for {
// For all but the first request, create the next
// request hop and replace req.
if len(reqs) > 0 {
		loc := resp.Header.Get("Location")
// ...此处省略代码...
		err = c.checkRedirect(req, reqs)
// ...此处省略很多代码...
	}

	reqs = append(reqs, req)
var err error
var didTimeout func() bool
if resp, didTimeout, err = c.send(req, deadline); err != nil {
// c.send() always closes req.Body
		reqBodyClosed = true
// ...此处省略代码...
return nil, uerr(err)
	}

var shouldRedirect bool
	redirectMethod, shouldRedirect, includeBody = redirectBehavior(req.Method, resp, reqs[0])
if !shouldRedirect {
return resp, nil
	}

	req.closeBody()
}

func defaultCheckRedirect(req *Request, via []*Request) error {
if len(via) >= 10 {
return errors.New("stopped after 10 redirects")
	}
return nil
}
// ...
func (c *Client) checkRedirect(req *Request, via []*Request) error {
	fn := c.CheckRedirect
if fn == nil {
		fn = defaultCheckRedirect
	}
return fn(req, via)
}

// didTimeout is non-nil only if err != nil.
func (c *Client) send(req *Request, deadline time.Time) (resp *Response, didTimeout func() bool, err error) {
if c.Jar != nil {
for _, cookie := range c.Jar.Cookies(req.URL) {
			req.AddCookie(cookie)
		}
	}
	resp, didTimeout, err = send(req, c.transport(), deadline)
if err != nil {
return nil, didTimeout, err
	}
if c.Jar != nil {
if rc := resp.Cookies(); len(rc) > 0 {
			c.Jar.SetCookies(req.URL, rc)
		}
	}
return resp, nil, nil
}

func (c *Client) transport() RoundTripper {
if c.Transport != nil {
return c.Transport
	}
return DefaultTransport
}

func send(ireq *Request, rt RoundTripper, deadline time.Time) (resp *Response, didTimeout func() bool, err error) {
	req := ireq // req is either the original request, or a modified fork
// ...此处省略代码...
if u := req.URL.User; u != nil && req.Header.Get("Authorization") == "" {
		username := u.Username()
		password, _ := u.Password()
		forkReq()
		req.Header = cloneOrMakeHeader(ireq.Header)
		req.Header.Set("Authorization", "Basic "+basicAuth(username, password))
	}

if !deadline.IsZero() {
		forkReq()
	}
	stopTimer, didTimeout := setRequestCancel(req, rt, deadline)

	resp, err = rt.RoundTrip(req)
if err != nil {
// ...此处省略代码...
return nil, didTimeout, err
	}
// ...此处省略代码...
return resp, nil, nil
}

func (t *Transport) RoundTrip(req *Request) (*Response, error) {
return t.roundTrip(req)
}
func (t *Transport) roundTrip(req *Request) (*Response, error) {
// ...此处省略校验header头和headervalue的代码以及其他代码...

for {
select {
case <-ctx.Done():
			req.closeBody()
return nil, ctx.Err()
default:
		}

// treq gets modified by roundTrip, so we need to recreate for each retry.
		treq := &transportRequest{Request: req, trace: trace}
		cm, err := t.connectMethodForRequest(treq)
// ...此处省略代码...
		pconn, err := t.getConn(treq, cm)
if err != nil {
			t.setReqCanceler(req, nil)
			req.closeBody()
return nil, err
		}

var resp *Response
if pconn.alt != nil {
// HTTP/2 path.
			t.setReqCanceler(req, nil) // not cancelable with CancelRequest
			resp, err = pconn.alt.RoundTrip(req)
		} else {
			resp, err = pconn.roundTrip(treq)
		}
if err == nil {
return resp, nil
		}

// ...此处省略判断是否重试请求的代码逻辑...
	}
}

func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (pc *persistConn, err error) {
	req := treq.Request
	trace := treq.trace
	ctx := req.Context()
// ...此处省略代码...
	w := &wantConn{
		cm:         cm,
		key:        cm.key(),
		ctx:        ctx,
		ready:      make(chan struct{}, 1),
		beforeDial: testHookPrePendingDial,
		afterDial:  testHookPostPendingDial,
	}
// ...此处省略代码...
// Queue for idle connection.
if delivered := t.queueForIdleConn(w); delivered {
		pc := w.pc
// ...此处省略代码...
return pc, nil
	}

	cancelc := make(chan error, 1)
	t.setReqCanceler(req, func(err error) { cancelc <- err })

// Queue for permission to dial.
	t.queueForDial(w)

// Wait for completion or cancellation.
select {
case <-w.ready:
// Trace success but only for HTTP/1.
// HTTP/2 calls trace.GotConn itself.
if w.pc != nil && w.pc.alt == nil && trace != nil && trace.GotConn != nil {
			trace.GotConn(httptrace.GotConnInfo{Conn: w.pc.conn, Reused: w.pc.isReused()})
		}
// ...此处省略代码...
return w.pc, w.err
case <-req.Cancel:
return nil, errRequestCanceledConn
case <-req.Context().Done():
return nil, req.Context().Err()
case err := <-cancelc:
if err == errRequestCanceled {
			err = errRequestCanceledConn
		}
return nil, err
	}
}

type connectMethodKey struct {
	proxy, scheme, addr string
	onlyH1              bool
}

func (t *Transport) queueForIdleConn(w *wantConn) (delivered bool) {
// ...此处省略代码...
// Look for most recently-used idle connection.
if list, ok := t.idleConn[w.key]; ok {
		stop := false
		delivered := false
for len(list) > 0 && !stop {
			pconn := list[len(list)-1]

// See whether this connection has been idle too long, considering
// only the wall time (the Round(0)), in case this is a laptop or VM
// coming out of suspend with previously cached idle connections.
			tooOld := !oldTime.IsZero() && pconn.idleAt.Round(0).Before(oldTime)
// ...此处省略代码...
			delivered = w.tryDeliver(pconn, nil)
if delivered {
// ...此处省略代码...
			}
			stop = true
		}
if len(list) > 0 {
			t.idleConn[w.key] = list
		} else {
delete(t.idleConn, w.key)
		}
if stop {
return delivered
		}
	}

// Register to receive next connection that becomes idle.
if t.idleConnWait == nil {
		t.idleConnWait = make(map[connectMethodKey]wantConnQueue)
	}
	q := t.idleConnWait[w.key]
	q.cleanFront()
	q.pushBack(w)
	t.idleConnWait[w.key] = q
return false
}

func (t *Transport) queueForDial(w *wantConn) {
	w.beforeDial()
if t.MaxConnsPerHost <= 0 {
go t.dialConnFor(w)
return
	}

	t.connsPerHostMu.Lock()
defer t.connsPerHostMu.Unlock()

if n := t.connsPerHost[w.key]; n < t.MaxConnsPerHost {
if t.connsPerHost == nil {
			t.connsPerHost = make(map[connectMethodKey]int)
		}
		t.connsPerHost[w.key] = n + 1
go t.dialConnFor(w)
return
	}

if t.connsPerHostWait == nil {
		t.connsPerHostWait = make(map[connectMethodKey]wantConnQueue)
	}
	q := t.connsPerHostWait[w.key]
	q.cleanFront()
	q.pushBack(w)
	t.connsPerHostWait[w.key] = q
}

func (t *Transport) dialConnFor(w *wantConn) {
defer w.afterDial()

	pc, err := t.dialConn(w.ctx, w.cm)
	delivered := w.tryDeliver(pc, err)
if err == nil && (!delivered || pc.alt != nil) {
// pconn was not passed to w,
// or it is HTTP/2 and can be shared.
// Add to the idle connection pool.
		t.putOrCloseIdleConn(pc)
	}
if err != nil {
		t.decConnsPerHost(w.key)
	}
}

func (t *Transport) putOrCloseIdleConn(pconn *persistConn) {
if err := t.tryPutIdleConn(pconn); err != nil {
		pconn.close(err)
	}
}
func (t *Transport) tryPutIdleConn(pconn *persistConn) error {
if t.DisableKeepAlives || t.MaxIdleConnsPerHost < 0 {
return errKeepAlivesDisabled
	}
// ...此处省略代码...
	t.idleMu.Lock()
defer t.idleMu.Unlock()
// ...此处省略代码...

// Deliver pconn to goroutine waiting for idle connection, if any.
// (They may be actively dialing, but this conn is ready first.
// Chrome calls this socket late binding.
// See https://insouciant.org/tech/connection-management-in-chromium/.)
	key := pconn.cacheKey
if q, ok := t.idleConnWait[key]; ok {
		done := false
if pconn.alt == nil {
// HTTP/1.
// Loop over the waiting list until we find a w that isn't done already, and hand it pconn.
for q.len() > 0 {
				w := q.popFront()
if w.tryDeliver(pconn, nil) {
					done = true
break
				}
			}
		} else {
// HTTP/2.
// Can hand the same pconn to everyone in the waiting list,
// and we still won't be done: we want to put it in the idle
// list unconditionally, for any future clients too.
for q.len() > 0 {
				w := q.popFront()
				w.tryDeliver(pconn, nil)
			}
		}
if q.len() == 0 {
delete(t.idleConnWait, key)
		} else {
			t.idleConnWait[key] = q
		}
if done {
return nil
		}
	}

if t.closeIdle {
return errCloseIdle
	}
if t.idleConn == nil {
		t.idleConn = make(map[connectMethodKey][]*persistConn)
	}
	idles := t.idleConn[key]
if len(idles) >= t.maxIdleConnsPerHost() {
return errTooManyIdleHost
	}
// ...此处省略代码...
	t.idleConn[key] = append(idles, pconn)
	t.idleLRU.add(pconn)
// ...此处省略代码...
// Set idle timer, but only for HTTP/1 (pconn.alt == nil).
// The HTTP/2 implementation manages the idle timer itself
// (see idleConnTimeout in h2_bundle.go).
if t.IdleConnTimeout > 0 && pconn.alt == nil {
if pconn.idleTimer != nil {
			pconn.idleTimer.Reset(t.IdleConnTimeout)
		} else {
			pconn.idleTimer = time.AfterFunc(t.IdleConnTimeout, pconn.closeConnIfStillIdle)
		}
	}
	pconn.idleAt = time.Now()
return nil
}
func (t *Transport) maxIdleConnsPerHost() int {
if v := t.MaxIdleConnsPerHost; v != 0 {
return v
	}
return DefaultMaxIdleConnsPerHost // 2
}

func (t *Transport) decConnsPerHost(key connectMethodKey) {
// ...此处省略代码...
	t.connsPerHostMu.Lock()
defer t.connsPerHostMu.Unlock()
	n := t.connsPerHost[key]
// ...此处省略代码...

// Can we hand this count to a goroutine still waiting to dial?
// (Some goroutines on the wait list may have timed out or
// gotten a connection another way. If they're all gone,
// we don't want to kick off any spurious dial operations.)
if q := t.connsPerHostWait[key]; q.len() > 0 {
		done := false
for q.len() > 0 {
			w := q.popFront()
if w.waiting() {
go t.dialConnFor(w)
				done = true
break
			}
		}
if q.len() == 0 {
delete(t.connsPerHostWait, key)
		} else {
// q is a value (like a slice), so we have to store
// the updated q back into the map.
			t.connsPerHostWait[key] = q
		}
if done {
return
		}
	}

// Otherwise, decrement the recorded count.
if n--; n == 0 {
delete(t.connsPerHost, key)
	} else {
		t.connsPerHost[key] = n
	}
}

func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (pconn *persistConn, err error) {
	pconn = &persistConn{
		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{}),
	}
// ...此处省略代码...
if cm.scheme() == "https" && t.hasCustomTLSDialer() {
// ...此处省略代码...
	} else {
		conn, err := t.dial(ctx, "tcp", cm.addr())
if err != nil {
return nil, wrapErr(err)
		}
		pconn.conn = conn
if cm.scheme() == "https" {
var firstTLSHost string
if firstTLSHost, _, err = net.SplitHostPort(cm.addr()); err != nil {
return nil, wrapErr(err)
			}
if err = pconn.addTLS(firstTLSHost, trace); err != nil {
return nil, wrapErr(err)
			}
		}
	}

// Proxy setup.
switch { // ...此处省略代码... }

if cm.proxyURL != nil && cm.targetScheme == "https" {
// ...此处省略代码...
	}

if s := pconn.tlsState; s != nil && s.NegotiatedProtocolIsMutual && s.NegotiatedProtocol != "" {
// ...此处省略代码...
	}

	pconn.br = bufio.NewReaderSize(pconn, t.readBufferSize())
	pconn.bw = bufio.NewWriterSize(persistConnWriter{pconn}, t.writeBufferSize())

go pconn.readLoop()
go pconn.writeLoop()
return pconn, nil
}
func (t *Transport) writeBufferSize() int {
if t.WriteBufferSize > 0 {
return t.WriteBufferSize
	}
return 4 << 10
}

func (t *Transport) readBufferSize() int {
if t.ReadBufferSize > 0 {
return t.ReadBufferSize
	}
return 4 << 10
}

func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err error) {
// ...此处省略代码...

	gone := make(chan struct{})
defer close(gone)
// ...此处省略代码...
const debugRoundTrip = false

// Write the request concurrently with waiting for a response,
// in case the server decides to reply before reading our full
// request body.
	startBytesWritten := pc.nwrite
	writeErrCh := make(chan error, 1)
	pc.writech <- writeRequest{req, writeErrCh, continueCh}

	resc := make(chan responseAndError)
	pc.reqch <- requestAndChan{
		req:        req.Request,
		ch:         resc,
		addedGzip:  requestedGzip,
		continueCh: continueCh,
		callerGone: gone,
	}

var respHeaderTimer <-chan time.Time
	cancelChan := req.Request.Cancel
	ctxDoneChan := req.Context().Done()
for {
		testHookWaitResLoop()
select {
case err := <-writeErrCh:
// ...此处省略代码...
if err != nil {
				pc.close(fmt.Errorf("write error: %v", err))
return nil, pc.mapRoundTripError(req, startBytesWritten, err)
			}
// ...此处省略代码...
case <-pc.closech:
// ...此处省略代码...
return nil, pc.mapRoundTripError(req, startBytesWritten, pc.closed)
case <-respHeaderTimer:
// ...此处省略代码...
return nil, errTimeout
case re := <-resc:
if (re.res == nil) == (re.err == nil) {
panic(fmt.Sprintf("internal error: exactly one of res or err should be set; nil=%v", re.res == nil))
			}
if debugRoundTrip {
				req.logf("resc recv: %p, %T/%#v", re.res, re.err, re.err)
			}
if re.err != nil {
return nil, pc.mapRoundTripError(req, startBytesWritten, re.err)
			}
return re.res, nil
case <-cancelChan:
			pc.t.CancelRequest(req.Request)
			cancelChan = nil
case <-ctxDoneChan:
			pc.t.cancelRequest(req.Request, req.Context().Err())
			cancelChan = nil
			ctxDoneChan = nil
		}
	}
}

func (pc *persistConn) writeLoop() {
defer close(pc.writeLoopDone)
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))
if bre, ok := err.(requestBodyReadError); ok {
				err = bre.error
				wr.req.setError(err)
			}
if err == nil {
				err = pc.bw.Flush()
			}
if err != nil {
				wr.req.Request.closeBody()
if pc.nwrite == startBytesWritten {
					err = nothingWrittenError{err}
				}
			}
			pc.writeErrCh <- err // to the body reader, which might recycle us
			wr.ch <- err         // to the roundTrip function
if err != nil {
				pc.close(err)
return
			}
case <-pc.closech:
return
		}
	}
}

func (pc *persistConn) readLoop() {
	closeErr := errReadLoopExiting // default value, if not changed below
defer func() {
		pc.close(closeErr)
		pc.t.removeIdleConn(pc)
	}()

	tryPutIdleConn := func(trace *httptrace.ClientTrace) bool {
if err := pc.t.tryPutIdleConn(pc); err != nil {
// ...此处省略代码...
		}
// ...此处省略代码...
return true
	}
// ...此处省略代码...
	alive := true
for alive {
// ...此处省略代码...
	    rc := <-pc.reqch
	    trace := httptrace.ContextClientTrace(rc.req.Context())

var resp *Response
if err == nil {
			resp, err = pc.readResponse(rc, trace)
		} else {
			err = transportReadFromServerError{err}
			closeErr = err
		}

// ...此处省略代码...
		bodyWritable := resp.bodyIsWritable()
		hasBody := rc.req.Method != "HEAD" && resp.ContentLength != 0

if resp.Close || rc.req.Close || resp.StatusCode <= 199 || bodyWritable {
// Don't do keep-alive on error if either party requested a close
// or we get an unexpected informational (1xx) response.
// StatusCode 100 is already handled above.
			alive = false
		}

if !hasBody || bodyWritable {
// ...此处省略代码...
continue
		}

		waitForBodyRead := make(chan bool, 2)
		body := &bodyEOFSignal{
			body: resp.Body,
			earlyCloseFn: func() error {
				waitForBodyRead <- false
				<-eofc // will be closed by deferred call at the end of the function
return nil

			},
			fn: func(err error) error {
				isEOF := err == io.EOF
				waitForBodyRead <- isEOF
if isEOF {
					<-eofc // see comment above eofc declaration
				} else if err != nil {
if cerr := pc.canceled(); cerr != nil {
return cerr
					}
				}
return err
			},
		}

		resp.Body = body
// ...此处省略代码...

select {
case rc.ch <- responseAndError{res: resp}:
case <-rc.callerGone:
return
		}

// Before looping back to the top of this function and peeking on
// the bufio.Reader, wait for the caller goroutine to finish
// reading the response body. (or for cancellation or death)
select {
case bodyEOF := <-waitForBodyRead:
			pc.t.setReqCanceler(rc.req, nil) // before pc might return to idle pool
			alive = alive &&
				bodyEOF &&
				!pc.sawEOF &&
				pc.wroteRequest() &&
				tryPutIdleConn(trace)
if bodyEOF {
				eofc <- struct{}{}
			}
case <-rc.req.Cancel:
			alive = false
			pc.t.CancelRequest(rc.req)
case <-rc.req.Context().Done():
			alive = false
			pc.t.cancelRequest(rc.req, rc.req.Context().Err())
case <-pc.closech:
			alive = false
		}

		testHookReadLoopBeforeNextRead()
	}
}

func (es *bodyEOFSignal) Read(p []byte) (n int, err error) {
// ...此处省略代码...
	n, err = es.body.Read(p)
if err != nil {
		es.mu.Lock()
defer es.mu.Unlock()
if es.rerr == nil {
			es.rerr = err
		}
		err = es.condfn(err)
	}
return
}

func (es *bodyEOFSignal) Close() error {
	es.mu.Lock()
defer es.mu.Unlock()
if es.closed {
return nil
	}
	es.closed = true
if es.earlyCloseFn != nil && es.rerr != io.EOF {
return es.earlyCloseFn()
	}
	err := es.body.Close()
return es.condfn(err)
}

// caller must hold es.mu.
func (es *bodyEOFSignal) condfn(err error) error {
if es.fn == nil {
return err
	}
	err = es.fn(err)
	es.fn = nil
return err
}

io.CopyN(ioutil.Discard, resp.Body, 2 << 10)
resp.Body.Close()