golang http/transport 代码分析

谁借莪１个温暖的怀抱￠ 2021-12-18 07:53 280阅读 0赞

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。

## 推荐阅读 ##

[Go HTTP Client 持久连接][Go HTTP Client]

转载于:https://www.cnblogs.com/gaorong/p/10887021.html

[Go HTTP Client]: https://serholiu.com/go-http-client-keepalive