODO: care? unlike http/1, it won't mess up our framing, so it's
// more safe smuggling-wise to ignore.
    }
  } else if len(clens) > 1 {
// TODO: care? unlike http/1, it won't mess up our framing, so it's
// more safe smuggling-wise to ignore.
  }
}

cs.bufPipe = http2pipe{b: &http2dataBuffer{expected: res.ContentLength}}
cs.bytesRemain = res.ContentLength
res.Body = http2transportResponseBody{cs}
go cs.awaitRequestCancel(cs.req)

if cs.requestedGzip && res.Header.Get("Content-Encoding") == "gzip" {
  res.Header.Del("Content-Encoding")
  res.Header.Del("Content-Length")
  res.ContentLength = -1
  res.Body = &http2gzipReader{body: res.Body}
  res.Uncompressed = true
}

cc := rl.cc
cs := cc.streamByID(f.StreamID, f.StreamEnded())
data := f.Data()
if cs == nil {
  cc.mu.Lock()
  neverSent := cc.nextStreamID
  cc.mu.Unlock()
// 此处省略代码
if f.Length > 0 {
    cc.mu.Lock()
    cc.inflow.add(int32(f.Length))
    cc.mu.Unlock()

    cc.wmu.Lock()
    cc.fr.WriteWindowUpdate(0, uint32(f.Length))
    cc.bw.Flush()
    cc.wmu.Unlock()
  }
return nil
}

if cs.req.Method == "HEAD" && len(data) > 0 {
  cc.logf("protocol error: received DATA on a HEAD request")
  rl.endStreamError(cs, http2StreamError{
    StreamID: f.StreamID,
    Code:     http2ErrCodeProtocol,
  })
return nil
}

cc.mu.Lock()
if cs.inflow.available() >= int32(f.Length) {
  cs.inflow.take(int32(f.Length))
} else {
  cc.mu.Unlock()
return http2ConnectionError(http2ErrCodeFlowControl)
}

var refund int
if pad := int(f.Length) - len(data); pad > 0 {
  refund += pad
}
// Return len(data) now if the stream is already closed,
// since data will never be read.
didReset := cs.didReset
if didReset {
  refund += len(data)
}
if refund > 0 {
  cc.inflow.add(int32(refund))
  cc.wmu.Lock()
  cc.fr.WriteWindowUpdate(0, uint32(refund))
if !didReset {
    cs.inflow.add(int32(refund))
    cc.fr.WriteWindowUpdate(cs.ID, uint32(refund))
  }
  cc.bw.Flush()
  cc.wmu.Unlock()
}
cc.mu.Unlock()

if len(data) > 0 && !didReset {
if _, err := cs.bufPipe.Write(data); err != nil {
    rl.endStreamError(cs, err)
return err
  }
}

cc.mu.Lock()
defer cc.mu.Unlock()
var connAdd, streamAdd int32
// Check the conn-level first, before the stream-level.
if v := cc.inflow.available(); v < http2transportDefaultConnFlow/2 {
  connAdd = http2transportDefaultConnFlow - v
  cc.inflow.add(connAdd)
}

if err == nil { // No need to refresh if the stream is over or failed.
// Consider any buffered body data (read from the conn but not
// consumed by the client) when computing flow control for this
// stream.
  v := int(cs.inflow.available()) + cs.bufPipe.Len()
if v < http2transportDefaultStreamFlow-http2transportDefaultStreamMinRefresh {
    streamAdd = int32(http2transportDefaultStreamFlow - v)
    cs.inflow.add(streamAdd)
  }
}

if connAdd != 0 || streamAdd != 0 {
  cc.wmu.Lock()
defer cc.wmu.Unlock()
if connAdd != 0 {
    cc.fr.WriteWindowUpdate(0, http2mustUint31(connAdd))
  }
if streamAdd != 0 {
    cc.fr.WriteWindowUpdate(cs.ID, http2mustUint31(streamAdd))
  }
  cc.bw.Flush()
}

func (cs *http2clientStream) writeRequestBody(body io.Reader, bodyCloser io.Closer) (err error) {
	cc := cs.cc
	sentEnd := false // whether we sent the final DATA frame w/ END_STREAM
// 此处省略代码
	req := cs.req
	hasTrailers := req.Trailer != nil
	remainLen := http2actualContentLength(req)
	hasContentLen := remainLen != -1

var sawEOF bool
for !sawEOF {
		n, err := body.Read(buf[:len(buf)-1])
// 此处省略代码
		remain := buf[:n]
for len(remain) > 0 && err == nil {
var allowed int32
			allowed, err = cs.awaitFlowControl(len(remain))
switch {
case err == http2errStopReqBodyWrite:
return err
case err == http2errStopReqBodyWriteAndCancel:
				cc.writeStreamReset(cs.ID, http2ErrCodeCancel, nil)
return err
case err != nil:
return err
			}
			cc.wmu.Lock()
			data := remain[:allowed]
			remain = remain[allowed:]
			sentEnd = sawEOF && len(remain) == 0 && !hasTrailers
			err = cc.fr.WriteData(cs.ID, sentEnd, data)
if err == nil {
				err = cc.bw.Flush()
			}
			cc.wmu.Unlock()
		}
if err != nil {
return err
		}
	}
// 此处省略代码
return err
}

func (cs *http2clientStream) awaitFlowControl(maxBytes int) (taken int32, err error) {
	cc := cs.cc
	cc.mu.Lock()
defer cc.mu.Unlock()
for {
if cc.closed {
return 0, http2errClientConnClosed
		}
if cs.stopReqBody != nil {
return 0, cs.stopReqBody
		}
if err := cs.checkResetOrDone(); err != nil {
return 0, err
		}
if a := cs.flow.available(); a > 0 {
			take := a
if int(take) > maxBytes {

				take = int32(maxBytes) // can't truncate int; take is int32
			}
if take > int32(cc.maxFrameSize) {
				take = int32(cc.maxFrameSize)
			}
			cs.flow.take(take)
return take, nil
		}
		cc.cond.Wait()
	}
}