Go的http client

go封裝了http客戶端，請求遠端資料非常方便，看些原始碼底層如何實現。

resp, err := http.Get("https://baidu.com") 
if err != nil {
    fmt.Printf("發起請求失敗：%v", err)
    return 
}

defer resp.Body.Close() 
io.Copy(os.Stdout, resp.Body)

請求的大致流程

1.根據請求條件，構建request物件

2.所有的client請求，都會經過client.do()處理

func (c *Client) do(req *Request) (retres *Response, reterr error) 

2.1 request請求使用client.send()處理

func (c *Client) send(req *Request, deadline time.Time) (resp *Response, didTimeout func() bool, err error)

resp, didTimeout, err = send(req, c.transport(), deadline)//預設傳DefaultTransport

3.send函式

func send(ireq *Request, rt RoundTripper, deadline time.Time) (resp *Response, didTimeout func() bool, err error) {
    resp, err = rt.RoundTrip(req) 
}

4.DefaultTransport的RoundTrip方法，實際就是Transport的RoundTrip方法

func (t *Transport) roundTrip(req *Request) (*Response, error) {
    treq := &transportRequest{Request: req, trace: trace} //封裝新的request
    cm, err := t.connectMethodForRequest(treq)
    pconn, err := t.getConn(treq, cm) //使用連線池技術，獲取連線物件*persistConn，
    resp, err = pconn.roundTrip(treq) //使用連線物件獲取response
}

5.使用連線池技術，獲取連線物件*persistConn

func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (pc *persistConn, err error) {
    w := &wantConn{ //構建連線物件
        cm:         cm,
        key:        cm.key(),
        ctx:        ctx,
        ready:      make(chan struct{}, 1),
        beforeDial: testHookPrePendingDial,
        afterDial:  testHookPostPendingDial,
    }
    if delivered := t.queueForIdleConn(w); delivered {//從連線池獲取符合的連線物件，有就返回
        pc := w.pc        
        return pc, nil
    }    
    t.queueForDial(w)//發起連線

    select {
    case <-w.ready:    //連線準備好，就返回連線物件    
        return w.pc, w.err

}

5.1 Transport.queueForDial發起連線

func (t *Transport) queueForDial(w *wantConn) {
    go t.dialConnFor(w)
}

5.2 發起撥號dialConnFor

func (t *Transport) dialConnFor(w *wantConn) {
    pc, err := t.dialConn(w.ctx, w.cm) //發起撥號，返回連線物件
    delivered := w.tryDeliver(pc, err)
}

5.3 發起撥號

func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (pconn *persistConn, err error) {
    pconn = &persistConn{ //構建連線物件
        t:             t,
        cacheKey:      cm.key(),
        reqch:         make(chan requestAndChan, 1),
        writech:       make(chan writeRequest, 1),
        closech:       make(chan struct{}),
        writeErrCh:    make(chan error, 1),
        writeLoopDone: make(chan struct{}),
    }
    conn, err := t.dial(ctx, "tcp", cm.addr()) //tcp連線，獲取到net.conn物件

    pconn.br = bufio.NewReaderSize(pconn, t.readBufferSize())//可以從conn讀
    pconn.bw = bufio.NewWriterSize(persistConnWriter{pconn}, t.writeBufferSize())//寫到conn

    go pconn.readLoop()//開啟讀協程
    go pconn.writeLoop()//開啟寫協程
    return pconn, nil
}

5.4讀協程,雖然是for迴圈，但是一次性就把請求的response讀完了，如果沒有關閉，就會造成協程洩露了

func (pc *persistConn) readLoop() {
    alive := true
    for alive {
        rc := <-pc.reqch //讀取request,寫入的地方在步驟6

        resp, err = pc.readResponse(rc, trace) //返回response
        //response的body是否可寫，伺服器code101才可寫，所以正常這個是false
        bodyWritable := resp.bodyIsWritable()

        //response.Close設定迴圈結束，退出協程
        if resp.Close || rc.req.Close || resp.StatusCode <= 199 || bodyWritable {                    alive = false
        }          

        //把response寫入通道，在步驟6會讀取這個通道
        select {
        case rc.ch <- responseAndError{res: resp}:
        case <-rc.callerGone:
            return
        }
        //迴圈結束的一些情況
        select {
        case bodyEOF := <-waitForBodyRead: //讀完body也會自動結束            
        case <-rc.req.Cancel:
        case <-rc.req.Context().Done():
        case <-pc.closech:
            alive = false
            pc.t.CancelRequest(rc.req)
        }
    }

5.4.1 pc.readResponse 獲取response

func (pc *persistConn) readResponse(rc requestAndChan, trace *httptrace.ClientTrace) (resp *Response, err error) {
    for{
        resp, err = ReadResponse(pc.br, rc.req) //獲取response
    }
}

5.4.2 ReadResponse讀取response

func ReadResponse(r *bufio.Reader, req *Request) (*Response, error) {
    tp := textproto.NewReader(r) //可以處理HTTP, NNTP, SMTP協議的內容，方便讀取
    resp := &Response{
        Request: req,
    }    
    line, err := tp.ReadLine()//讀取第一行，獲取協議，狀態碼
    resp.Proto = line[:i]
    resp.Status = strings.TrimLeft(line[i+1:], " ")

    mimeHeader, err := tp.ReadMIMEHeader()//讀取header頭
    resp.Header = Header(mimeHeader)
}

5.5 寫協程

func (pc *persistConn) writeLoop() {
    for {
        select {
        case wr := <-pc.writech:
            startBytesWritten := pc.nwrite
            err := wr.req.Request.write(pc.bw, pc.isProxy, wr.req.extra, pc.waitForContinue(wr.continueCh))            
    }
}

6.使用連線物件*persistConn獲取response

func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err error) {
    var continueCh chan struct{}
    resc := make(chan responseAndError) //response通道

    pc.writech <- writeRequest{req, writeErrCh, continueCh}//written by roundTrip; read by writeLoop   

    pc.reqch <- requestAndChan{ //written by roundTrip; read by readLoop
        req:        req.Request,
        ch:         resc,
        addedGzip:  requestedGzip,
        continueCh: continueCh,
        callerGone: gone,
    }
    for { //監聽這些通道
        testHookWaitResLoop()
        select {
        case err := <-writeErrCh:            
        case <-pc.closech:            
        case re := <-resc: //監聽 response通道，返回response         
            return re.res, nil
        }
    }
}

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