golang源码阅读-net.Dial

1.基本使用

//获取链接
conn, err := net.Dial("tcp", "0.0.0.0:8080")if err != nil {fmt.Println("Dial error ", err)return}//通过链接写入数据conn.Write([]byte("hello everyone"))

2.Dial源码

func Dial(network, address string) (Conn, error) {var d Dialer
//调用Dialer的Dial方法return d.Dial(network, address)
}

3.Dialer的Dial方法源码

func (d *Dialer) Dial(network, address string) (Conn, error) {return d.DialContext(context.Background(), network, address)
}

4. DialContext源码

func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn, error) {
//判断上下文是否为空if ctx == nil {panic("nil context")}
//获取deadline, 获取dialer和上下文的最早的deadlinedeadline := d.deadline(ctx, time.Now())if !deadline.IsZero() {
//如果不是日期的 0//如果上下文没有deadline 或者 上下文的deadline早于dialer的if d, ok := ctx.Deadline(); !ok || deadline.Before(d) {//构建子上下文subCtx, cancel := context.WithDeadline(ctx, deadline)defer cancel()ctx = subCtx}}
//如果dialer有cancel 转成上下文 WithCancelif oldCancel := d.Cancel; oldCancel != nil {subCtx, cancel := context.WithCancel(ctx)defer cancel()go func() {select {case <-oldCancel:cancel()case <-subCtx.Done():}}()ctx = subCtx}// Shadow the nettrace (if any) during resolve so Connect events don't fire for DNS lookups.resolveCtx := ctxif trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace); trace != nil {shadow := *traceshadow.ConnectStart = nilshadow.ConnectDone = nilresolveCtx = context.WithValue(resolveCtx, nettrace.TraceKey{}, &shadow)}//解析出地址列表addrs, err := d.resolver().resolveAddrList(resolveCtx, "dial", network, address, d.LocalAddr)if err != nil {return nil, &OpError{Op: "dial", Net: network, Source: nil, Addr: nil, Err: err}}//实例sd := &sysDialer{Dialer:  *d,network: network,address: address,}//ipv6回退ipv4的处理var primaries, fallbacks addrListif d.dualStack() && network == "tcp" {primaries, fallbacks = addrs.partition(isIPv4)} else {primaries = addrs}var c Connif len(fallbacks) > 0 {c, err = sd.dialParallel(ctx, primaries, fallbacks)} else {//获取链接c, err = sd.dialSerial(ctx, primaries)}if err != nil {return nil, err}//如果是tcp链接 设置 KeepAliveif tc, ok := c.(*TCPConn); ok && d.KeepAlive >= 0 {setKeepAlive(tc.fd, true)ka := d.KeepAliveif d.KeepAlive == 0 {ka = 15 * time.Second}setKeepAlivePeriod(tc.fd, ka)testHookSetKeepAlive(ka)}return c, nil
}

5.sd.dialSerial源码

func (sd *sysDialer) dialSerial(ctx context.Context, ras addrList) (Conn, error) {//链接的第一个错误
var firstErr error // The error from the first address is most relevant.//遍历地址for i, ra := range ras {//上下文 是否取消select {case <-ctx.Done():return nil, &OpError{Op: "dial", Net: sd.network, Source: sd.LocalAddr, Addr: ra, Err: mapErr(ctx.Err())}default:}//上下文的deadlinedeadline, _ := ctx.Deadline()//处理deadlinepartialDeadline, err := partialDeadline(time.Now(), deadline, len(ras)-i)if err != nil {// Ran out of time.if firstErr == nil {firstErr = &OpError{Op: "dial", Net: sd.network, Source: sd.LocalAddr, Addr: ra, Err: err}}break}dialCtx := ctx//处理后deadline 早于上下文的deadlineif partialDeadline.Before(deadline) {var cancel context.CancelFunc//新的WithDeadlinedialCtx, cancel = context.WithDeadline(ctx, partialDeadline)defer cancel()}//链接一个地址c, err := sd.dialSingle(dialCtx, ra)//如果成功直接返回if err == nil {return c, nil}if firstErr == nil {firstErr = err}}if firstErr == nil {firstErr = &OpError{Op: "dial", Net: sd.network, Source: nil, Addr: nil, Err: errMissingAddress}}return nil, firstErr
}

6.处理deadlien 的方法 partialDeadline源码

func partialDeadline(now, deadline time.Time, addrsRemaining int) (time.Time, error) {
//如果时间是0if deadline.IsZero() {return deadline, nil}
//deadline和当前时间的间距timeRemaining := deadline.Sub(now)if timeRemaining <= 0 {return time.Time{}, poll.ErrTimeout}// Tentatively allocate equal time to each remaining address.//时间间距除以 i(第几个地址) timeout := timeRemaining / time.Duration(addrsRemaining)// If the time per address is too short, steal from the end of the list.//2秒
const saneMinimum = 2 * time.Second
//小于2if timeout < saneMinimum {//总的deadlien小于2if timeRemaining < saneMinimum {timeout = timeRemaining} else {//一个链接的超时等于2timeout = saneMinimum}}return now.Add(timeout), nil
}

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