golang http/transport 代码分析
请结合源码阅读，本文只是总结一下，源码里有详细的注释。基于：go1.12.4
http.Client 表示一个http client端，用来处理HTTP相关的工作，例如cookies, redirect, timeout等工作，其内部包含一个Transport，为RountTripper interface类型。
type Client struct {
// Transport specifies the mechanism by which individual
// HTTP requests are made.
// If nil, DefaultTransport is used.
Transport RoundTripper
...
}
RountTripper定义了执行一次http请求时，如何根据reueqest返回response，它必须是支持并发的一个结构体，允许多个groutine同时调用：
type RoundTripper interface {
RoundTrip(*Request) (*Response, error)
}
如果不给http.Client显式指定RoundTripper则会创建一个默认的DefaultTransport。Transport是用来保存多个请求过程中的一些状态，用来缓存tcp连接，客户可以重用这些连接，防止每次新建，transport需要同时支持http, https, 并且需要http/1.1, http/2。DefaultTransport默认就支持http/2.0，如果需要显式指定则调用ConfigureTransport。
transport必须实现interface中的roundTrip方法：
// roundTrip implements a RoundTripper over HTTP.
func (t *Transport) roundTrip(req *Request) (*Response, error) {
...
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)
if err != nil {
req.closeBody()
return nil, err
}
// 获取一个连接
// Get the cached or newly-created connection to either the
// host (for http or https), the http proxy, or the http proxy
// pre-CONNECTed to https server. In any case, we'll be ready
// to send it requests.
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.decHostConnCount(cm.key()) // don't count cached http2 conns toward conns per host
t.setReqCanceler(req, nil) // not cancelable with CancelRequest
resp, err = pconn.alt.RoundTrip(req)
} else {
// 开始调用该pconn的rountTrip方法取得response
resp, err = pconn.roundTrip(treq)
}
if err == nil {
return resp, nil
}
if !pconn.shouldRetryRequest(req, err) {
// Issue 16465: return underlying net.Conn.Read error from peek,
// as we've historically done.
if e, ok := err.(transportReadFromServerError); ok {
err = e.err
}
return nil, err
}
testHookRoundTripRetried()
// Rewind the body if we're able to.
if req.GetBody != nil {
newReq := *req
var err error
newReq.Body, err = req.GetBody()
if err != nil {
return nil, err
}
req = &newReq
}
}
}
roundTrip其实就是通过getConn用于获取一个连接persisConn并调用其roundTrip方法返回repsonse。其中getConn的实现如下：
// 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) (*persistConn, error) {
req := treq.Request
trace := treq.trace
ctx := req.Context()
if trace != nil && trace.GetConn != nil {
trace.GetConn(cm.addr())
}
// 首先从idleConn空闲连接池中尝试获取闲置的连接
if pc, idleSince := t.getIdleConn(cm); pc != nil {
if trace != nil && trace.GotConn != nil {
trace.GotConn(pc.gotIdleConnTrace(idleSince))
}
// 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
}
type dialRes struct {
pc *persistConn
err error
}
dialc := make(chan dialRes) // 连接创建完成之后会从该管道异步通知
cmKey := cm.key() // 标识一个连接的key
// Copy these hooks so we don't race on the postPendingDial in
// the goroutine we launch. Issue 11136.
testHookPrePendingDial := testHookPrePendingDial
testHookPostPendingDial := testHookPostPendingDial
handlePendingDial := func() {
testHookPrePendingDial()
go func() {
if v := <-dialc; v.err == nil {
t.putOrCloseIdleConn(v.pc)
} else {
t.decHostConnCount(cmKey)
}
testHookPostPendingDial()
}()
}
cancelc := make(chan error, 1)
t.setReqCanceler(req, func(err error) { cancelc <- err })
// 一边增加记录的连接数，一边尝试获取连接，一边监听取消事件
if t.MaxConnsPerHost > 0 {
select {
case <-t.incHostConnCount(cmKey):
// count below conn per host limit; proceed
case pc := <-t.getIdleConnCh(cm):
if trace != nil && trace.GotConn != nil {
trace.GotConn(httptrace.GotConnInfo{Conn: pc.conn, Reused: pc.isReused()})
}
return pc, nil
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
}
}
// 异步发起连接操作
go func() {
pc, err := t.dialConn(ctx, cm)
dialc <- dialRes{pc, err}
}()
// 监听多个事件来源
// 1. 新创建成功
// 2. 其它连接结束，闲置连接池中有连接可以复用
// 3. 连接被取消
// 第一种情况和第二种情况谁先成功就直接返回
// 除了新建连接成功，其它所有情况都需要处理调用`handlePendingDial`，该函数决定新建连接返回后该如何处理
idleConnCh := t.getIdleConnCh(cm)
select {
case v := <-dialc: // 如果新建连接结束后会从该channel发送过来
// Our dial finished.
if v.pc != nil {
if trace != nil && trace.GotConn != nil && v.pc.alt == nil {
trace.GotConn(httptrace.GotConnInfo{Conn: v.pc.conn})
}
return v.pc, nil
}
// Our dial failed. See why to return a nicer error
// value.
t.decHostConnCount(cmKey)
select {
case <-req.Cancel:
// It was an error due to cancelation, so prioritize that
// error value. (Issue 16049)
return nil, errRequestCanceledConn
case <-req.Context().Done():
return nil, req.Context().Err()
case err := <-cancelc:
if err == errRequestCanceled {
err = errRequestCanceledConn
}
return nil, err
default:
// It wasn't an error due to cancelation, so
// return the original error message:
return nil, v.err
}
case pc := <-idleConnCh: // 如果从空闲连接池中有了可用的连接，直接返回
// Another request finished first and its net.Conn
// became available before our dial. Or somebody
// else's dial that they didn't use.
// But our dial is still going, so give it away
// when it finishes:
handlePendingDial()
if trace != nil && trace.GotConn != nil {
trace.GotConn(httptrace.GotConnInfo{Conn: pc.conn, Reused: pc.isReused()})
}
return pc, nil
case <-req.Cancel:
handlePendingDial()
return nil, errRequestCanceledConn
case <-req.Context().Done():
handlePendingDial()
return nil, req.Context().Err()
case err := <-cancelc:
handlePendingDial()
if err == errRequestCanceled {
err = errRequestCanceledConn
}
return nil, err
}
}
getConn首先从空闲连接池中获取连接，如果没有，则新建连接。在新建过程中，如果连接池中有空闲连接则也复用空闲连接。
继续看一下dialConn是如何建立连接的：
func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (*persistConn, error) {
// 注意这里初始化的各种channle
pconn := &persistConn{
t: t,
cacheKey: cm.key(),
reqch: make(chan requestAndChan, 1), // 用于给readLoop发送request
writech: make(chan writeRequest, 1), // 用于给writeLoop发送request
closech: make(chan struct{}), // 当连接关闭是用于传递信息
writeErrCh: make(chan error, 1), // 由writeLoop返回给roundTrip错误信息
writeLoopDone: make(chan struct{}), // 当writeLoop结束的时候会关闭该channel
}
trace := httptrace.ContextClientTrace(ctx)
wrapErr := func(err error) error {
if cm.proxyURL != nil {
// Return a typed error, per Issue 16997
return &net.OpError{Op: "proxyconnect", Net: "tcp", Err: err}
}
return err
}
conn, err := t.dial(ctx, "tcp", cm.addr())
if err != nil {
return nil, wrapErr(err)
}
pconn.conn = conn
// 包装一个请求成另一个结构体，方便后续处理
if t.MaxConnsPerHost > 0 {
pconn.conn = &connCloseListener{Conn: pconn.conn, t: t, cmKey: pconn.cacheKey}
}
// 包装读写conn并开启读取和写入groutine
pconn.br = bufio.NewReader(pconn)
pconn.bw = bufio.NewWriter(persistConnWriter{pconn})
go pconn.readLoop()
go pconn.writeLoop()
return pconn, nil
}
可以看到首先调用dial函数，获取一个conn对象，然后封装为pconn的, 启动readLoop和wirteLoop后将该pconn返回。
以readLoop为例，看看是如何从一个pc中读取response的：
func (pc *persistConn) readLoop() {
// 默认是失败，如果失败则进行处理，移除该连接，使用defer语句表示在程序退出的时候执行，也就是说该groutine在正常情况下不会退出，是个死循环，通过channel与其它groutine通信，处理请求
closeErr := errReadLoopExiting // default value, if not changed below
defer func() {
pc.close(closeErr)
pc.t.removeIdleConn(pc)
}()
// 尝试将该连接重新返回闲置连接池
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
}
// 用来保证先后次序，先归还连接再读取response.Body
// eofc is used to block caller goroutines reading from Response.Body
// at EOF until this goroutines has (potentially) added the connection
// back to the idle pool.
eofc := make(chan struct{})
defer close(eofc) // unblock reader on errors
// Read this once, before loop starts. (to avoid races in tests)
testHookMu.Lock()
testHookReadLoopBeforeNextRead := testHookReadLoopBeforeNextRead
testHookMu.Unlock()
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()
// 获取一个新连接来处理
rc := <-pc.reqch
trace := httptrace.ContextClientTrace(rc.req.Context())
var resp *Response
if err == nil {
// 读取返回结果
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
}
// bodyEOFSignal实现了io.ReadCloser interface, 保证读取的时候，该response已经收到了eof
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 {
// 将分装好的repsponse发送回去
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 cancelation 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{}{} //前面所有检查完毕，通知对端开始读取
}
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()
}
}
上面readLoop中从一个channel中读取出来需要处理的request, 然后读取readResponse并通过管道返回回去。那接受到的request是从哪个地方发送过来的呐?
回到最开始的Transport.roundTrip函数里，它先调用getConn返回一个pconn后然后调用pconn.roundTrip方法，就是在这里面发送的，我们看看：
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())
}
...
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
// 通过writech发送该请求
// 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)
// 发送当前正在处理的请求给readLoop，readLoop中从channle中读取出该请求，进行readResponse
// 其中的requestAndChan.ch是response返回的channel
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: // writeLoop出现错误
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: // readLoop会将读取的结果通过resc channel发送回来
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
}
}
}
该函数中会将request进行封装，然后分别通过channel发送给readLoop和writeLoop，并监听各个channel的事件，分别进行不同的处理。
整体流程走完之后，我们回顾一下两个比较重要的结构体：persistConn和Transport的成员
// 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 // 当前连接对应的key， 也是idleConns map中的key
conn net.Conn // 被封装的conn对象
tlsState *tls.ConnectionState
br *bufio.Reader // from conn // bufio.Reader 对象，封装conn
bw *bufio.Writer // to conn // bufio.Writer 对象，封装conn
nwrite int64 // bytes written // 记录写入的长度
reqch chan requestAndChan // written by roundTrip; read by readLoop // rountTrip在创建一个请求的时候会讲请求通过该chenel发送给readLoop, readLoop后面解释
writech chan writeRequest // written by roundTrip; read by writeLoop // writeTrop 从中读取写入请求并执行写入
closech chan struct{} // closed when conn closed // 连接关闭的时候从该channle通信
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 //表示当期期望的返回response数目
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 Transport struct {
idleMu sync.Mutex // 互斥锁，用于保护下面空闲连接池
wantIdle bool // user has requested to close all idle conns// 标识是否idle
idleConn map[connectMethodKey][]*persistConn // most recently used at end // 空闲连接池
idleConnCh map[connectMethodKey]chan *persistConn // 用于在groutine中间传递空闲的连接，一般用于当连接池中没有连接，但是还有请求需要处理，当连接池中出现空闲连接时通过该channel通知
idleLRU connLRU
reqMu sync.Mutex
reqCanceler map[*Request]func(error)
altMu sync.Mutex // guards changing altProto only
altProto atomic.Value // of nil or map[string]RoundTripper, key is URI scheme
connCountMu sync.Mutex
connPerHostCount map[connectMethodKey]int
connPerHostAvailable map[connectMethodKey]chan struct{}
// Proxy specifies a function to return a proxy for a given
// Request. If the function returns a non-nil error, the
// request is aborted with the provided error.
//
// The proxy type is determined by the URL scheme. "http",
// "https", and "socks5" are supported. If the scheme is empty,
// "http" is assumed.
//
// If Proxy is nil or returns a nil *URL, no proxy is used.
Proxy func(*Request) (*url.URL, error)
// DialContext specifies the dial function for creating unencrypted TCP connections.
// If DialContext is nil (and the deprecated Dial below is also nil),
// then the transport dials using package net.
//
// DialContext runs concurrently with calls to RoundTrip.
// A RoundTrip call that initiates a dial may end up using
// a connection dialed previously when the earlier connection
// becomes idle before the later DialContext completes.
DialContext func(ctx context.Context, network, addr string) (net.Conn, error)// 用于新建连接时使用
// Dial specifies the dial function for creating unencrypted TCP connections.
//
// Dial runs concurrently with calls to RoundTrip.
// A RoundTrip call that initiates a dial may end up using
// a connection dialed previously when the earlier connection
// becomes idle before the later Dial completes.
//
// Deprecated: Use DialContext instead, which allows the transport
// to cancel dials as soon as they are no longer needed.
// If both are set, DialContext takes priority.
Dial func(network, addr string) (net.Conn, error)
// DialTLS specifies an optional dial function for creating
// TLS connections for non-proxied HTTPS requests.
//
// If DialTLS is nil, Dial and TLSClientConfig are used.
//
// If DialTLS is set, the Dial hook is not used for HTTPS
// requests and the TLSClientConfig and TLSHandshakeTimeout
// are ignored. The returned net.Conn is assumed to already be
// past the TLS handshake.
DialTLS func(network, addr string) (net.Conn, error)
// TLSClientConfig specifies the TLS configuration to use with
// tls.Client.
// If nil, the default configuration is used.
// If non-nil, HTTP/2 support may not be enabled by default.
TLSClientConfig *tls.Config
// TLSHandshakeTimeout specifies the maximum amount of time waiting to
// wait for a TLS handshake. Zero means no timeout.
TLSHandshakeTimeout time.Duration
// DisableKeepAlives, if true, disables HTTP keep-alives and
// will only use the connection to the server for a single
// HTTP request.
//
// This is unrelated to the similarly named TCP keep-alives.
DisableKeepAlives bool
// DisableCompression, if true, prevents the Transport from
// requesting compression with an "Accept-Encoding: gzip"
// request header when the Request contains no existing
// Accept-Encoding value. If the Transport requests gzip on
// its own and gets a gzipped response, it's transparently
// decoded in the Response.Body. However, if the user
// explicitly requested gzip it is not automatically
// uncompressed.
DisableCompression bool
// MaxIdleConns controls the maximum number of idle (keep-alive)
// connections across all hosts. Zero means no limit.
MaxIdleConns int
// MaxIdleConnsPerHost, if non-zero, controls the maximum idle
// (keep-alive) connections to keep per-host. If zero,
// DefaultMaxIdleConnsPerHost is used.
MaxIdleConnsPerHost int
// MaxConnsPerHost optionally limits the total number of
// connections per host, including connections in the dialing,
// active, and idle states. On limit violation, dials will block.
//
// Zero means no limit.
//
// For HTTP/2, this currently only controls the number of new
// connections being created at a time, instead of the total
// number. In practice, hosts using HTTP/2 only have about one
// idle connection, though.
MaxConnsPerHost int
// IdleConnTimeout is the maximum amount of time an idle
// (keep-alive) connection will remain idle before closing
// itself.
// Zero means no limit.
IdleConnTimeout time.Duration
// ResponseHeaderTimeout, if non-zero, specifies the amount of
// time to wait for a server's response headers after fully
// writing the request (including its body, if any). This
// time does not include the time to read the response body.
ResponseHeaderTimeout time.Duration
// ExpectContinueTimeout, if non-zero, specifies the amount of
// time to wait for a server's first response headers after fully
// writing the request headers if the request has an
// "Expect: 100-continue" header. Zero means no timeout and
// causes the body to be sent immediately, without
// waiting for the server to approve.
// This time does not include the time to send the request header.
ExpectContinueTimeout time.Duration
// TLSNextProto specifies how the Transport switches to an
// alternate protocol (such as HTTP/2) after a TLS NPN/ALPN
// protocol negotiation. If Transport dials an TLS connection
// with a non-empty protocol name and TLSNextProto contains a
// map entry for that key (such as "h2"), then the func is
// called with the request's authority (such as "example.com"
// or "example.com:1234") and the TLS connection. The function
// must return a RoundTripper that then handles the request.
// If TLSNextProto is not nil, HTTP/2 support is not enabled
// automatically.
TLSNextProto map[string]func(authority string, c *tls.Conn) RoundTripper
// ProxyConnectHeader optionally specifies headers to send to
// proxies during CONNECT requests.
ProxyConnectHeader Header
// MaxResponseHeaderBytes specifies a limit on how many
// response bytes are allowed in the server's response
// header.
//
// Zero means to use a default limit.
MaxResponseHeaderBytes int64
// nextProtoOnce guards initialization of TLSNextProto and
// h2transport (via onceSetNextProtoDefaults)
nextProtoOnce sync.Once
h2transport h2Transport // non-nil if http2 wired up
}
如此便是整个流程，其实还是很清晰的，最后总结一下：
tranport用来建立一个连接，其中维护了一个空闲连接池idleConn map[connectMethodKey][]*persistConn，其中的每个成员都是一个persistConn对象，persistConn是个具体的连接实例，包含了连接的上下文，会启动两个groutine分别执行readLoop和writeLoop, 每当transport调用roundTrip的时候，就会从连接池中选择一个空闲的persistConn，然后调用其roundTrip方法，将读写请求通过channel分别发送到readLoop和writeLoop中，然后会进行select各个channel的信息，包括连接关闭，请求超时，writeLoop出错， readLoop返回读取结果等。在writeLoop中发送请求，在readLoop中获取response并通过channe返回给roundTrip函数中，并再次将自己加入到idleConn中，等待下次请求到来。
