golang net之http server

golang 版本：1.12.9

简单的HTTP服务器代码：

package main

import (
    "net/http"
)

type TestHandler struct {
    str string
}

func (th *TestHandler)ServeHTTP(w http.ResponseWriter, r *http.Request){
    w.Write([]byte(string(th.str+",welcome")))
}

func main(){
    http.Handle("/", &TestHandler{"Hi,Stranger"})
    http.HandleFunc("/test",func(w http.ResponseWriter,r *http.Request){
        w.Write([]byte("Hi,Tester"))
    })
    http.ListenAndServe(":8000",nil)}

在浏览器输入“http://127.0.0.1:8000”得到输出“Hi,Stranger,welcome”；输入“http://127.0.0.1:8000/test”得到输出“Hi,Tester”

handler的注册

handler的相关方法如下：

func NewServeMux() *ServeMux
func (mux *ServeMux) Handle(pattern string, handler Handler)                               //注册handler
func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request))    //注册handler
func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string)                       //在mux.m中根据pattern查找handler
func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request)                               //handler的具体实现

http使用handler定义请求的路径以及请求的处理。每个handler都必须实现ServeHTTP方法，该方法将请求分发到不同的handler进行处理，每个handler处理一条请求路径。有两种注册handler的方式：http.Handle和http.HandleFunc，两种实现本质上是一致的，前者需要明确写出ServeHTTP方法的实现，后者由内置方法实现(见下文)。

Handler的接口定义如下：

// net/http/server.go
type Handler interface {
    ServeHTTP(ResponseWriter, *Request)
}

http.HandleFunc的第二个参数被定义为HandlerFunc，实现了Handler接口。

// net/http/server.go
type HandlerFunc func(ResponseWriter, *Request)

// ServeHTTP calls f(w, r).
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
    f(w, r)
}

当http.ListenAndServe的第二个参数为nil，则使用http.Handle和http.HandleFunc方法注册的handler，默认保存在http.DefaultServeMux.m中(注册方法为ServeMux.Handle/ServeMux.HandleFunc)。当http server接收到一个request时，会在serverHandler.ServeHTTP中调用DefaultServeMux.ServeHTTP来处理接收到的request，分为两步：

• • 调用ServeMux.Handler函数，在ServeMux.m中根据pattern遍历查合适的handler
  • 调用handler的ServeHTTP方法

serverHandler.ServeHTTP的源码如下：

// net/http/server.go
func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {
    // 如果有自注册的handler则使用自注册的，否则使用默认的handler处理请求
    handler := sh.srv.Handler
    if handler == nil {
        handler = DefaultServeMux
    }
    if req.RequestURI == "*" && req.Method == "OPTIONS" {
        handler = globalOptionsHandler{}
    }
    handler.ServeHTTP(rw, req)
}

DefaultServeMux的结构体定义如下：

var DefaultServeMux = &defaultServeMux
var defaultServeMux ServeMux

// net/http/server.go
type ServeMux struct {
    mu    sync.RWMutex
    m     map[string]muxEntry
    es    []muxEntry // slice of entries sorted from longest to shortest.
    hosts bool       // whether any patterns contain hostnames
}

默认的handler的ServeHTTP方法实现如下，主要实现查找handler并处理请求

func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
    if r.RequestURI == "*" {
        if r.ProtoAtLeast(1, 1) {
            w.Header().Set("Connection", "close")
        }
        w.WriteHeader(StatusBadRequest)
        return
    }
    // 根据请求的路径查找注册的handler
    h, _ := mux.Handler(r)
    // 调用注册的handler处理请求,对应上面例子的
    // http.HandleFunc("/test",func(w http.ResponseWriter,r *http.Request){w.Write([]byte("Hi,Tester"))})
    h.ServeHTTP(w, r)
}

// 本函数根据请求中的路径找到合适的handler或者重定向(请求路径格式不正确)
func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string) {

    // CONNECT requests are not canonicalized.
    // 对CONNECT请求的处理,代理场景
    if r.Method == "CONNECT" {
        // If r.URL.Path is /tree and its handler is not registered,
        // the /tree -> /tree/ redirect applies to CONNECT requests
        // but the path canonicalization does not.
        // redirectToPathSlash函数主要用于自动检测是否重定向URL并修改重定向URL路径，当注册的URL路径为/tree/,而请求URL路径为/tree，
        // redirectToPathSlash函数无法在mux.m中查找注册的handler，则将设请求URL设置为/tree/
        if u, ok := mux.redirectToPathSlash(r.URL.Host, r.URL.Path, r.URL); ok {
            return RedirectHandler(u.String(), StatusMovedPermanently), u.Path
        }

        return mux.handler(r.Host, r.URL.Path)
    }

    // All other requests have any port stripped and path cleaned
    // before passing to mux.handler.
    host := stripHostPort(r.Host)
    path := cleanPath(r.URL.Path)

    // 非代理场景重定向的处理，与"CONNECT"逻辑相同
    if u, ok := mux.redirectToPathSlash(host, path, r.URL); ok {
        return RedirectHandler(u.String(), StatusMovedPermanently), u.Path
    }
    // 如果请求路径不等于处理后的路径，如请求路径为"//test/",处理后的路径为"/test/",执行重定向并返回URL路径，重定向
    // 通过http.redirectHandler.ServeHTTP函数进行处理，如下：
    /*

        < HTTP/1.1 301 Moved Permanently
        < Content-Type: text/html; charset=utf-8
        < Location: /test/
        < Date: Fri, 06 Dec 2019 03:35:59 GMT
        < Content-Length: 41
        <
        <a href="/test/">Moved Permanently</a>.

      */

if path != r.URL.Path {
        _, pattern = mux.handler(host, path)
        url := *r.URL
        url.Path = path
        return RedirectHandler(url.String(), StatusMovedPermanently), pattern
    }
    // 在mux.m和mux.es中根据host/url.path找到对应的handler
    return mux.handler(host, r.URL.Path)
}

func (rh *redirectHandler) ServeHTTP(w ResponseWriter, r *Request) {
    Redirect(w, r, rh.url, rh.code)
}

通常使用http.HandleFunc注册handler，使用DefaultServeMux的方法分发处理请求即可。也可以通过http.NewServeMux()创建一个自定义的serverHandler，并实现Serve HTTP方法。

import (
    "net/http"
)

type TestHandler struct {
    str string
}

func (th *TestHandler)ServeHTTP(w http.ResponseWriter, r *http.Request){
    w.Write([]byte(string(th.str+",welcome")))
}

func main(){
    serverHandler := http.NewServeMux()
    serverHandler.Handle("/", &TestHandler{"Hi,Stranger"})
    serverHandler.HandleFunc("/test",func(w http.ResponseWriter,r *http.Request){
        w.Write([]byte("Hi,Tester"))
    })

    http.ListenAndServe(":8000",serverHandler)
}

http.server

调用下面函数进行监听，主要创建监听socket并接收该socket上的连接。通常调用如下接口即可：

func ListenAndServe(addr string, handler Handler) error {
    server := &Server{Addr: addr, Handler: handler}
    return server.ListenAndServe()
}

一个Server结构体表示一个启用监听端口的真实服务

type Server struct {
    Addr    string  // TCP address to listen on, ":http" if empty
    Handler Handler // handler to invoke, http.DefaultServeMux if nil

    // TLSConfig optionally provides a TLS configuration for use
    // by ServeTLS and ListenAndServeTLS. Note that this value is
    // cloned by ServeTLS and ListenAndServeTLS, so it's not
    // possible to modify the configuration with methods like
    // tls.Config.SetSessionTicketKeys. To use
    // SetSessionTicketKeys, use Server.Serve with a TLS Listener
    // instead.
    TLSConfig *tls.Config

    // ReadTimeout is the maximum duration for reading the entire
    // request, including the body.
    //
    // Because ReadTimeout does not let Handlers make per-request
    // decisions on each request body's acceptable deadline or
    // upload rate, most users will prefer to use
    // ReadHeaderTimeout. It is valid to use them both.
    ReadTimeout time.Duration

    // ReadHeaderTimeout is the amount of time allowed to read
    // request headers. The connection's read deadline is reset
    // after reading the headers and the Handler can decide what
    // is considered too slow for the body. If ReadHeaderTimeout
    // is zero, the value of ReadTimeout is used. If both are
    // zero, there is no timeout.
    ReadHeaderTimeout time.Duration

    // WriteTimeout is the maximum duration before timing out
    // writes of the response. It is reset whenever a new
    // request's header is read. Like ReadTimeout, it does not
    // let Handlers make decisions on a per-request basis.
    WriteTimeout time.Duration

    // IdleTimeout is the maximum amount of time to wait for the
    // next request when keep-alives are enabled. If IdleTimeout
    // is zero, the value of ReadTimeout is used. If both are
    // zero, there is no timeout.
    IdleTimeout time.Duration

    // MaxHeaderBytes controls the maximum number of bytes the
    // server will read parsing the request header's keys and
    // values, including the request line. It does not limit the
    // size of the request body.
    // If zero, DefaultMaxHeaderBytes is used.
    MaxHeaderBytes int

    // TLSNextProto optionally specifies a function to take over
    // ownership of the provided TLS connection when an NPN/ALPN
    // protocol upgrade has occurred. The map key is the protocol
    // name negotiated. The Handler argument should be used to
    // handle HTTP requests and will initialize the Request's TLS
    // and RemoteAddr if not already set. The connection is
    // automatically closed when the function returns.
    // If TLSNextProto is not nil, HTTP/2 support is not enabled
    // automatically.
    TLSNextProto map[string]func(*Server, *tls.Conn, Handler)

    // ConnState specifies an optional callback function that is
    // called when a client connection changes state. See the
    // ConnState type and associated constants for details.
    ConnState func(net.Conn, ConnState)

    // ErrorLog specifies an optional logger for errors accepting
    // connections, unexpected behavior from handlers, and
    // underlying FileSystem errors.
    // If nil, logging is done via the log package's standard logger.
    ErrorLog *log.Logger

    // BaseContext optionally specifies a function that returns
    // the base context for incoming requests on this server.
    // The provided Listener is the specific Listener that's
    // about to start accepting requests.
    // If BaseContext is nil, the default is context.Background().
    // If non-nil, it must return a non-nil context.
    BaseContext func(net.Listener) context.Context

    // ConnContext optionally specifies a function that modifies
    // the context used for a new connection c. The provided ctx
    // is derived from the base context and has a ServerContextKey
    // value.
    ConnContext func(ctx context.Context, c net.Conn) context.Context

    disableKeepAlives int32     // accessed atomically.
    inShutdown        int32     // accessed atomically (non-zero means we're in Shutdown)
    nextProtoOnce     sync.Once // guards setupHTTP2_* init
    nextProtoErr      error     // result of http2.ConfigureServer if used

    mu         sync.Mutex
    listeners  map[*net.Listener]struct{}
    activeConn map[*conn]struct{}
    doneChan   chan struct{}
    onShutdown []func()
}

 ListenAndServe在创建监听socket后调用Serve等待连接

func (srv *Server) ListenAndServe() error {
    // 服务器调用Server.Close或Server.Shutdown关闭连接时会设置shuttingDown为1，表示该服务正在停止，不可提供服务。
    // Close会直接关闭底层tcp连接，Shutdown则会调用服务提供的函数Server.onShutdown平滑关闭。推荐使用Shutdown
    if srv.shuttingDown() {
        return ErrServerClosed
    }
    addr := srv.Addr
    if addr == "" {
        addr = ":http"
    }
    ln, err := net.Listen("tcp", addr)
    if err != nil {
        return err
    }
    return srv.Serve(ln)
}

ListenAndServeTLS与ListenAndServe类似，只是入参多了证书参数

func (srv *Server) ListenAndServeTLS(certFile, keyFile string) error {
    if srv.shuttingDown() {
        return ErrServerClosed
    }
    addr := srv.Addr
    if addr == "" {
        addr = ":https"
    }

    ln, err := net.Listen("tcp", addr)
    if err != nil {
        return err
    }

    defer ln.Close()

    return srv.ServeTLS(ln, certFile, keyFile)
}

ServeTLS函数中会调用tls.NewListener创建一个tls类型的监听socket，后续会调用tls的Accetp函数接收客户端连接

func (srv *Server) ServeTLS(l net.Listener, certFile, keyFile string) error {
    // Setup HTTP/2 before srv.Serve, to initialize srv.TLSConfig
    // before we clone it and create the TLS Listener.
    if err := srv.setupHTTP2_ServeTLS(); err != nil {
        return err
    }

    config := cloneTLSConfig(srv.TLSConfig)
    if !strSliceContains(config.NextProtos, "http/1.1") {
        config.NextProtos = append(config.NextProtos, "http/1.1")
    }

    configHasCert := len(config.Certificates) > 0 || config.GetCertificate != nil
    if !configHasCert || certFile != "" || keyFile != "" {
        var err error
        config.Certificates = make([]tls.Certificate, 1)
        config.Certificates[0], err = tls.LoadX509KeyPair(certFile, keyFile)
        if err != nil {
            return err
        }
    }

    tlsListener := tls.NewListener(l, config)
    return srv.Serve(tlsListener)
}

// src/crypto/tls/tls.go
// tls的Accept仅仅在处理Server函数是增加了证书相关的参数
func (l *listener) Accept() (net.Conn, error) {
    c, err := l.Listener.Accept()
    if err != nil {
        return nil, err
    }
    return Server(c, l.config), nil
}

Serve主要实现如下。通过Accept与客户端创建连接后，通过newConn函数初始化一个HTTP连接，该连接包含HTTP的描述(监听地址，URL等)和一个TCP连接，然后处理来自客户的HTTP请求。

func (srv *Server) Serve(l net.Listener) error {
    ...
    ctx := context.WithValue(baseCtx, ServerContextKey, srv)
    for {
        // Accept()返回底层TCP的连接
        rw, e := l.Accept()
        if e != nil {
            select {
            case <-srv.getDoneChan():
                return ErrServerClosed
            default:
            }
            if ne, ok := e.(net.Error); ok && ne.Temporary() {
                // 处理accept因为网络失败之后的等待时间
                if tempDelay == 0 {
                    tempDelay = 5 * time.Millisecond
                } else {
                    tempDelay *= 2
                }
                if max := 1 * time.Second; tempDelay > max {
                    tempDelay = max
                }
                srv.logf("http: Accept error: %v; retrying in %v", e, tempDelay)
                time.Sleep(tempDelay)
                continue
            }
            return e
        }
        if cc := srv.ConnContext; cc != nil {
            ctx = cc(ctx, rw)
            if ctx == nil {
                panic("ConnContext returned nil")
            }
        }
        tempDelay = 0
        //构造HTTP连接
        c := srv.newConn(rw)
        c.setState(c.rwc, StateNew) // before Serve can return
        //在另外的goroutine中处理基于该TCP的HTTP请求，本goroutine可以继续accept TCP连接
        go c.serve(ctx)
    }
}

Accept返回的底层的连接结构如下

type Conn interface {
    // Read reads data from the connection.
    // Read can be made to time out and return an Error with Timeout() == true
    // after a fixed time limit; see SetDeadline and SetReadDeadline.
    Read(b []byte) (n int, err error)

    // Write writes data to the connection.
    // Write can be made to time out and return an Error with Timeout() == true
    // after a fixed time limit; see SetDeadline and SetWriteDeadline.
    Write(b []byte) (n int, err error)

    // Close closes the connection.
    // Any blocked Read or Write operations will be unblocked and return errors.
    Close() error

    // LocalAddr returns the local network address.
    LocalAddr() Addr

    // RemoteAddr returns the remote network address.
    RemoteAddr() Addr

    // SetDeadline sets the read and write deadlines associated
    // with the connection. It is equivalent to calling both
    // SetReadDeadline and SetWriteDeadline.
    //
    // A deadline is an absolute time after which I/O operations
    // fail with a timeout (see type Error) instead of
    // blocking. The deadline applies to all future and pending
    // I/O, not just the immediately following call to Read or
    // Write. After a deadline has been exceeded, the connection
    // can be refreshed by setting a deadline in the future.
    //
    // An idle timeout can be implemented by repeatedly extending
    // the deadline after successful Read or Write calls.
    //
    // A zero value for t means I/O operations will not time out.
    //
    // Note that if a TCP connection has keep-alive turned on,
    // which is the default unless overridden by Dialer.KeepAlive
    // or ListenConfig.KeepAlive, then a keep-alive failure may
    // also return a timeout error. On Unix systems a keep-alive
    // failure on I/O can be detected using
    // errors.Is(err, syscall.ETIMEDOUT).
    SetDeadline(t time.Time) error

    // SetReadDeadline sets the deadline for future Read calls
    // and any currently-blocked Read call.
    // A zero value for t means Read will not time out.
    SetReadDeadline(t time.Time) error

    // SetWriteDeadline sets the deadline for future Write calls
    // and any currently-blocked Write call.
    // Even if write times out, it may return n > 0, indicating that
    // some of the data was successfully written.
    // A zero value for t means Write will not time out.
    SetWriteDeadline(t time.Time) error
}

实现如上接口的有tcpsock的TCPConn以及unixsock的UnixConn，通常使用TCPConn

type TCPConn struct {
    conn
}

type UnixConn struct {
    conn
}

newConn生成的HTTP结构体如下，它表示一条基于TCP的HTTP连接，封装了3个重要的数据结构：server表示HTTP server的"server"；rwc表示底层连接结构体rwc net.Conn；r用于读取http数据的connReader（从rwc读取数据）。后续的request和response都基于该结构体

type conn struct {
    // server is the server on which the connection arrived.
    // Immutable; never nil.
    server *Server

    // cancelCtx cancels the connection-level context.
    cancelCtx context.CancelFunc

    // rwc is the underlying network connection.
    // This is never wrapped by other types and is the value given out
    // to CloseNotifier callers. It is usually of type *net.TCPConn or
    // *tls.Conn.
    rwc net.Conn

    // remoteAddr is rwc.RemoteAddr().String(). It is not populated synchronously
    // inside the Listener's Accept goroutine, as some implementations block.
    // It is populated immediately inside the (*conn).serve goroutine.
    // This is the value of a Handler's (*Request).RemoteAddr.
    remoteAddr string

    // tlsState is the TLS connection state when using TLS.
    // nil means not TLS.
    tlsState *tls.ConnectionState

    // werr is set to the first write error to rwc.
    // It is set via checkConnErrorWriter{w}, where bufw writes.
    werr error

    // r is bufr's read source. It's a wrapper around rwc that provides
    // io.LimitedReader-style limiting (while reading request headers)
    // and functionality to support CloseNotifier. See *connReader docs.
    r *connReader

    // bufr reads from r.
    bufr *bufio.Reader

    // bufw writes to checkConnErrorWriter{c}, which populates werr on error.
    bufw *bufio.Writer

    // lastMethod is the method of the most recent request
    // on this connection, if any.
    lastMethod string

    curReq atomic.Value // of *response (which has a Request in it)

    curState struct{ atomic uint64 } // packed (unixtime<<8|uint8(ConnState))

    // mu guards hijackedv
    mu sync.Mutex

    // hijackedv is whether this connection has been hijacked
    // by a Handler with the Hijacker interface.
    // It is guarded by mu.
    hijackedv bool
}

connReader中的conn就是上面表示http连接的结构体

type connReader struct {
    conn *conn

    mu      sync.Mutex // guards following
    hasByte bool
    byteBuf [1]byte
    cond    *sync.Cond
    inRead  bool
    aborted bool  // set true before conn.rwc deadline is set to past
    remain  int64 // bytes remaining
}

在下面的server函数中处理请求并返回响应

func (c *conn) serve(ctx context.Context) {
    c.remoteAddr = c.rwc.RemoteAddr().String()
    ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr())
    defer func() {
        if err := recover(); err != nil && err != ErrAbortHandler {
            const size = 64 << 10
            buf := make([]byte, size)
            buf = buf[:runtime.Stack(buf, false)]
            c.server.logf("http: panic serving %v: %v
%s", c.remoteAddr, err, buf)
        }
        if !c.hijacked() {
            c.close()
            c.setState(c.rwc, StateClosed)
        }
    }()

    // 处理ServeTLS accept的连接
    if tlsConn, ok := c.rwc.(*tls.Conn); ok {
        if d := c.server.ReadTimeout; d != 0 {
            // 设置TCP的读超时时间
            c.rwc.SetReadDeadline(time.Now().Add(d))
        }
        if d := c.server.WriteTimeout; d != 0 {
            // 设置TCP的写超时时间
            c.rwc.SetWriteDeadline(time.Now().Add(d))
        }
        // tls协商并判断协商结果
        if err := tlsConn.Handshake(); err != nil {
            // If the handshake failed due to the client not speaking
            // TLS, assume they're speaking plaintext HTTP and write a
            // 400 response on the TLS conn's underlying net.Conn.
            if re, ok := err.(tls.RecordHeaderError); ok && re.Conn != nil && tlsRecordHeaderLooksLikeHTTP(re.RecordHeader) {
                io.WriteString(re.Conn, "HTTP/1.0 400 Bad Request

Client sent an HTTP request to an HTTPS server.
")
                re.Conn.Close()
                return
            }
            c.server.logf("http: TLS handshake error from %s: %v", c.rwc.RemoteAddr(), err)
            return
        }
        c.tlsState = new(tls.ConnectionState)
        *c.tlsState = tlsConn.ConnectionState()
        // 用于判断是否使用TLS的NPN扩展协商出非http/1.1和http/1.0的上层协议，如果存在则使用server.TLSNextProto处理请求
        if proto := c.tlsState.NegotiatedProtocol; validNPN(proto) {
            if fn := c.server.TLSNextProto[proto]; fn != nil {
                h := initNPNRequest{ctx, tlsConn, serverHandler{c.server}}
                fn(c.server, tlsConn, h)
            }
            return
        }
    }

    // 下面处理HTTP/1.x的请求
    ctx, cancelCtx := context.WithCancel(ctx)
    c.cancelCtx = cancelCtx
    defer cancelCtx()
　　
　 　// 为c.bufr创建read源，使用sync.pool提高存取效率
    c.r = &connReader{conn: c}
    // read buf长度默认为4096,创建ioReader为c.r的bufio.Reader。用于读取HTTP的request
    c.bufr = newBufioReader(c.r)
　　 // c.bufw默认长度为4096，4<<10=4096，用于发送response
    c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)
    // 循环处理HTTP请求
    for {
　　　　 // 处理请求并返回封装好的响应
        w, err := c.readRequest(ctx)
        // 判断是否有读取过数据,如果读取过数据则设置TCP状态为active
        if c.r.remain != c.server.initialReadLimitSize() {
            // If we read any bytes off the wire, we're active.
            c.setState(c.rwc, StateActive)
        }
        // 处理http请求错误
        if err != nil {
            const errorHeaders = "
Content-Type: text/plain; charset=utf-8
Connection: close

"

            switch {
            case err == errTooLarge:
                // Their HTTP client may or may not be
                // able to read this if we're
                // responding to them and hanging up
                // while they're still writing their
                // request. Undefined behavior.
                const publicErr = "431 Request Header Fields Too Large"
                fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
                c.closeWriteAndWait()
                return
            // 直接return会断开底层TCP连接(GC？)
            case isUnsupportedTEError(err):
                // Respond as per RFC 7230 Section 3.3.1 which says,
                //      A server that receives a request message with a
                //      transfer coding it does not understand SHOULD
                //      respond with 501 (Unimplemented).
                code := StatusNotImplemented

                // We purposefully aren't echoing back the transfer-encoding's value,
                // so as to mitigate the risk of cross side scripting by an attacker.
                fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s%sUnsupported transfer encoding", code, StatusText(code), errorHeaders)
                return

            case isCommonNetReadError(err):
                return // don't reply

            default:
                publicErr := "400 Bad Request"
                if v, ok := err.(badRequestError); ok {
                    publicErr = publicErr + ": " + string(v)
                }

                fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
                return
            }
        }

        // Expect 100 Continue support
        req := w.req
        // 如果http首部包含"100-continue"请求
        if req.expectsContinue() {
            // "100-continue"的首部要求http1.1版本以上，且http.body长度不为0
            if req.ProtoAtLeast(1, 1) && req.ContentLength != 0 {
                // Wrap the Body reader with one that replies on the connection
                req.Body = &expectContinueReader{readCloser: req.Body, resp: w}
            }
        // 非"100-continue"但首部包含"Expect"字段的请求为非法请求
        } else if req.Header.get("Expect") != "" {
            w.sendExpectationFailed()
            return
        }
        // curReq保存了当前的response，当前代码中主要用于在读失败后调用response中的closeNotifyCh传递信号，此时连接断开
        c.curReq.Store(w)
        // 判断是否有后续的数据，req.Body在http.readTransfer函数中设置为http.body类型，registerOnHitEOF注册的就是
        // 遇到EOF时执行的函数http.body.onHitEOF
        if requestBodyRemains(req.Body) {
            registerOnHitEOF(req.Body, w.conn.r.startBackgroundRead)
        } else {
            // 如果没有后续的数据，调用下面函数在新的goroutine中阻塞等待数据的到来，通知finishRequest
            w.conn.r.startBackgroundRead()
        }

        // HTTP cannot have multiple simultaneous active requests.[*]
        // Until the server replies to this request, it can't read another,
        // so we might as well run the handler in this goroutine.
        // [*] Not strictly true: HTTP pipelining. We could let them all process
        // in parallel even if their responses need to be serialized.
        // But we're not going to implement HTTP pipelining because it
        // was never deployed in the wild and the answer is HTTP/2.
        // 通过请求找到匹配的handler，然后处理请求并发送响应
        serverHandler{c.server}.ServeHTTP(w, w.req)
        w.cancelCtx()
        if c.hijacked() {
            return
        }
        // 该函数中会结束HTTP请求，发送response
        w.finishRequest()
        // 判断是否需要重用底层TCP连接，即是否退出本函数的for循环，推出for循环将断开连接
        if !w.shouldReuseConnection() {
            // 不可重用底层连接时,如果请求数据过大或设置提前取消读取数据，则调用closeWriteAndWait平滑关闭TCP连接
            if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
                c.closeWriteAndWait()
            }
            return
        }
        // 重用连接，设置底层状态为idle
        c.setState(c.rwc, StateIdle) 
        c.curReq.Store((*response)(nil))
        
        // 如果没有通过SetKeepAlivesEnabled设置HTTP keepalive或底层连接已经通过如Server.Close关闭，则直接退出
        if !w.conn.server.doKeepAlives() {
            // We're in shutdown mode. We might've replied
            // to the user without "Connection: close" and
            // they might think they can send another
            // request, but such is life with HTTP/1.1.
            return
        }

        if d := c.server.idleTimeout(); d != 0 {
            // 如果设置了idle状态超时时间，则调用SetReadDeadline设置底层连接deadline，并调用bufr.Peek等待请求
            c.rwc.SetReadDeadline(time.Now().Add(d))
            if _, err := c.bufr.Peek(4); err != nil {
                return
            }
        }
        c.rwc.SetReadDeadline(time.Time{})
    }
}

readRequest函数处理http请求

func (c *conn) readRequest(ctx context.Context) (w *response, err error) {
    if c.hijacked() {
        return nil, ErrHijacked
    }

    var (
        wholeReqDeadline time.Time // or zero if none
        hdrDeadline      time.Time // or zero if none
    )
    t0 := time.Now()
    // 设置读取HTTP的超时时间
    if d := c.server.readHeaderTimeout(); d != 0 {
        hdrDeadline = t0.Add(d)
    }
    // 设置读取整个HTTP的超时时间
    if d := c.server.ReadTimeout; d != 0 {
        wholeReqDeadline = t0.Add(d)
    }
    // 通过SetReadDeadline设置TCP读超时时间
    c.rwc.SetReadDeadline(hdrDeadline)
    if d := c.server.WriteTimeout; d != 0 {
        // 通过defer设置TCP写超时时间，本函数主要处理读请求，在本函数处理完request之后再设置写超时时间
        defer func() {
            c.rwc.SetWriteDeadline(time.Now().Add(d))
        }()
    }
    // 设置读取请求的最大字节数，为DefaultMaxHeaderBytes+4096=1052672，用于防止超大报文攻击
    c.r.setReadLimit(c.server.initialReadLimitSize())
    // 处理老设备的client
    if c.lastMethod == "POST" {
        // RFC 7230 section 3.5 Message Parsing Robustness tolerance for old buggy clients.
        peek, _ := c.bufr.Peek(4) // ReadRequest will get err below
        c.bufr.Discard(numLeadingCRorLF(peek))
    }
    // 从bufr读取request，并返回结构体格式的请求
    req, err := readRequest(c.bufr, keepHostHeader)
    if err != nil {
        // 如果读取的报文超过限制，则返回错误
        if c.r.hitReadLimit() {
            return nil, errTooLarge
        }
        return nil, err
    }
    // 判断是否是go服务所支持的HTTP/1.x的请求
    if !http1ServerSupportsRequest(req) {
        return nil, badRequestError("unsupported protocol version")
    }

    c.lastMethod = req.Method
    c.r.setInfiniteReadLimit()

    hosts, haveHost := req.Header["Host"]
    isH2Upgrade := req.isH2Upgrade()
    // 判断是否需要Host首部字段
    if req.ProtoAtLeast(1, 1) && (!haveHost || len(hosts) == 0) && !isH2Upgrade && req.Method != "CONNECT" {
        return nil, badRequestError("missing required Host header")
    }
    // 多个Host首部字段
    if len(hosts) > 1 {
        return nil, badRequestError("too many Host headers")
    }
    // 非法Host首部字段值
    if len(hosts) == 1 && !httpguts.ValidHostHeader(hosts[0]) {
        return nil, badRequestError("malformed Host header")
    }
    // 判断首部字段值是否有非法字符
    for k, vv := range req.Header {
        if !httpguts.ValidHeaderFieldName(k) {
            return nil, badRequestError("invalid header name")
        }
        for _, v := range vv {
            if !httpguts.ValidHeaderFieldValue(v) {
                return nil, badRequestError("invalid header value")
            }
        }
    }
    // 响应报文中不包含Host字段
    delete(req.Header, "Host")

    ctx, cancelCtx := context.WithCancel(ctx)
    req.ctx = ctx
    req.RemoteAddr = c.remoteAddr
    req.TLS = c.tlsState
    if body, ok := req.Body.(*body); ok {
        body.doEarlyClose = true
    }

    // 判断是否超过请求的最大值
    if !hdrDeadline.Equal(wholeReqDeadline) {
        c.rwc.SetReadDeadline(wholeReqDeadline)
    }

    w = &response{
        conn:          c,
        cancelCtx:     cancelCtx,
        req:           req,
        reqBody:       req.Body,
        handlerHeader: make(Header),
        contentLength: -1,
        closeNotifyCh: make(chan bool, 1),

        // We populate these ahead of time so we're not
        // reading from req.Header after their Handler starts
        // and maybe mutates it (Issue 14940)
        wants10KeepAlive: req.wantsHttp10KeepAlive(),
        wantsClose:       req.wantsClose(),
    }
    if isH2Upgrade {
        w.closeAfterReply = true
    }
    // w.cw.res中保存了response的信息，而response中又保存了底层连接conn，后续将通过w.cw.res.conn写数据
    w.cw.res = w
    // 创建2048字节的写bufio，用于发送response
    w.w = newBufioWriterSize(&w.cw, bufferBeforeChunkingSize)
    return w, nil
}

读取HTTP请求，并将其结构化为http.Request

func readRequest(b *bufio.Reader, deleteHostHeader bool) (req *Request, err error) {
    // 封装为textproto.Reader，该结构体实现了读取HTTP的相关方法
    tp := newTextprotoReader(b)
    // 初始化一个Request结构体，该函数后续工作就是填充该变量并返回
    req = new(Request)

    // First line: GET /index.html HTTP/1.0
    var s string
    // ReadLine会调用<textproto.(*Reader).ReadLine->textproto.(*Reader).readLineSlice->bufio.(*Reader).ReadLine->
    // bufio.(*Reader).ReadSlic->bufio.(*Reader).fill->http.(*connReader).Read>读取HTTP的请求并填充b.buf，并返回以"
"作为
    // 分隔符的首行字符串
    if s, err = tp.ReadLine(); err != nil {
        return nil, err
    }
    // putTextprotoReader函数使用sync.pool来保存textproto.Reader变量，通过重用内存来提升在大量HTTP请求下执行效率。
    // 对应函数首部的newTextprotoReader
    defer func() {
        putTextprotoReader(tp)
        if err == io.EOF {
            err = io.ErrUnexpectedEOF
        }
    }()

    var ok bool
    // 解析请求方法，请求URL，请求协议
    req.Method, req.RequestURI, req.Proto, ok = parseRequestLine(s)
    if !ok {
        return nil, &badStringError{"malformed HTTP request", s}
    }
    // 判断方法是否包含非法字符
    if !validMethod(req.Method) {
        return nil, &badStringError{"invalid method", req.Method}
    }
    // 获取请求路径，如HTTP请求为"http://127.0.0.1:8000/test"时，rawurl为"/test"
    rawurl := req.RequestURI
    // 判断HTTP协议版本有效性，通常为支持HTTP/1.x
    if req.ProtoMajor, req.ProtoMinor, ok = ParseHTTPVersion(req.Proto); !ok {
        return nil, &badStringError{"malformed HTTP version", req.Proto}
    }

    // CONNECT requests are used two different ways, and neither uses a full URL:
    // The standard use is to tunnel HTTPS through an HTTP proxy.
    // It looks like "CONNECT www.google.com:443 HTTP/1.1", and the parameter is
    // just the authority section of a URL. This information should go in req.URL.Host.
    //
    // The net/rpc package also uses CONNECT, but there the parameter is a path
    // that starts with a slash. It can be parsed with the regular URL parser,
    // and the path will end up in req.URL.Path, where it needs to be in order for
    // RPC to work.
    // 处理代理场景，使用"CONNECT"与代理建立连接时会使用完整的URL(带host)
    justAuthority := req.Method == "CONNECT" && !strings.HasPrefix(rawurl, "/")
    if justAuthority {
        rawurl = "http://" + rawurl
    }

    if req.URL, err = url.ParseRequestURI(rawurl); err != nil {
        return nil, err
    }

    if justAuthority {
        // Strip the bogus "http://" back off.
        req.URL.Scheme = ""
    }

    // 解析request首部的key：value
    mimeHeader, err := tp.ReadMIMEHeader()
    if err != nil {
        return nil, err
    }
    req.Header = Header(mimeHeader)

    // RFC 7230, section 5.3: Must treat
    //    GET /index.html HTTP/1.1
    //    Host: www.google.com
    // and
    //    GET http://www.google.com/index.html HTTP/1.1
    //    Host: doesntmatter
    // the same. In the second case, any Host line is ignored.
    req.Host = req.URL.Host
    // 如果是上面注释中的第一种需要从req.Header中获取"Host"字段
    if req.Host == "" {
        req.Host = req.Header.get("Host")
    }
    // "Host"字段仅存在于request中，在接收到之后需要删除首部的Host字段，更多参见该变量注释
    if deleteHostHeader {
        delete(req.Header, "Host")
    }
    //处理"Cache-Control"首部
    fixPragmaCacheControl(req.Header)
    // 判断是否是长连接，如果是，则保持连接，反之则断开并删除"Connection"首部
    req.Close = shouldClose(req.ProtoMajor, req.ProtoMinor, req.Header, false)
    // 解析首部字段并填充req内容
    err = readTransfer(req, b)
    if err != nil {
        return nil, err
    }
    // 当HTTP1.1服务尝试解析HTTP2的消息时使用"PRI"方法
    if req.isH2Upgrade() {
        // Because it's neither chunked, nor declared:
        req.ContentLength = -1

        // We want to give handlers a chance to hijack the
        // connection, but we need to prevent the Server from
        // dealing with the connection further if it's not
        // hijacked. Set Close to ensure that:
        req.Close = true
    }
    return req, nil
}

func shouldClose(major, minor int, header Header, removeCloseHeader bool) bool {
    // HTTP/1.x以下不支持"connection"指定长连接
    if major < 1 {
        return true
    }

    conv := header["Connection"]
    // 如果首部包含"Connection: close"则断开连接
    hasClose := httpguts.HeaderValuesContainsToken(conv, "close")
    // 使用HTTP/1.0时，如果包含"Connection: close"或不包含"Connection: keep-alive"，则使用短连接；
    // HTTP/1.1中不指定"Connection"，默认使用长连接
    if major == 1 && minor == 0 {
        return hasClose || !httpguts.HeaderValuesContainsToken(conv, "keep-alive")
    }
    // 如果使用非长连接，且需要删除首部中的Connection字段。在经过proxy或gateway时必须移除Connection首部字段
    if hasClose && removeCloseHeader {
        header.Del("Connection")
    }

    return hasClose
}

func readTransfer(msg interface{}, r *bufio.Reader) (err error) {
    t := &transferReader{RequestMethod: "GET"}

    // Unify input
    isResponse := false
    switch rr := msg.(type) {
    // 消息为响应时的赋值
    case *Response:
        t.Header = rr.Header
        t.StatusCode = rr.StatusCode
        t.ProtoMajor = rr.ProtoMajor
        t.ProtoMinor = rr.ProtoMinor
        // 响应中不需要Connection首部字段，下面函数最后一个参数设置为true，删除该首部字段
        t.Close = shouldClose(t.ProtoMajor, t.ProtoMinor, t.Header, true)
        isResponse = true
        if rr.Request != nil {
            t.RequestMethod = rr.Request.Method
        }
    // 消息为请求时的赋值
    case *Request:
        t.Header = rr.Header
        t.RequestMethod = rr.Method
        t.ProtoMajor = rr.ProtoMajor
        t.ProtoMinor = rr.ProtoMinor
        // Transfer semantics for Requests are exactly like those for
        // Responses with status code 200, responding to a GET method
        t.StatusCode = 200
        t.Close = rr.Close
    default:
        panic("unexpected type")
    }

    // Default to HTTP/1.1
    if t.ProtoMajor == 0 && t.ProtoMinor == 0 {
        t.ProtoMajor, t.ProtoMinor = 1, 1
    }

    // 处理"Transfer-Encoding"首部
    err = t.fixTransferEncoding()
    if err != nil {
        return err
    }
    // 处理"Content-Length"首部,注意此处返回的是真实的消息载体长度
    realLength, err := fixLength(isResponse, t.StatusCode, t.RequestMethod, t.Header, t.TransferEncoding)
    if err != nil {
        return err
    }
    // 如果该消息为响应且对应的请求方法为HEAD，如果响应首部包含Content-Length字段，则将此作为响应的ContentLength的值，表示server
    // 可以接收到的数据的最大长度，由于该响应没有有效载体，此时不能使用fixLength返回的真实长度0
    if isResponse && t.RequestMethod == "HEAD" {
        if n, err := parseContentLength(t.Header.get("Content-Length")); err != nil {
            return err
        } else {
            t.ContentLength = n
        }
    } else {
        t.ContentLength = realLength
    }

    // 处理Trailer首部字段，主要进行有消息校验
    t.Trailer, err = fixTrailer(t.Header, t.TransferEncoding)
    if err != nil {
        return err
    }

    // If there is no Content-Length or chunked Transfer-Encoding on a *Response
    // and the status is not 1xx, 204 or 304, then the body is unbounded.
    // See RFC 7230, section 3.3.
    // 含body但不是chunked且不包含length字段的响应称为unbounded(无法衡量长度的消息)消息，根据RFC 7230会被关闭
    switch msg.(type) {
    case *Response:
        if realLength == -1 &&
            !chunked(t.TransferEncoding) &&
            bodyAllowedForStatus(t.StatusCode) {
            // Unbounded body.
            t.Close = true
        }
    }

    // Prepare body reader. ContentLength < 0 means chunked encoding
    // or close connection when finished, since multipart is not supported yet
    // 给t.Body赋值
    switch {
    // chunked 场景处理
    case chunked(t.TransferEncoding):
        // 如果请求为HEAD或响应状态码为1xx, 204 or 304，则消息不包含有效载体
        if noResponseBodyExpected(t.RequestMethod) || !bodyAllowedForStatus(t.StatusCode) {
            t.Body = NoBody
        } else {
            // 下面会创建chunkedReader
            t.Body = &body{src: internal.NewChunkedReader(r), hdr: msg, r: r, closing: t.Close}
        }
    case realLength == 0:
        t.Body = NoBody
    // 非chunked且包含有效载体(对应Content-Length)，创建limitReader
    case realLength > 0:
        t.Body = &body{src: io.LimitReader(r, realLength), closing: t.Close}
    default:
        // realLength < 0, i.e. "Content-Length" not mentioned in header
        // 此处对于消息有效载体unbounded场景，断开底层连接
        if t.Close {
            // Close semantics (i.e. HTTP/1.0)
            t.Body = &body{src: r, closing: t.Close}
        } else {
            // Persistent connection (i.e. HTTP/1.1) 好像走不到该分支。。。
            t.Body = NoBody
        }
    }

    // 为请求/响应结构体赋值并通过指针返回
    switch rr := msg.(type) {
    case *Request:
        rr.Body = t.Body
        rr.ContentLength = t.ContentLength
        rr.TransferEncoding = t.TransferEncoding
        rr.Close = t.Close
        rr.Trailer = t.Trailer
    case *Response:
        rr.Body = t.Body
        rr.ContentLength = t.ContentLength
        rr.TransferEncoding = t.TransferEncoding
        rr.Close = t.Close
        rr.Trailer = t.Trailer
    }

    return nil
}

// 1.13.3版本的本函数描述有误，下面代码来自最新master分支
func (t *transferReader) fixTransferEncoding() error {
    // 本函数主要处理"Transfer-Encoding"首部，如果不存在，则直接退出
    raw, present := t.Header["Transfer-Encoding"]
    if !present {
        return nil
    }
    delete(t.Header, "Transfer-Encoding")

    // Issue 12785; ignore Transfer-Encoding on HTTP/1.0 requests.
    // HTTP/1.0不处理此首部
    if !t.protoAtLeast(1, 1) {
        return nil
    }

    // "Transfer-Encoding"首部字段使用逗号分割
    encodings := strings.Split(raw[0], ",")
    te := make([]string, 0, len(encodings))

    // When adding new encodings, please maintain the invariant:
    //   if chunked encoding is present, it must always
    //   come last and it must be applied only once.
    // See RFC 7230 Section 3.3.1 Transfer-Encoding.
    // 循环处理各个传输编码，目前仅实现了"chunked"
    for i, encoding := range encodings {
        encoding = strings.ToLower(strings.TrimSpace(encoding))

        if encoding == "identity" {
            // "identity" should not be mixed with other transfer-encodings/compressions
            // because it means "no compression, no transformation".
            if len(encodings) != 1 {
                return &badStringError{`"identity" when present must be the only transfer encoding`, strings.Join(encodings, ",")}
            }
            // "identity" is not recorded.
            break
        }

        switch {
        case encoding == "chunked":
            // "chunked" MUST ALWAYS be the last
            // encoding as per the  loop invariant.
            // That is:
            //     Invalid: [chunked, gzip]
            //     Valid:   [gzip, chunked]
            if i+1 != len(encodings) {
                return &badStringError{"chunked must be applied only once, as the last encoding", strings.Join(encodings, ",")}
            }
            // Supported otherwise.

        case isGzipTransferEncoding(encoding):
            // Supported

        default:
            return &unsupportedTEError{fmt.Sprintf("unsupported transfer encoding: %q", encoding)}
        }

        te = te[0 : len(te)+1]
        te[len(te)-1] = encoding
    }

    if len(te) > 0 {
        // RFC 7230 3.3.2 says "A sender MUST NOT send a
        // Content-Length header field in any message that
        // contains a Transfer-Encoding header field."
        //
        // but also:
        // "If a message is received with both a
        // Transfer-Encoding and a Content-Length header
        // field, the Transfer-Encoding overrides the
        // Content-Length. Such a message might indicate an
        // attempt to perform request smuggling (Section 9.5)
        // or response splitting (Section 9.4) and ought to be
        // handled as an error. A sender MUST remove the
        // received Content-Length field prior to forwarding
        // such a message downstream."
        //
        // Reportedly, these appear in the wild.
        // "Transfer-Encoding"就是为了解决"Content-Length"不存在才出现了，因此当存在"Transfer-Encoding"时无需处理"Content-Length"，
        // 此处删除"Content-Length"首部，不在fixLength函数中处理

        delete(t.Header, "Content-Length")
        t.TransferEncoding = te
        return nil
    }

    return nil
}

// 本函数处理Content-Length首部，并返回真实的消息载体长度
func fixLength(isResponse bool, status int, requestMethod string, header Header, te []string) (int64, error) {
    isRequest := !isResponse
    contentLens := header["Content-Length"]

    // Hardening against HTTP request smuggling
    if len(contentLens) > 1 {
        // Per RFC 7230 Section 3.3.2, prevent multiple
        // Content-Length headers if they differ in value.
        // If there are dups of the value, remove the dups.
        // See Issue 16490.
        // 下面按照RFC 7230的建议进行处理，如果一个Content-Length包含多个不同的value，则认为该消息无效
        first := strings.TrimSpace(contentLens[0])
        for _, ct := range contentLens[1:] {
            if first != strings.TrimSpace(ct) {
                return 0, fmt.Errorf("http: message cannot contain multiple Content-Length headers; got %q", contentLens)
            }
        }

        // 如果一个Content-Length包含多个相同的value，则仅保留一个
        header.Del("Content-Length")
        header.Add("Content-Length", first)

        contentLens = header["Content-Length"]
    }

    // 处理HEAD请求
    if noResponseBodyExpected(requestMethod) {
        // For HTTP requests, as part of hardening against request
        // smuggling (RFC 7230), don't allow a Content-Length header for
        // methods which don't permit bodies. As an exception, allow
        // exactly one Content-Length header if its value is "0".
        // 当HEAD请求中的Content-Length为0时允许存在该字段
        if isRequest && len(contentLens) > 0 && !(len(contentLens) == 1 && contentLens[0] == "0") {
            return 0, fmt.Errorf("http: method cannot contain a Content-Length; got %q", contentLens)
        }
        return 0, nil
    }
    // 处理状态码为1xx的响应，不包含消息体
    if status/100 == 1 {
        return 0, nil
    }
    // 处理状态码为204和304的响应，不包含消息体
    switch status {
    case 204, 304:
        return 0, nil
    }

    // 包含Transfer-Encoding时无法衡量数据长度,以Transfer-Encoding为准，设置返回长度为-1，直接返回
    if chunked(te) {
        return -1, nil
    }
    
    var cl string
    // 获取Content-Length字段值
    if len(contentLens) == 1 {
        cl = strings.TrimSpace(contentLens[0])
    }
    // 对Content-Length字段的值进行有效性验证,如果有效则返回该值的整型，无效返回错误
    if cl != "" {
        n, err := parseContentLength(cl)
        if err != nil {
            return -1, err
        }
        return n, nil
    }
    // 数值为空，删除该首部字段
    header.Del("Content-Length")

    // 请求中没有Content-Length且没有Transfer-Encoding字段的请求被认为没有有效载体
    if isRequest {
        // RFC 7230 neither explicitly permits nor forbids an
        // entity-body on a GET request so we permit one if
        // declared, but we default to 0 here (not -1 below)
        // if there's no mention of a body.
        // Likewise, all other request methods are assumed to have
        // no body if neither Transfer-Encoding chunked nor a
        // Content-Length are set.
        return 0, nil
    }

    // Body-EOF logic based on other methods (like closing, or chunked coding)
    // 消息为响应，该场景后续会在readTransfer被close处理
    return -1, nil
}

func (cr *connReader) startBackgroundRead() {
    cr.lock()
    defer cr.unlock()
    // 表示该连接正在被读取
    if cr.inRead {
        panic("invalid concurrent Body.Read call")
    }
    // 表示该连接上是否还有数据
    if cr.hasByte {
        return
    }
    cr.inRead = true
    // 设置底层连接deadline为1<<64 -1
    cr.conn.rwc.SetReadDeadline(time.Time{})
    // 在新的goroutine中等待数据
    go cr.backgroundRead()
}

func (cr *connReader) backgroundRead() {
    // 阻塞等待读取一个字节的数
    n, err := cr.conn.rwc.Read(cr.byteBuf[:])
    cr.lock()
    // 如果存在数据则设置cr.hasByte为true,byteBuf容量为1
    if n == 1 {
        cr.hasByte = true
        // We were past the end of the previous request's body already
        // (since we wouldn't be in a background read otherwise), so
        // this is a pipelined HTTP request. Prior to Go 1.11 we used to
        // send on the CloseNotify channel and cancel the context here,
        // but the behavior was documented as only "may", and we only
        // did that because that's how CloseNotify accidentally behaved
        // in very early Go releases prior to context support. Once we
        // added context support, people used a Handler's
        // Request.Context() and passed it along. Having that context
        // cancel on pipelined HTTP requests caused problems.
        // Fortunately, almost nothing uses HTTP/1.x pipelining.
        // Unfortunately, apt-get does, or sometimes does.
        // New Go 1.11 behavior: don't fire CloseNotify or cancel
        // contexts on pipelined requests. Shouldn't affect people, but
        // fixes cases like Issue 23921. This does mean that a client
        // closing their TCP connection after sending a pipelined
        // request won't cancel the context, but we'll catch that on any
        // write failure (in checkConnErrorWriter.Write).
        // If the server never writes, yes, there are still contrived
        // server & client behaviors where this fails to ever cancel the
        // context, but that's kinda why HTTP/1.x pipelining died
        // anyway.
    }
    if ne, ok := err.(net.Error); ok && cr.aborted && ne.Timeout() {
        // Ignore this error. It's the expected error from
        // another goroutine calling abortPendingRead.
    } else if err != nil {
        cr.handleReadError(err)
    }
    cr.aborted = false
    cr.inRead = false
    cr.unlock()
    // 当有数据时，通知cr.cond.Wait解锁
    cr.cond.Broadcast()
}

func (w *response) finishRequest() {
    w.handlerDone.setTrue()
    // wroteHeader表示是否已经将响应首部写入，没有则写入
    if !w.wroteHeader {
        w.WriteHeader(StatusOK)
    }
    // 此处调用w.cw.write(checkConnErrorWriter) -> c.rwc.write发送数据，即调用底层连接的write将buf中的数据发送出去
    w.w.Flush()
    // 将w.w重置并放入sync.pool中，待后续重用
    putBufioWriter(w.w)
    
    // 主要构造chunked的结束符:"0
"，"
"，通过cw.chunking判断是否是chunked编码
    w.cw.close()
    // 发送bufw缓存的数据
    w.conn.bufw.Flush()
    // 用于等待处理未读取完的数据，与connReader.backgroundRead中的cr.cond.Broadcast()对应
    w.conn.r.abortPendingRead()

    // Close the body (regardless of w.closeAfterReply) so we can
    // re-use its bufio.Reader later safely.
    w.reqBody.Close()

    if w.req.MultipartForm != nil {
        w.req.MultipartForm.RemoveAll()
    }
}

func (w *response) shouldReuseConnection() bool {
    // 表示是否需要在响应之后关闭底层连接。requestTooLarge，isH2Upgrade或包含首部字段"Connection:close"时置位
    if w.closeAfterReply {
        // The request or something set while executing the
        // handler indicated we shouldn't reuse this
        // connection.
        return false
    }
    // 写入数据与"content-length"不匹配,为避免不同步，不重用连接
    if w.req.Method != "HEAD" && w.contentLength != -1 && w.bodyAllowed() && w.contentLength != w.written {
        // Did not write enough. Avoid getting out of sync.
        return false
    }

    // There was some error writing to the underlying connection
    // during the request, so don't re-use this conn.
    // 底层连接出现错误，不可重用
    if w.conn.werr != nil {
        return false
    }
    // 判断是否在读取完数据前执行关闭
    if w.closedRequestBodyEarly() {
        return false
    }

    return true
}

// closeWrite flushes any outstanding data and sends a FIN packet (if
// client is connected via TCP), signalling that we're done. We then
// pause for a bit, hoping the client processes it before any
// subsequent RST.
//
// See https://golang.org/issue/3595
func (c *conn) closeWriteAndWait() {
   // 在关闭写之前将缓冲区中的数据发送出去
   c.finalFlush()
   if tcp, ok := c.rwc.(closeWriter); ok {
      // 执行tcpsock.go中的TCPConn.CloseWrite，调用SHUT_WR关闭写
      tcp.CloseWrite()
   }
   time.Sleep(rstAvoidanceDelay)
}

func (c *conn) finalFlush() {
    // 本函数中如果c.bufr或c.bufw不为空，都会重置并重用这部分内存
    if c.bufr != nil {
        // Steal the bufio.Reader (~4KB worth of memory) and its associated
        // reader for a future connection.
        putBufioReader(c.bufr)
        c.bufr = nil
    }

    if c.bufw != nil {
        // 将缓存区中的数据全部通过底层发送出去
        // respose写数据调用为c.bufw.wr.Write -> checkConnErrorWriter.write -> c.rwc.write，最终通过底层write发送数据
        c.bufw.Flush()
        // Steal the bufio.Writer (~4KB worth of memory) and its associated
        // writer for a future connection.
        putBufioWriter(c.bufw)
        c.bufw = nil
    }
}

http.transport

 参见详解transport

NetPoll

　参见详解golang net之netpoll

参考：

https://golang.org/pkg/net/http/

https://lanre.wtf/blog/2017/07/24/roundtripper-go/

https://lanre.wtf/blog/2017/04/03/http-in-go/