golang http timeout源码解析

package main

import (
	"fmt"
	"net/http"
	"net/url"
	"time"
	"net"
)

func send_http_request(addr string, port int) error {
	client := &http.Client{
		Transport: &http.Transport{
			Proxy: http.ProxyFromEnvironment,
			DialContext: (&net.Dialer{
				Timeout:   30 * time.Second,
				KeepAlive: 30 * time.Second,
				DualStack: true,
			}).DialContext,
			MaxIdleConns:        100,
			MaxIdleConnsPerHost: 100,
			IdleConnTimeout:     90 * time.Second,
		},
	}

	// construct encoded endpoint
	Url, err := url.Parse(fmt.Sprintf("http://%s:%d", addr, port))
	if err != nil {
		return err
	}
	Url.Path += "/index"
	endpoint := Url.String()
	req, err := http.NewRequest("GET", endpoint, nil)
	if err != nil {
		return err
	}
	// use httpClient to send request
	rsp, err := client.Do(req)
	if err != nil {
		return err
	}
	// close the connection to reuse it
	defer rsp.Body.Close()
	// check status code
	if rsp.StatusCode != http.StatusOK {
		return fmt.Errorf("get rsp error: %v", rsp)
	}
	return err
}

func main() {
	send_http_request("xxx", 8080)
}

func (c *Client) do(req *Request) (retres *Response, reterr error) {
	if testHookClientDoResult != nil {
		defer func() { testHookClientDoResult(retres, reterr) }()
	}
	if req.URL == nil {
		req.closeBody()
		return nil, &url.Error{
			Op:  urlErrorOp(req.Method),
			Err: errors.New("http: nil Request.URL"),
		}
	}

	var (
		deadline      = c.deadline()
		reqs          []*Request
		resp          *Response
		copyHeaders   = c.makeHeadersCopier(req)
		reqBodyClosed = false // have we closed the current req.Body?

		// Redirect behavior:
		redirectMethod string
		includeBody    bool
	)
	uerr := func(err error) error {
		// the body may have been closed already by c.send()
		if !reqBodyClosed {
			req.closeBody()
		}
		var urlStr string
		if resp != nil && resp.Request != nil {
			urlStr = stripPassword(resp.Request.URL)
		} else {
			urlStr = stripPassword(req.URL)
		}
		return &url.Error{
			Op:  urlErrorOp(reqs[0].Method),
			URL: urlStr,
			Err: err,
		}
	}
	for {
		// For all but the first request, create the next
		// request hop and replace req.
		if len(reqs) > 0 {
			loc := resp.Header.Get("Location")
			if loc == "" {
				resp.closeBody()
				return nil, uerr(fmt.Errorf("%d response missing Location header", resp.StatusCode))
			}
			u, err := req.URL.Parse(loc)
			if err != nil {
				resp.closeBody()
				return nil, uerr(fmt.Errorf("failed to parse Location header %q: %v", loc, err))
			}
			host := ""
			if req.Host != "" && req.Host != req.URL.Host {
				// If the caller specified a custom Host header and the
				// redirect location is relative, preserve the Host header
				// through the redirect. See issue #22233.
				if u, _ := url.Parse(loc); u != nil && !u.IsAbs() {
					host = req.Host
				}
			}
			ireq := reqs[0]
			req = &Request{
				Method:   redirectMethod,
				Response: resp,
				URL:      u,
				Header:   make(Header),
				Host:     host,
				Cancel:   ireq.Cancel,
				ctx:      ireq.ctx,
			}
			if includeBody && ireq.GetBody != nil {
				req.Body, err = ireq.GetBody()
				if err != nil {
					resp.closeBody()
					return nil, uerr(err)
				}
				req.ContentLength = ireq.ContentLength
			}

			// Copy original headers before setting the Referer,
			// in case the user set Referer on their first request.
			// If they really want to override, they can do it in
			// their CheckRedirect func.
			copyHeaders(req)

			// Add the Referer header from the most recent
			// request URL to the new one, if it's not https->http:
			if ref := refererForURL(reqs[len(reqs)-1].URL, req.URL); ref != "" {
				req.Header.Set("Referer", ref)
			}
			err = c.checkRedirect(req, reqs)

			// Sentinel error to let users select the
			// previous response, without closing its
			// body. See Issue 10069.
			if err == ErrUseLastResponse {
				return resp, nil
			}

			// Close the previous response's body. But
			// read at least some of the body so if it's
			// small the underlying TCP connection will be
			// re-used. No need to check for errors: if it
			// fails, the Transport won't reuse it anyway.
			const maxBodySlurpSize = 2 << 10
			if resp.ContentLength == -1 || resp.ContentLength <= maxBodySlurpSize {
				io.CopyN(ioutil.Discard, resp.Body, maxBodySlurpSize)
			}
			resp.Body.Close()

			if err != nil {
				// Special case for Go 1 compatibility: return both the response
				// and an error if the CheckRedirect function failed.
				// See https://golang.org/issue/3795
				// The resp.Body has already been closed.
				ue := uerr(err)
				ue.(*url.Error).URL = loc
				return resp, ue
			}
		}

		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
			if !deadline.IsZero() && didTimeout() {
				err = &httpError{
					// TODO: early in cycle: s/Client.Timeout exceeded/timeout or context cancellation/
					err:     err.Error() + " (Client.Timeout exceeded while awaiting headers)",
					timeout: true,
				}
			}
			return nil, uerr(err)
		}

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

		req.closeBody()
	}
}

func (c *Client) deadline() time.Time {
	if c.Timeout > 0 {
		return time.Now().Add(c.Timeout)
	}
	return time.Time{}
}

// 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
}

// send issues an HTTP request.
// Caller should close resp.Body when done reading from it.
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

	if rt == nil {
		req.closeBody()
		return nil, alwaysFalse, errors.New("http: no Client.Transport or DefaultTransport")
	}

	if req.URL == nil {
		req.closeBody()
		return nil, alwaysFalse, errors.New("http: nil Request.URL")
	}

	if req.RequestURI != "" {
		req.closeBody()
		return nil, alwaysFalse, errors.New("http: Request.RequestURI can't be set in client requests.")
	}

	// forkReq forks req into a shallow clone of ireq the first
	// time it's called.
	forkReq := func() {
		if ireq == req {
			req = new(Request)
			*req = *ireq // shallow clone
		}
	}

	// Most the callers of send (Get, Post, et al) don't need
	// Headers, leaving it uninitialized. We guarantee to the
	// Transport that this has been initialized, though.
	if req.Header == nil {
		forkReq()
		req.Header = make(Header)
	}

	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 {
		stopTimer()
		if resp != nil {
			log.Printf("RoundTripper returned a response & error; ignoring response")
		}
		if tlsErr, ok := err.(tls.RecordHeaderError); ok {
			// If we get a bad TLS record header, check to see if the
			// response looks like HTTP and give a more helpful error.
			// See golang.org/issue/11111.
			if string(tlsErr.RecordHeader[:]) == "HTTP/" {
				err = errors.New("http: server gave HTTP response to HTTPS client")
			}
		}
		return nil, didTimeout, err
	}
	if !deadline.IsZero() {
		resp.Body = &cancelTimerBody{
			stop:          stopTimer,
			rc:            resp.Body,
			reqDidTimeout: didTimeout,
		}
	}
	return resp, nil, nil
}

// setRequestCancel sets the Cancel field of req, if deadline is
// non-zero. The RoundTripper's type is used to determine whether the legacy
// CancelRequest behavior should be used.
//
// As background, there are three ways to cancel a request:
// First was Transport.CancelRequest. (deprecated)
// Second was Request.Cancel (this mechanism).
// Third was Request.Context.
func setRequestCancel(req *Request, rt RoundTripper, deadline time.Time) (stopTimer func(), didTimeout func() bool) {
	if deadline.IsZero() {
		return nop, alwaysFalse
	}

	initialReqCancel := req.Cancel // the user's original Request.Cancel, if any

	cancel := make(chan struct{})
	req.Cancel = cancel

	doCancel := func() {
		// The newer way (the second way in the func comment):
		close(cancel)

		// The legacy compatibility way, used only
		// for RoundTripper implementations written
		// before Go 1.5 or Go 1.6.
		type canceler interface {
			CancelRequest(*Request)
		}
		switch v := rt.(type) {
		case *Transport, *http2Transport:
			// Do nothing. The net/http package's transports
			// support the new Request.Cancel channel
		case canceler:
			v.CancelRequest(req)
		}
	}

	stopTimerCh := make(chan struct{})
	var once sync.Once
	stopTimer = func() { once.Do(func() { close(stopTimerCh) }) }

	timer := time.NewTimer(time.Until(deadline))
	var timedOut atomicBool

	go func() {
		select {
		case <-initialReqCancel:
			doCancel()
			timer.Stop()
		case <-timer.C:
			timedOut.setTrue()
			doCancel()
		case <-stopTimerCh:
			timer.Stop()
		}
	}()

	return stopTimer, timedOut.isSet
}

timer := time.NewTimer(time.Until(deadline))
var timedOut atomicBool

go func() {
    select {
    case <-initialReqCancel:
        doCancel()
        timer.Stop()
    case <-timer.C:
        timedOut.setTrue()
        doCancel()
    case <-stopTimerCh:
        timer.Stop()
    }
}()

cancel := make(chan struct{})
req.Cancel = cancel

doCancel := func() {
    // The newer way (the second way in the func comment):
    close(cancel)

    // The legacy compatibility way, used only
    // for RoundTripper implementations written
    // before Go 1.5 or Go 1.6.
    type canceler interface {
        CancelRequest(*Request)
    }
    switch v := rt.(type) {
    case *Transport, *http2Transport:
        // Do nothing. The net/http package's transports
        // support the new Request.Cancel channel
    case canceler:
        v.CancelRequest(req)
    }
}

// send issues an HTTP request.
// Caller should close resp.Body when done reading from it.
func send(ireq *Request, rt RoundTripper, deadline time.Time) (resp *Response, didTimeout func() bool, err error) {
    ...
	if !deadline.IsZero() {
		forkReq()
	}
	stopTimer, didTimeout := setRequestCancel(req, rt, deadline)

	resp, err = rt.RoundTrip(req)
	if err != nil {
		stopTimer()
		if resp != nil {
			log.Printf("RoundTripper returned a response & error; ignoring response")
		}
		if tlsErr, ok := err.(tls.RecordHeaderError); ok {
			// If we get a bad TLS record header, check to see if the
			// response looks like HTTP and give a more helpful error.
			// See golang.org/issue/11111.
			if string(tlsErr.RecordHeader[:]) == "HTTP/" {
				err = errors.New("http: server gave HTTP response to HTTPS client")
			}
		}
		return nil, didTimeout, err
	}
	if !deadline.IsZero() {
		resp.Body = &cancelTimerBody{
			stop:          stopTimer,
			rc:            resp.Body,
			reqDidTimeout: didTimeout,
		}
	}
	return resp, nil, nil
}

func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err error) {
    ...
	if !pc.t.replaceReqCanceler(req.Request, pc.cancelRequest) {
		pc.t.putOrCloseIdleConn(pc)
		return nil, errRequestCanceled
	}
	defer func() {
		if err != nil {
			pc.t.setReqCanceler(req.Request, nil)
		}
	}()

	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 <-pc.closech:
			if debugRoundTrip {
				req.logf("closech recv: %T %#v", pc.closed, pc.closed)
			}
			return nil, pc.mapRoundTripError(req, startBytesWritten, pc.closed)
		case <-respHeaderTimer:
			if debugRoundTrip {
				req.logf("timeout waiting for response headers.")
			}
			pc.close(errTimeout)
			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
		}
	}
}

type Transport struct {
	idleMu       sync.Mutex
	closeIdle    bool                                // user has requested to close all idle conns
	idleConn     map[connectMethodKey][]*persistConn // most recently used at end
	idleConnWait map[connectMethodKey]wantConnQueue  // waiting getConns
	idleLRU      connLRU

	reqMu       sync.Mutex
	reqCanceler map[*Request]func(error)
    ...
}

// replaceReqCanceler replaces an existing cancel function. If there is no cancel function
// for the request, we don't set the function and return false.
// Since CancelRequest will clear the canceler, we can use the return value to detect if
// the request was canceled since the last setReqCancel call.
func (t *Transport) replaceReqCanceler(r *Request, fn func(error)) bool {
	t.reqMu.Lock()
	defer t.reqMu.Unlock()
	_, ok := t.reqCanceler[r]
	if !ok {
		return false
	}
	if fn != nil {
		t.reqCanceler[r] = fn
	} else {
		delete(t.reqCanceler, r)
	}
	return true
}

// CancelRequest cancels an in-flight request by closing its connection.
// CancelRequest should only be called after RoundTrip has returned.
//
// Deprecated: Use Request.WithContext to create a request with a
// cancelable context instead. CancelRequest cannot cancel HTTP/2
// requests.
func (t *Transport) CancelRequest(req *Request) {
	t.cancelRequest(req, errRequestCanceled)
}

// Cancel an in-flight request, recording the error value.
func (t *Transport) cancelRequest(req *Request, err error) {
	t.reqMu.Lock()
	cancel := t.reqCanceler[req]
	delete(t.reqCanceler, req)
	t.reqMu.Unlock()
	if cancel != nil {
		cancel(err)
	}
}

func (pc *persistConn) cancelRequest(err error) {
	pc.mu.Lock()
	defer pc.mu.Unlock()
	pc.canceledErr = err
	pc.closeLocked(errRequestCanceled)
}

func (pc *persistConn) closeLocked(err error) {
	if err == nil {
		panic("nil error")
	}
	pc.broken = true
	if pc.closed == nil {
		pc.closed = err
		pc.t.decConnsPerHost(pc.cacheKey)
		// Close HTTP/1 (pc.alt == nil) connection.
		// HTTP/2 closes its connection itself.
		if pc.alt == nil {
			if err != errCallerOwnsConn {
				pc.conn.Close()
			}
			close(pc.closech)
		}
	}
	pc.mutateHeaderFunc = nil
}

// decConnsPerHost decrements the per-host connection count for key,
// which may in turn give a different waiting goroutine permission to dial.
func (t *Transport) decConnsPerHost(key connectMethodKey) {
	if t.MaxConnsPerHost <= 0 {
		return
	}

	t.connsPerHostMu.Lock()
	defer t.connsPerHostMu.Unlock()
	n := t.connsPerHost[key]
	if n == 0 {
		// Shouldn't happen, but if it does, the counting is buggy and could
		// easily lead to a silent deadlock, so report the problem loudly.
		panic("net/http: internal error: connCount underflow")
	}

	// 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 (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err error) {
	testHookEnterRoundTrip()
	if !pc.t.replaceReqCanceler(req.Request, pc.cancelRequest) {
		pc.t.putOrCloseIdleConn(pc)
		return nil, errRequestCanceled
	}
	pc.mu.Lock()
	pc.numExpectedResponses++
	headerFn := pc.mutateHeaderFunc
	pc.mu.Unlock()

	if headerFn != nil {
		headerFn(req.extraHeaders())
	}

	// Ask for a compressed version if the caller didn't set their
	// own value for Accept-Encoding. We only attempt to
	// uncompress the gzip stream if we were the layer that
	// requested it.
	requestedGzip := false
	if !pc.t.DisableCompression &&
		req.Header.Get("Accept-Encoding") == "" &&
		req.Header.Get("Range") == "" &&
		req.Method != "HEAD" {
		// Request gzip only, not deflate. Deflate is ambiguous and
		// not as universally supported anyway.
		// See: https://zlib.net/zlib_faq.html#faq39
		//
		// Note that we don't request this for HEAD requests,
		// due to a bug in nginx:
		//   https://trac.nginx.org/nginx/ticket/358
		//   https://golang.org/issue/5522
		//
		// We don't request gzip if the request is for a range, since
		// auto-decoding a portion of a gzipped document will just fail
		// anyway. See https://golang.org/issue/8923
		requestedGzip = true
		req.extraHeaders().Set("Accept-Encoding", "gzip")
	}

	var continueCh chan struct{}
	if req.ProtoAtLeast(1, 1) && req.Body != nil && req.expectsContinue() {
		continueCh = make(chan struct{}, 1)
	}

	if pc.t.DisableKeepAlives && !req.wantsClose() {
		req.extraHeaders().Set("Connection", "close")
	}

	gone := make(chan struct{})
	defer close(gone)

	defer func() {
		if err != nil {
			pc.t.setReqCanceler(req.Request, nil)
		}
	}()

	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:
			if debugRoundTrip {
				req.logf("writeErrCh resv: %T/%#v", err, err)
			}
			if err != nil {
				pc.close(fmt.Errorf("write error: %v", err))
				return nil, pc.mapRoundTripError(req, startBytesWritten, err)
			}
			if d := pc.t.ResponseHeaderTimeout; d > 0 {
				if debugRoundTrip {
					req.logf("starting timer for %v", d)
				}
				timer := time.NewTimer(d)
				defer timer.Stop() // prevent leaks
				respHeaderTimer = timer.C
			}
		case <-pc.closech:
			if debugRoundTrip {
				req.logf("closech recv: %T %#v", pc.closed, pc.closed)
			}
			return nil, pc.mapRoundTripError(req, startBytesWritten, pc.closed)
		case <-respHeaderTimer:
			if debugRoundTrip {
				req.logf("timeout waiting for response headers.")
			}
			pc.close(errTimeout)
			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
		}
	}
}

// mapRoundTripError returns the appropriate error value for
// persistConn.roundTrip.
//
// The provided err is the first error that (*persistConn).roundTrip
// happened to receive from its select statement.
//
// The startBytesWritten value should be the value of pc.nwrite before the roundTrip
// started writing the request.
func (pc *persistConn) mapRoundTripError(req *transportRequest, startBytesWritten int64, err error) error {
	if err == nil {
		return nil
	}

	// If the request was canceled, that's better than network
	// failures that were likely the result of tearing down the
	// connection.
	if cerr := pc.canceled(); cerr != nil {
		return cerr
	}

	// See if an error was set explicitly.
	req.mu.Lock()
	reqErr := req.err
	req.mu.Unlock()
	if reqErr != nil {
		return reqErr
	}

	if err == errServerClosedIdle {
		// Don't decorate
		return err
	}

	if _, ok := err.(transportReadFromServerError); ok {
		// Don't decorate
		return err
	}
	if pc.isBroken() {
		<-pc.writeLoopDone
		if pc.nwrite == startBytesWritten {
			return nothingWrittenError{err}
		}
		return fmt.Errorf("net/http: HTTP/1.x transport connection broken: %v", err)
	}
	return err
}

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 {
			closeErr = err
			if trace != nil && trace.PutIdleConn != nil && err != errKeepAlivesDisabled {
				trace.PutIdleConn(err)
			}
			return false
		}
		if trace != nil && trace.PutIdleConn != nil {
			trace.PutIdleConn(nil)
		}
		return true
	}
    
    ...

	alive := true
	for alive {
		pc.readLimit = pc.maxHeaderResponseSize()
		_, err := pc.br.Peek(1)

		pc.mu.Lock()
		if pc.numExpectedResponses == 0 {
			pc.readLoopPeekFailLocked(err)
			pc.mu.Unlock()
			return
		}
		pc.mu.Unlock()

		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
		}

		if err != nil {
			if pc.readLimit <= 0 {
				err = fmt.Errorf("net/http: server response headers exceeded %d bytes; aborted", pc.maxHeaderResponseSize())
			}

			select {
			case rc.ch <- responseAndError{err: err}:
			case <-rc.callerGone:
				return
			}
			return
		}
		pc.readLimit = maxInt64 // effictively no limit for response bodies

		pc.mu.Lock()
		pc.numExpectedResponses--
		pc.mu.Unlock()

		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 {
			pc.t.setReqCanceler(rc.req, nil)

			// Put the idle conn back into the pool before we send the response
			// so if they process it quickly and make another request, they'll
			// get this same conn. But we use the unbuffered channel 'rc'
			// to guarantee that persistConn.roundTrip got out of its select
			// potentially waiting for this persistConn to close.
			// but after
			alive = alive &&
				!pc.sawEOF &&
				pc.wroteRequest() &&
				tryPutIdleConn(trace)

			if bodyWritable {
				closeErr = errCallerOwnsConn
			}

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

			// Now that they've read from the unbuffered channel, they're safely
			// out of the select that also waits on this goroutine to die, so
			// we're allowed to exit now if needed (if alive is false)
			testHookReadLoopBeforeNextRead()
			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
		if rc.addedGzip && strings.EqualFold(resp.Header.Get("Content-Encoding"), "gzip") {
			resp.Body = &gzipReader{body: body}
			resp.Header.Del("Content-Encoding")
			resp.Header.Del("Content-Length")
			resp.ContentLength = -1
			resp.Uncompressed = true
		}

		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{}{}
			}
        }
		testHookReadLoopBeforeNextRead()
	}
}

// tryPutIdleConn adds pconn to the list of idle persistent connections awaiting
// a new request.
// If pconn is no longer needed or not in a good state, tryPutIdleConn returns
// an error explaining why it wasn't registered.
// tryPutIdleConn does not close pconn. Use putOrCloseIdleConn instead for that.
func (t *Transport) tryPutIdleConn(pconn *persistConn) error {
	if t.DisableKeepAlives || t.MaxIdleConnsPerHost < 0 {
		return errKeepAlivesDisabled
	}
	if pconn.isBroken() {
		return errConnBroken
	}
	pconn.markReused()

	t.idleMu.Lock()
	defer t.idleMu.Unlock()

	// HTTP/2 (pconn.alt != nil) connections do not come out of the idle list,
	// because multiple goroutines can use them simultaneously.
	// If this is an HTTP/2 connection being “returned,” we're done.
	if pconn.alt != nil && t.idleLRU.m[pconn] != nil {
		return nil
	}

	// 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
	}
	for _, exist := range idles {
		if exist == pconn {
			log.Fatalf("dup idle pconn %p in freelist", pconn)
		}
	}
	t.idleConn[key] = append(idles, pconn)
	t.idleLRU.add(pconn)
	if t.MaxIdleConns != 0 && t.idleLRU.len() > t.MaxIdleConns {
		oldest := t.idleLRU.removeOldest()
		oldest.close(errTooManyIdle)
		t.removeIdleConnLocked(oldest)
	}

	// 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
}

// A wantConn records state about a wanted connection
// (that is, an active call to getConn).
// The conn may be gotten by dialing or by finding an idle connection,
// or a cancellation may make the conn no longer wanted.
// These three options are racing against each other and use
// wantConn to coordinate and agree about the winning outcome.
type wantConn struct {
	cm    connectMethod
	key   connectMethodKey // cm.key()
	ctx   context.Context  // context for dial
	ready chan struct{}    // closed when pc, err pair is delivered

	// hooks for testing to know when dials are done
	// beforeDial is called in the getConn goroutine when the dial is queued.
	// afterDial is called when the dial is completed or cancelled.
	beforeDial func()
	afterDial  func()

	mu  sync.Mutex // protects pc, err, close(ready)
	pc  *persistConn
	err error
}

// persistConn wraps a connection, usually a persistent one
// (but may be used for non-keep-alive requests as well)
type persistConn struct {
	// alt optionally specifies the TLS NextProto RoundTripper.
	// This is used for HTTP/2 today and future protocols later.
	// If it's non-nil, the rest of the fields are unused.
	alt RoundTripper

	t         *Transport
	cacheKey  connectMethodKey
	conn      net.Conn
	tlsState  *tls.ConnectionState
	br        *bufio.Reader       // from conn
	bw        *bufio.Writer       // to conn
	nwrite    int64               // bytes written
	reqch     chan requestAndChan // written by roundTrip; read by readLoop
	writech   chan writeRequest   // written by roundTrip; read by writeLoop
	closech   chan struct{}       // closed when conn closed
	isProxy   bool
	sawEOF    bool  // whether we've seen EOF from conn; owned by readLoop
	readLimit int64 // bytes allowed to be read; owned by readLoop
	// writeErrCh passes the request write error (usually nil)
	// from the writeLoop goroutine to the readLoop which passes
	// it off to the res.Body reader, which then uses it to decide
	// whether or not a connection can be reused. Issue 7569.
	writeErrCh chan error

	writeLoopDone chan struct{} // closed when write loop ends

	// Both guarded by Transport.idleMu:
	idleAt    time.Time   // time it last become idle
	idleTimer *time.Timer // holding an AfterFunc to close it

	mu                   sync.Mutex // guards following fields
	numExpectedResponses int
	closed               error // set non-nil when conn is closed, before closech is closed
	canceledErr          error // set non-nil if conn is canceled
	broken               bool  // an error has happened on this connection; marked broken so it's not reused.
	reused               bool  // whether conn has had successful request/response and is being reused.
	// mutateHeaderFunc is an optional func to modify extra
	// headers on each outbound request before it's written. (the
	// original Request given to RoundTrip is not modified)
	mutateHeaderFunc func(Header)
}

type requestAndChan struct {
	req *Request
	ch  chan responseAndError // unbuffered; always send in select on callerGone

	// whether the Transport (as opposed to the user client code)
	// added the Accept-Encoding gzip header. If the Transport
	// set it, only then do we transparently decode the gzip.
	addedGzip bool

	// Optional blocking chan for Expect: 100-continue (for send).
	// If the request has an "Expect: 100-continue" header and
	// the server responds 100 Continue, readLoop send a value
	// to writeLoop via this chan.
	continueCh chan<- struct{}

	callerGone <-chan struct{} // closed when roundTrip caller has returned
}

// A writeRequest is sent by the readLoop's goroutine to the
// writeLoop's goroutine to write a request while the read loop
// concurrently waits on both the write response and the server's
// reply.
type writeRequest struct {
	req *transportRequest
	ch  chan<- error

	// Optional blocking chan for Expect: 100-continue (for receive).
	// If not nil, writeLoop blocks sending request body until
	// it receives from this chan.
	continueCh <-chan struct{}
}

// tryDeliver attempts to deliver pc, err to w and reports whether it succeeded.
func (w *wantConn) tryDeliver(pc *persistConn, err error) bool {
	w.mu.Lock()
	defer w.mu.Unlock()

	if w.pc != nil || w.err != nil {
		return false
	}

	w.pc = pc
	w.err = err
	if w.pc == nil && w.err == nil {
		panic("net/http: internal error: misuse of tryDeliver")
	}
	close(w.ready)
	return true
}

// 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()

...
// closeConnIfStillIdle closes the connection if it's still sitting idle.
// This is what's called by the persistConn's idleTimer, and is run in its
// own goroutine.
func (pc *persistConn) closeConnIfStillIdle() {
	t := pc.t
	t.idleMu.Lock()
	defer t.idleMu.Unlock()
	if _, ok := t.idleLRU.m[pc]; !ok {
		// Not idle.
		return
	}
	t.removeIdleConnLocked(pc)
	pc.close(errIdleConnTimeout)
}

// t.idleMu must be held.
func (t *Transport) removeIdleConnLocked(pconn *persistConn) {
	if pconn.idleTimer != nil {
		pconn.idleTimer.Stop()
	}
	t.idleLRU.remove(pconn)
	key := pconn.cacheKey
	pconns := t.idleConn[key]
	switch len(pconns) {
	case 0:
		// Nothing
	case 1:
		if pconns[0] == pconn {
			delete(t.idleConn, key)
		}
	default:
		for i, v := range pconns {
			if v != pconn {
				continue
			}
			// Slide down, keeping most recently-used
			// conns at the end.
			copy(pconns[i:], pconns[i+1:])
			t.idleConn[key] = pconns[:len(pconns)-1]
			break
		}
	}
}

// closeConnIfStillIdle closes the connection if it's still sitting idle.
// This is what's called by the persistConn's idleTimer, and is run in its
// own goroutine.
func (pc *persistConn) closeConnIfStillIdle() {
	t := pc.t
	t.idleMu.Lock()
	defer t.idleMu.Unlock()
	if _, ok := t.idleLRU.m[pc]; !ok {
		// Not idle.
		return
	}
	t.removeIdleConnLocked(pc)
	pc.close(errIdleConnTimeout)
}

// getConn dials and creates a new persistConn to the target as
// specified in the connectMethod. This includes doing a proxy CONNECT
// and/or setting up TLS.  If this doesn't return an error, the persistConn
// is ready to write requests to.
func (t *Transport) getConn(treq *transportRequest, cm connectMethod) (pc *persistConn, err error) {
	req := treq.Request
	trace := treq.trace
	ctx := req.Context()
	if trace != nil && trace.GetConn != nil {
		trace.GetConn(cm.addr())
	}

	w := &wantConn{
		cm:         cm,
		key:        cm.key(),
		ctx:        ctx,
		ready:      make(chan struct{}, 1),
		beforeDial: testHookPrePendingDial,
		afterDial:  testHookPostPendingDial,
	}
	defer func() {
		if err != nil {
			w.cancel(t, err)
		}
	}()

	// Queue for idle connection.
	if delivered := t.queueForIdleConn(w); delivered {
		pc := w.pc
		// Trace only for HTTP/1.
		// HTTP/2 calls trace.GotConn itself.
		if pc.alt == nil && trace != nil && trace.GotConn != nil {
			trace.GotConn(pc.gotIdleConnTrace(pc.idleAt))
		}
		// set request canceler to some non-nil function so we
		// can detect whether it was cleared between now and when
		// we enter roundTrip
		t.setReqCanceler(req, func(error) {})
		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()})
		}
		if w.err != nil {
			// If the request has been cancelled, that's probably
			// what caused w.err; if so, prefer to return the
			// cancellation error (see golang.org/issue/16049).
			select {
			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
			default:
				// return below
			}
		}
		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
	}
}

// queueForIdleConn queues w to receive the next idle connection for w.cm.
// As an optimization hint to the caller, queueForIdleConn reports whether
// it successfully delivered an already-idle connection.
func (t *Transport) queueForIdleConn(w *wantConn) (delivered bool) {
	if t.DisableKeepAlives {
		return false
	}

	t.idleMu.Lock()
	defer t.idleMu.Unlock()

	// Stop closing connections that become idle - we might want one.
	// (That is, undo the effect of t.CloseIdleConnections.)
	t.closeIdle = false

	if w == nil {
		// Happens in test hook.
		return false
	}

	// 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]
			if pconn.isBroken() {
				// persistConn.readLoop has marked the connection broken,
				// but Transport.removeIdleConn has not yet removed it from the idle list.
				// Drop on floor on behalf of Transport.removeIdleConn.
				list = list[:len(list)-1]
				continue
			}
			delivered = w.tryDeliver(pconn, nil)
			if delivered {
				if pconn.alt != nil {
					// HTTP/2: multiple clients can share pconn.
					// Leave it in the list.
				} else {
					// HTTP/1: only one client can use pconn.
					// Remove it from the list.
					t.idleLRU.remove(pconn)
					list = list[:len(list)-1]
				}
			}
			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
}

// DialContext connects to the address on the named network using
// the provided context.
//
// The provided Context must be non-nil. If the context expires before
// the connection is complete, an error is returned. Once successfully
// connected, any expiration of the context will not affect the
// connection.
//
// When using TCP, and the host in the address parameter resolves to multiple
// network addresses, any dial timeout (from d.Timeout or ctx) is spread
// over each consecutive dial, such that each is given an appropriate
// fraction of the time to connect.
// For example, if a host has 4 IP addresses and the timeout is 1 minute,
// the connect to each single address will be given 15 seconds to complete
// before trying the next one.
//
// See func Dial for a description of the network and address
// parameters.
func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn, error) {
	if ctx == nil {
		panic("nil context")
	}
	deadline := d.deadline(ctx, time.Now())
	if !deadline.IsZero() {
		if d, ok := ctx.Deadline(); !ok || deadline.Before(d) {
			subCtx, cancel := context.WithDeadline(ctx, deadline)
			defer cancel()
			ctx = subCtx
		}
	}
	if oldCancel := d.Cancel; oldCancel != nil {
		subCtx, cancel := context.WithCancel(ctx)
		defer cancel()
		go func() {
			select {
			case <-oldCancel:
				cancel()
			case <-subCtx.Done():
			}
		}()
		ctx = subCtx
	}

	// Shadow the nettrace (if any) during resolve so Connect events don't fire for DNS lookups.
	resolveCtx := ctx
	if trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace); trace != nil {
		shadow := *trace
		shadow.ConnectStart = nil
		shadow.ConnectDone = nil
		resolveCtx = context.WithValue(resolveCtx, nettrace.TraceKey{}, &shadow)
	}

	addrs, err := d.resolver().resolveAddrList(resolveCtx, "dial", network, address, d.LocalAddr)
	if err != nil {
		return nil, &OpError{Op: "dial", Net: network, Source: nil, Addr: nil, Err: err}
	}

	sd := &sysDialer{
		Dialer:  *d,
		network: network,
		address: address,
	}

	var primaries, fallbacks addrList
	if d.dualStack() && network == "tcp" {
		primaries, fallbacks = addrs.partition(isIPv4)
	} else {
		primaries = addrs
	}

	var c Conn
	if len(fallbacks) > 0 {
		c, err = sd.dialParallel(ctx, primaries, fallbacks)
	} else {
		c, err = sd.dialSerial(ctx, primaries)
	}
	if err != nil {
		return nil, err
	}

	if tc, ok := c.(*TCPConn); ok && d.KeepAlive >= 0 {
		setKeepAlive(tc.fd, true)
		ka := d.KeepAlive
		if d.KeepAlive == 0 {
			ka = defaultTCPKeepAlive
		}
		setKeepAlivePeriod(tc.fd, ka)
		testHookSetKeepAlive(ka)
	}
	return c, nil
}

// deadline returns the earliest of:
//   - now+Timeout
//   - d.Deadline
//   - the context's deadline
// Or zero, if none of Timeout, Deadline, or context's deadline is set.
func (d *Dialer) deadline(ctx context.Context, now time.Time) (earliest time.Time) {
	if d.Timeout != 0 { // including negative, for historical reasons
		earliest = now.Add(d.Timeout)
	}
	if d, ok := ctx.Deadline(); ok {
		earliest = minNonzeroTime(earliest, d)
	}
	return minNonzeroTime(earliest, d.Deadline)
}

// dialSerial connects to a list of addresses in sequence, returning
// either the first successful connection, or the first error.
func (sd *sysDialer) dialSerial(ctx context.Context, ras addrList) (Conn, error) {
	var firstErr error // The error from the first address is most relevant.

	for i, ra := range ras {
		select {
		case <-ctx.Done():
			return nil, &OpError{Op: "dial", Net: sd.network, Source: sd.LocalAddr, Addr: ra, Err: mapErr(ctx.Err())}
		default:
		}

		deadline, _ := ctx.Deadline()
		partialDeadline, err := partialDeadline(time.Now(), deadline, len(ras)-i)
		if err != nil {
			// Ran out of time.
			if firstErr == nil {
				firstErr = &OpError{Op: "dial", Net: sd.network, Source: sd.LocalAddr, Addr: ra, Err: err}
			}
			break
		}
		dialCtx := ctx
		if partialDeadline.Before(deadline) {
			var cancel context.CancelFunc
			dialCtx, cancel = context.WithDeadline(ctx, partialDeadline)
			defer cancel()
		}

		c, err := sd.dialSingle(dialCtx, ra)
		if err == nil {
			return c, nil
		}
		if firstErr == nil {
			firstErr = err
		}
	}

	if firstErr == nil {
		firstErr = &OpError{Op: "dial", Net: sd.network, Source: nil, Addr: nil, Err: errMissingAddress}
	}
	return nil, firstErr
}

// dialSingle attempts to establish and returns a single connection to
// the destination address.
func (sd *sysDialer) dialSingle(ctx context.Context, ra Addr) (c Conn, err error) {
	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
	if trace != nil {
		raStr := ra.String()
		if trace.ConnectStart != nil {
			trace.ConnectStart(sd.network, raStr)
		}
		if trace.ConnectDone != nil {
			defer func() { trace.ConnectDone(sd.network, raStr, err) }()
		}
	}
	la := sd.LocalAddr
	switch ra := ra.(type) {
	case *TCPAddr:
		la, _ := la.(*TCPAddr)
		c, err = sd.dialTCP(ctx, la, ra)
	case *UDPAddr:
		la, _ := la.(*UDPAddr)
		c, err = sd.dialUDP(ctx, la, ra)
	case *IPAddr:
		la, _ := la.(*IPAddr)
		c, err = sd.dialIP(ctx, la, ra)
	case *UnixAddr:
		la, _ := la.(*UnixAddr)
		c, err = sd.dialUnix(ctx, la, ra)
	default:
		return nil, &OpError{Op: "dial", Net: sd.network, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: sd.address}}
	}
	if err != nil {
		return nil, &OpError{Op: "dial", Net: sd.network, Source: la, Addr: ra, Err: err} // c is non-nil interface containing nil pointer
	}
	return c, nil
}

func (sd *sysDialer) dialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPConn, error) {
	if testHookDialTCP != nil {
		return testHookDialTCP(ctx, sd.network, laddr, raddr)
	}
	return sd.doDialTCP(ctx, laddr, raddr)
}

func (sd *sysDialer) doDialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPConn, error) {
	fd, err := internetSocket(ctx, sd.network, laddr, raddr, syscall.SOCK_STREAM, 0, "dial", sd.Dialer.Control)

	// TCP has a rarely used mechanism called a 'simultaneous connection' in
	// which Dial("tcp", addr1, addr2) run on the machine at addr1 can
	// connect to a simultaneous Dial("tcp", addr2, addr1) run on the machine
	// at addr2, without either machine executing Listen. If laddr == nil,
	// it means we want the kernel to pick an appropriate originating local
	// address. Some Linux kernels cycle blindly through a fixed range of
	// local ports, regardless of destination port. If a kernel happens to
	// pick local port 50001 as the source for a Dial("tcp", "", "localhost:50001"),
	// then the Dial will succeed, having simultaneously connected to itself.
	// This can only happen when we are letting the kernel pick a port (laddr == nil)
	// and when there is no listener for the destination address.
	// It's hard to argue this is anything other than a kernel bug. If we
	// see this happen, rather than expose the buggy effect to users, we
	// close the fd and try again. If it happens twice more, we relent and
	// use the result. See also:
	//	https://golang.org/issue/2690
	//	https://stackoverflow.com/questions/4949858/
	//
	// The opposite can also happen: if we ask the kernel to pick an appropriate
	// originating local address, sometimes it picks one that is already in use.
	// So if the error is EADDRNOTAVAIL, we have to try again too, just for
	// a different reason.
	//
	// The kernel socket code is no doubt enjoying watching us squirm.
	for i := 0; i < 2 && (laddr == nil || laddr.Port == 0) && (selfConnect(fd, err) || spuriousENOTAVAIL(err)); i++ {
		if err == nil {
			fd.Close()
		}
		fd, err = internetSocket(ctx, sd.network, laddr, raddr, syscall.SOCK_STREAM, 0, "dial", sd.Dialer.Control)
	}

	if err != nil {
		return nil, err
	}
	return newTCPConn(fd), nil
}

func internetSocket(ctx context.Context, net string, laddr, raddr sockaddr, sotype, proto int, mode string, ctrlFn func(string, string, syscall.RawConn) error) (fd *netFD, err error) {
	if (runtime.GOOS == "aix" || runtime.GOOS == "windows" || runtime.GOOS == "openbsd" || runtime.GOOS == "nacl") && mode == "dial" && raddr.isWildcard() {
		raddr = raddr.toLocal(net)
	}
	family, ipv6only := favoriteAddrFamily(net, laddr, raddr, mode)
	return socket(ctx, net, family, sotype, proto, ipv6only, laddr, raddr, ctrlFn)
}

// socket returns a network file descriptor that is ready for
// asynchronous I/O using the network poller.
func socket(ctx context.Context, net string, family, sotype, proto int, ipv6only bool, laddr, raddr sockaddr, ctrlFn func(string, string, syscall.RawConn) error) (fd *netFD, err error) {
	s, err := sysSocket(family, sotype, proto)
	if err != nil {
		return nil, err
	}
	if err = setDefaultSockopts(s, family, sotype, ipv6only); err != nil {
		poll.CloseFunc(s)
		return nil, err
	}
	if fd, err = newFD(s, family, sotype, net); err != nil {
		poll.CloseFunc(s)
		return nil, err
	}

	// This function makes a network file descriptor for the
	// following applications:
	//
	// - An endpoint holder that opens a passive stream
	//   connection, known as a stream listener
	//
	// - An endpoint holder that opens a destination-unspecific
	//   datagram connection, known as a datagram listener
	//
	// - An endpoint holder that opens an active stream or a
	//   destination-specific datagram connection, known as a
	//   dialer
	//
	// - An endpoint holder that opens the other connection, such
	//   as talking to the protocol stack inside the kernel
	//
	// For stream and datagram listeners, they will only require
	// named sockets, so we can assume that it's just a request
	// from stream or datagram listeners when laddr is not nil but
	// raddr is nil. Otherwise we assume it's just for dialers or
	// the other connection holders.

	if laddr != nil && raddr == nil {
		switch sotype {
		case syscall.SOCK_STREAM, syscall.SOCK_SEQPACKET:
			if err := fd.listenStream(laddr, listenerBacklog(), ctrlFn); err != nil {
				fd.Close()
				return nil, err
			}
			return fd, nil
		case syscall.SOCK_DGRAM:
			if err := fd.listenDatagram(laddr, ctrlFn); err != nil {
				fd.Close()
				return nil, err
			}
			return fd, nil
		}
	}
	if err := fd.dial(ctx, laddr, raddr, ctrlFn); err != nil {
		fd.Close()
		return nil, err
	}
	return fd, nil
}

...
func (fd *netFD) connect(ctx context.Context, la, ra syscall.Sockaddr) (rsa syscall.Sockaddr, ret error) {
	// Do not need to call fd.writeLock here,
	// because fd is not yet accessible to user,
	// so no concurrent operations are possible.
	switch err := connectFunc(fd.pfd.Sysfd, ra); err {
	case syscall.EINPROGRESS, syscall.EALREADY, syscall.EINTR:
	case nil, syscall.EISCONN:
		select {
		case <-ctx.Done():
			return nil, mapErr(ctx.Err())
		default:
		}
		if err := fd.pfd.Init(fd.net, true); err != nil {
			return nil, err
		}
		runtime.KeepAlive(fd)
		return nil, nil
	case syscall.EINVAL:
		// On Solaris and illumos we can see EINVAL if the socket has
		// already been accepted and closed by the server.  Treat this
		// as a successful connection--writes to the socket will see
		// EOF.  For details and a test case in C see
		// https://golang.org/issue/6828.
		if runtime.GOOS == "solaris" || runtime.GOOS == "illumos" {
			return nil, nil
		}
		fallthrough
	default:
		return nil, os.NewSyscallError("connect", err)
	}
	if err := fd.pfd.Init(fd.net, true); err != nil {
		return nil, err
	}
	if deadline, _ := ctx.Deadline(); !deadline.IsZero() {
		fd.pfd.SetWriteDeadline(deadline)
		defer fd.pfd.SetWriteDeadline(noDeadline)
	}

	// Start the "interrupter" goroutine, if this context might be canceled.
	// (The background context cannot)
	//
	// The interrupter goroutine waits for the context to be done and
	// interrupts the dial (by altering the fd's write deadline, which
	// wakes up waitWrite).
	if ctx != context.Background() {
		// Wait for the interrupter goroutine to exit before returning
		// from connect.
		done := make(chan struct{})
		interruptRes := make(chan error)
		defer func() {
			close(done)
			if ctxErr := <-interruptRes; ctxErr != nil && ret == nil {
				// The interrupter goroutine called SetWriteDeadline,
				// but the connect code below had returned from
				// waitWrite already and did a successful connect (ret
				// == nil). Because we've now poisoned the connection
				// by making it unwritable, don't return a successful
				// dial. This was issue 16523.
				ret = mapErr(ctxErr)
				fd.Close() // prevent a leak
			}
		}()
		go func() {
			select {
			case <-ctx.Done():
				// Force the runtime's poller to immediately give up
				// waiting for writability, unblocking waitWrite
				// below.
				fd.pfd.SetWriteDeadline(aLongTimeAgo)
				testHookCanceledDial()
				interruptRes <- ctx.Err()
			case <-done:
				interruptRes <- nil
			}
		}()
	}

	for {
		// Performing multiple connect system calls on a
		// non-blocking socket under Unix variants does not
		// necessarily result in earlier errors being
		// returned. Instead, once runtime-integrated network
		// poller tells us that the socket is ready, get the
		// SO_ERROR socket option to see if the connection
		// succeeded or failed. See issue 7474 for further
		// details.
		if err := fd.pfd.WaitWrite(); err != nil {
			select {
			case <-ctx.Done():
				return nil, mapErr(ctx.Err())
			default:
			}
			return nil, err
		}
		nerr, err := getsockoptIntFunc(fd.pfd.Sysfd, syscall.SOL_SOCKET, syscall.SO_ERROR)
		if err != nil {
			return nil, os.NewSyscallError("getsockopt", err)
		}
		switch err := syscall.Errno(nerr); err {
		case syscall.EINPROGRESS, syscall.EALREADY, syscall.EINTR:
		case syscall.EISCONN:
			return nil, nil
		case syscall.Errno(0):
			// The runtime poller can wake us up spuriously;
			// see issues 14548 and 19289. Check that we are
			// really connected; if not, wait again.
			if rsa, err := syscall.Getpeername(fd.pfd.Sysfd); err == nil {
				return rsa, nil
			}
		default:
			return nil, os.NewSyscallError("connect", err)
		}
		runtime.KeepAlive(fd)
	}
}