src/net/dial.go - go - Git at Google

 // Copyright 2010 The Go Authors.  All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package net

 import (
 	"errors"
 	"time"
 )

 // A Dialer contains options for connecting to an address.
 //
 // The zero value for each field is equivalent to dialing
 // without that option. Dialing with the zero value of Dialer
 // is therefore equivalent to just calling the Dial function.
 type Dialer struct {
 	// Timeout is the maximum amount of time a dial will wait for
 	// a connect to complete. If Deadline is also set, it may fail
 	// earlier.
 	//
 	// The default is no timeout.
 	//
 	// When dialing a name with multiple IP addresses, the timeout
 	// may be divided between them.
 	//
 	// With or without a timeout, the operating system may impose
 	// its own earlier timeout. For instance, TCP timeouts are
 	// often around 3 minutes.
 	Timeout time.Duration

 	// Deadline is the absolute point in time after which dials
 	// will fail. If Timeout is set, it may fail earlier.
 	// Zero means no deadline, or dependent on the operating system
 	// as with the Timeout option.
 	Deadline time.Time

 	// LocalAddr is the local address to use when dialing an
 	// address. The address must be of a compatible type for the
 	// network being dialed.
 	// If nil, a local address is automatically chosen.
 	LocalAddr Addr

 	// DualStack enables RFC 6555-compliant "Happy Eyeballs" dialing
 	// when the network is "tcp" and the destination is a host name
 	// with both IPv4 and IPv6 addresses. This allows a client to
 	// tolerate networks where one address family is silently broken.
 	DualStack bool

 	// FallbackDelay specifies the length of time to wait before
 	// spawning a fallback connection, when DualStack is enabled.
 	// If zero, a default delay of 300ms is used.
 	FallbackDelay time.Duration

 	// KeepAlive specifies the keep-alive period for an active
 	// network connection.
 	// If zero, keep-alives are not enabled. Network protocols
 	// that do not support keep-alives ignore this field.
 	KeepAlive time.Duration
 }

 // Return either now+Timeout or Deadline, whichever comes first.
 // Or zero, if neither is set.
 func (d *Dialer) deadline(now time.Time) time.Time {
 	if d.Timeout == 0 {
 		return d.Deadline
 	}
 	timeoutDeadline := now.Add(d.Timeout)
 	if d.Deadline.IsZero() || timeoutDeadline.Before(d.Deadline) {
 		return timeoutDeadline
 	} else {
 		return d.Deadline
 	}
 }

 // partialDeadline returns the deadline to use for a single address,
 // when multiple addresses are pending.
 func partialDeadline(now, deadline time.Time, addrsRemaining int) (time.Time, error) {
 	if deadline.IsZero() {
 		return deadline, nil
 	}
 	timeRemaining := deadline.Sub(now)
 	if timeRemaining <= 0 {
 		return time.Time{}, errTimeout
 	}
 	// Tentatively allocate equal time to each remaining address.
 	timeout := timeRemaining / time.Duration(addrsRemaining)
 	// If the time per address is too short, steal from the end of the list.
 	const saneMinimum = 2 * time.Second
 	if timeout < saneMinimum {
 		if timeRemaining < saneMinimum {
 			timeout = timeRemaining
 		} else {
 			timeout = saneMinimum
 		}
 	}
 	return now.Add(timeout), nil
 }

 func (d *Dialer) fallbackDelay() time.Duration {
 	if d.FallbackDelay > 0 {
 		return d.FallbackDelay
 	} else {
 		return 300 * time.Millisecond
 	}
 }

 func parseNetwork(net string) (afnet string, proto int, err error) {
 	i := last(net, ':')
 	if i < 0 { // no colon
 		switch net {
 		case "tcp", "tcp4", "tcp6":
 		case "udp", "udp4", "udp6":
 		case "ip", "ip4", "ip6":
 		case "unix", "unixgram", "unixpacket":
 		default:
 			return "", 0, UnknownNetworkError(net)
 		}
 		return net, 0, nil
 	}
 	afnet = net[:i]
 	switch afnet {
 	case "ip", "ip4", "ip6":
 		protostr := net[i+1:]
 		proto, i, ok := dtoi(protostr, 0)
 		if !ok || i != len(protostr) {
 			proto, err = lookupProtocol(protostr)
 			if err != nil {
 				return "", 0, err
 			}
 		}
 		return afnet, proto, nil
 	}
 	return "", 0, UnknownNetworkError(net)
 }

 func resolveAddrList(op, net, addr string, deadline time.Time) (addrList, error) {
 	afnet, _, err := parseNetwork(net)
 	if err != nil {
 		return nil, err
 	}
 	if op == "dial" && addr == "" {
 		return nil, errMissingAddress
 	}
 	switch afnet {
 	case "unix", "unixgram", "unixpacket":
 		addr, err := ResolveUnixAddr(afnet, addr)
 		if err != nil {
 			return nil, err
 		}
 		return addrList{addr}, nil
 	}
 	return internetAddrList(afnet, addr, deadline)
 }

 // Dial connects to the address on the named network.
 //
 // Known networks are "tcp", "tcp4" (IPv4-only), "tcp6" (IPv6-only),
 // "udp", "udp4" (IPv4-only), "udp6" (IPv6-only), "ip", "ip4"
 // (IPv4-only), "ip6" (IPv6-only), "unix", "unixgram" and
 // "unixpacket".
 //
 // For TCP and UDP networks, addresses have the form host:port.
 // If host is a literal IPv6 address it must be enclosed
 // in square brackets as in "[::1]:80" or "[ipv6-host%zone]:80".
 // The functions JoinHostPort and SplitHostPort manipulate addresses
 // in this form.
 //
 // Examples:
 //	Dial("tcp", "12.34.56.78:80")
 //	Dial("tcp", "google.com:http")
 //	Dial("tcp", "[2001:db8::1]:http")
 //	Dial("tcp", "[fe80::1%lo0]:80")
 //
 // For IP networks, the network must be "ip", "ip4" or "ip6" followed
 // by a colon and a protocol number or name and the addr must be a
 // literal IP address.
 //
 // Examples:
 //	Dial("ip4:1", "127.0.0.1")
 //	Dial("ip6:ospf", "::1")
 //
 // For Unix networks, the address must be a file system path.
 func Dial(network, address string) (Conn, error) {
 	var d Dialer
 	return d.Dial(network, address)
 }

 // DialTimeout acts like Dial but takes a timeout.
 // The timeout includes name resolution, if required.
 func DialTimeout(network, address string, timeout time.Duration) (Conn, error) {
 	d := Dialer{Timeout: timeout}
 	return d.Dial(network, address)
 }

 // dialContext holds common state for all dial operations.
 type dialContext struct {
 	Dialer
 	network, address string
 	finalDeadline    time.Time
 }

 // Dial connects to the address on the named network.
 //
 // See func Dial for a description of the network and address
 // parameters.
 func (d *Dialer) Dial(network, address string) (Conn, error) {
 	finalDeadline := d.deadline(time.Now())
 	addrs, err := resolveAddrList("dial", network, address, finalDeadline)
 	if err != nil {
 		return nil, &OpError{Op: "dial", Net: network, Source: nil, Addr: nil, Err: err}
 	}

 	ctx := &dialContext{
 		Dialer:        *d,
 		network:       network,
 		address:       address,
 		finalDeadline: finalDeadline,
 	}

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

 	var c Conn
 	if len(fallbacks) == 0 {
 		// dialParallel can accept an empty fallbacks list,
 		// but this shortcut avoids the goroutine/channel overhead.
 		c, err = dialSerial(ctx, primaries, nil)
 	} else {
 		c, err = dialParallel(ctx, primaries, fallbacks)
 	}

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

 // dialParallel races two copies of dialSerial, giving the first a
 // head start. It returns the first established connection and
 // closes the others. Otherwise it returns an error from the first
 // primary address.
 func dialParallel(ctx *dialContext, primaries, fallbacks addrList) (Conn, error) {
 	results := make(chan dialResult) // unbuffered, so dialSerialAsync can detect race loss & cleanup
 	cancel := make(chan struct{})
 	defer close(cancel)

 	// Spawn the primary racer.
 	go dialSerialAsync(ctx, primaries, nil, cancel, results)

 	// Spawn the fallback racer.
 	fallbackTimer := time.NewTimer(ctx.fallbackDelay())
 	go dialSerialAsync(ctx, fallbacks, fallbackTimer, cancel, results)

 	var primaryErr error
 	for nracers := 2; nracers > 0; nracers-- {
 		res := <-results
 		// If we're still waiting for a connection, then hasten the delay.
 		// Otherwise, disable the Timer and let cancel take over.
 		if fallbackTimer.Stop() && res.error != nil {
 			fallbackTimer.Reset(0)
 		}
 		if res.error == nil {
 			return res.Conn, nil
 		}
 		if res.primary {
 			primaryErr = res.error
 		}
 	}
 	return nil, primaryErr
 }

 type dialResult struct {
 	Conn
 	error
 	primary bool
 }

 // dialSerialAsync runs dialSerial after some delay, and returns the
 // resulting connection through a channel. When racing two connections,
 // the primary goroutine uses a nil timer to omit the delay.
 func dialSerialAsync(ctx *dialContext, ras addrList, timer *time.Timer, cancel <-chan struct{}, results chan<- dialResult) {
 	if timer != nil {
 		// We're in the fallback goroutine; sleep before connecting.
 		select {
 		case <-timer.C:
 		case <-cancel:
 			return
 		}
 	}
 	c, err := dialSerial(ctx, ras, cancel)
 	select {
 	case results <- dialResult{c, err, timer == nil}:
 		// We won the race.
 	case <-cancel:
 		// The other goroutine won the race.
 		if c != nil {
 			c.Close()
 		}
 	}
 }

 // dialSerial connects to a list of addresses in sequence, returning
 // either the first successful connection, or the first error.
 func dialSerial(ctx *dialContext, ras addrList, cancel <-chan struct{}) (Conn, error) {
 	var firstErr error // The error from the first address is most relevant.

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

 		partialDeadline, err := partialDeadline(time.Now(), ctx.finalDeadline, len(ras)-i)
 		if err != nil {
 			// Ran out of time.
 			if firstErr == nil {
 				firstErr = &OpError{Op: "dial", Net: ctx.network, Source: ctx.LocalAddr, Addr: ra, Err: err}
 			}
 			break
 		}

 		// dialTCP does not support cancelation (see golang.org/issue/11225),
 		// so if cancel fires, we'll continue trying to connect until the next
 		// timeout, or return a spurious connection for the caller to close.
 		dialer := func(d time.Time) (Conn, error) {
 			return dialSingle(ctx, ra, d)
 		}
 		c, err := dial(ctx.network, ra, dialer, partialDeadline)
 		if err == nil {
 			return c, nil
 		}
 		if firstErr == nil {
 			firstErr = err
 		}
 	}

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

 // dialSingle attempts to establish and returns a single connection to
 // the destination address. This must be called through the OS-specific
 // dial function, because some OSes don't implement the deadline feature.
 func dialSingle(ctx *dialContext, ra Addr, deadline time.Time) (c Conn, err error) {
 	la := ctx.LocalAddr
 	if la != nil && la.Network() != ra.Network() {
 		return nil, &OpError{Op: "dial", Net: ctx.network, Source: la, Addr: ra, Err: errors.New("mismatched local address type " + la.Network())}
 	}
 	switch ra := ra.(type) {
 	case *TCPAddr:
 		la, _ := la.(*TCPAddr)
 		c, err = testHookDialTCP(ctx.network, la, ra, deadline)
 	case *UDPAddr:
 		la, _ := la.(*UDPAddr)
 		c, err = dialUDP(ctx.network, la, ra, deadline)
 	case *IPAddr:
 		la, _ := la.(*IPAddr)
 		c, err = dialIP(ctx.network, la, ra, deadline)
 	case *UnixAddr:
 		la, _ := la.(*UnixAddr)
 		c, err = dialUnix(ctx.network, la, ra, deadline)
 	default:
 		return nil, &OpError{Op: "dial", Net: ctx.network, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: ctx.address}}
 	}
 	if err != nil {
 		return nil, err // c is non-nil interface containing nil pointer
 	}
 	return c, nil
 }

 // Listen announces on the local network address laddr.
 // The network net must be a stream-oriented network: "tcp", "tcp4",
 // "tcp6", "unix" or "unixpacket".
 // See Dial for the syntax of laddr.
 func Listen(net, laddr string) (Listener, error) {
 	addrs, err := resolveAddrList("listen", net, laddr, noDeadline)
 	if err != nil {
 		return nil, &OpError{Op: "listen", Net: net, Source: nil, Addr: nil, Err: err}
 	}
 	var l Listener
 	switch la := addrs.first(isIPv4).(type) {
 	case *TCPAddr:
 		l, err = ListenTCP(net, la)
 	case *UnixAddr:
 		l, err = ListenUnix(net, la)
 	default:
 		return nil, &OpError{Op: "listen", Net: net, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: laddr}}
 	}
 	if err != nil {
 		return nil, err // l is non-nil interface containing nil pointer
 	}
 	return l, nil
 }

 // ListenPacket announces on the local network address laddr.
 // The network net must be a packet-oriented network: "udp", "udp4",
 // "udp6", "ip", "ip4", "ip6" or "unixgram".
 // See Dial for the syntax of laddr.
 func ListenPacket(net, laddr string) (PacketConn, error) {
 	addrs, err := resolveAddrList("listen", net, laddr, noDeadline)
 	if err != nil {
 		return nil, &OpError{Op: "listen", Net: net, Source: nil, Addr: nil, Err: err}
 	}
 	var l PacketConn
 	switch la := addrs.first(isIPv4).(type) {
 	case *UDPAddr:
 		l, err = ListenUDP(net, la)
 	case *IPAddr:
 		l, err = ListenIP(net, la)
 	case *UnixAddr:
 		l, err = ListenUnixgram(net, la)
 	default:
 		return nil, &OpError{Op: "listen", Net: net, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: laddr}}
 	}
 	if err != nil {
 		return nil, err // l is non-nil interface containing nil pointer
 	}
 	return l, nil
 }
	// Copyright 2010 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package net

	import (
	"errors"
	"time"
	)

	// A Dialer contains options for connecting to an address.
	//
	// The zero value for each field is equivalent to dialing
	// without that option. Dialing with the zero value of Dialer
	// is therefore equivalent to just calling the Dial function.
	type Dialer struct {
	// Timeout is the maximum amount of time a dial will wait for
	// a connect to complete. If Deadline is also set, it may fail
	// earlier.
	//
	// The default is no timeout.
	//
	// When dialing a name with multiple IP addresses, the timeout
	// may be divided between them.
	//
	// With or without a timeout, the operating system may impose
	// its own earlier timeout. For instance, TCP timeouts are
	// often around 3 minutes.
	Timeout time.Duration

	// Deadline is the absolute point in time after which dials
	// will fail. If Timeout is set, it may fail earlier.
	// Zero means no deadline, or dependent on the operating system
	// as with the Timeout option.
	Deadline time.Time

	// LocalAddr is the local address to use when dialing an
	// address. The address must be of a compatible type for the
	// network being dialed.
	// If nil, a local address is automatically chosen.
	LocalAddr Addr

	// DualStack enables RFC 6555-compliant "Happy Eyeballs" dialing
	// when the network is "tcp" and the destination is a host name
	// with both IPv4 and IPv6 addresses. This allows a client to
	// tolerate networks where one address family is silently broken.
	DualStack bool

	// FallbackDelay specifies the length of time to wait before
	// spawning a fallback connection, when DualStack is enabled.
	// If zero, a default delay of 300ms is used.
	FallbackDelay time.Duration

	// KeepAlive specifies the keep-alive period for an active
	// network connection.
	// If zero, keep-alives are not enabled. Network protocols
	// that do not support keep-alives ignore this field.
	KeepAlive time.Duration
	}

	// Return either now+Timeout or Deadline, whichever comes first.
	// Or zero, if neither is set.
	func (d *Dialer) deadline(now time.Time) time.Time {
	if d.Timeout == 0 {
	return d.Deadline
	}
	timeoutDeadline := now.Add(d.Timeout)
	if d.Deadline.IsZero() \|\| timeoutDeadline.Before(d.Deadline) {
	return timeoutDeadline
	} else {
	return d.Deadline
	}
	}

	// partialDeadline returns the deadline to use for a single address,
	// when multiple addresses are pending.
	func partialDeadline(now, deadline time.Time, addrsRemaining int) (time.Time, error) {
	if deadline.IsZero() {
	return deadline, nil
	}
	timeRemaining := deadline.Sub(now)
	if timeRemaining <= 0 {
	return time.Time{}, errTimeout
	}
	// Tentatively allocate equal time to each remaining address.
	timeout := timeRemaining / time.Duration(addrsRemaining)
	// If the time per address is too short, steal from the end of the list.
	const saneMinimum = 2 * time.Second
	if timeout < saneMinimum {
	if timeRemaining < saneMinimum {
	timeout = timeRemaining
	} else {
	timeout = saneMinimum
	}
	}
	return now.Add(timeout), nil
	}

	func (d *Dialer) fallbackDelay() time.Duration {
	if d.FallbackDelay > 0 {
	return d.FallbackDelay
	} else {
	return 300 * time.Millisecond
	}
	}

	func parseNetwork(net string) (afnet string, proto int, err error) {
	i := last(net, ':')
	if i < 0 { // no colon
	switch net {
	case "tcp", "tcp4", "tcp6":
	case "udp", "udp4", "udp6":
	case "ip", "ip4", "ip6":
	case "unix", "unixgram", "unixpacket":
	default:
	return "", 0, UnknownNetworkError(net)
	}
	return net, 0, nil
	}
	afnet = net[:i]
	switch afnet {
	case "ip", "ip4", "ip6":
	protostr := net[i+1:]
	proto, i, ok := dtoi(protostr, 0)
	if !ok \|\| i != len(protostr) {
	proto, err = lookupProtocol(protostr)
	if err != nil {
	return "", 0, err
	}
	}
	return afnet, proto, nil
	}
	return "", 0, UnknownNetworkError(net)
	}

	func resolveAddrList(op, net, addr string, deadline time.Time) (addrList, error) {
	afnet, _, err := parseNetwork(net)
	if err != nil {
	return nil, err
	}
	if op == "dial" && addr == "" {
	return nil, errMissingAddress
	}
	switch afnet {
	case "unix", "unixgram", "unixpacket":
	addr, err := ResolveUnixAddr(afnet, addr)
	if err != nil {
	return nil, err
	}
	return addrList{addr}, nil
	}
	return internetAddrList(afnet, addr, deadline)
	}

	// Dial connects to the address on the named network.
	//
	// Known networks are "tcp", "tcp4" (IPv4-only), "tcp6" (IPv6-only),
	// "udp", "udp4" (IPv4-only), "udp6" (IPv6-only), "ip", "ip4"
	// (IPv4-only), "ip6" (IPv6-only), "unix", "unixgram" and
	// "unixpacket".
	//
	// For TCP and UDP networks, addresses have the form host:port.
	// If host is a literal IPv6 address it must be enclosed
	// in square brackets as in "[::1]:80" or "[ipv6-host%zone]:80".
	// The functions JoinHostPort and SplitHostPort manipulate addresses
	// in this form.
	//
	// Examples:
	// Dial("tcp", "12.34.56.78:80")
	// Dial("tcp", "google.com:http")
	// Dial("tcp", "[2001:db8::1]:http")
	// Dial("tcp", "[fe80::1%lo0]:80")
	//
	// For IP networks, the network must be "ip", "ip4" or "ip6" followed
	// by a colon and a protocol number or name and the addr must be a
	// literal IP address.
	//
	// Examples:
	// Dial("ip4:1", "127.0.0.1")
	// Dial("ip6:ospf", "::1")
	//
	// For Unix networks, the address must be a file system path.
	func Dial(network, address string) (Conn, error) {
	var d Dialer
	return d.Dial(network, address)
	}

	// DialTimeout acts like Dial but takes a timeout.
	// The timeout includes name resolution, if required.
	func DialTimeout(network, address string, timeout time.Duration) (Conn, error) {
	d := Dialer{Timeout: timeout}
	return d.Dial(network, address)
	}

	// dialContext holds common state for all dial operations.
	type dialContext struct {
	Dialer
	network, address string
	finalDeadline time.Time
	}

	// Dial connects to the address on the named network.
	//
	// See func Dial for a description of the network and address
	// parameters.
	func (d *Dialer) Dial(network, address string) (Conn, error) {
	finalDeadline := d.deadline(time.Now())
	addrs, err := resolveAddrList("dial", network, address, finalDeadline)
	if err != nil {
	return nil, &OpError{Op: "dial", Net: network, Source: nil, Addr: nil, Err: err}
	}

	ctx := &dialContext{
	Dialer: *d,
	network: network,
	address: address,
	finalDeadline: finalDeadline,
	}

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

	var c Conn
	if len(fallbacks) == 0 {
	// dialParallel can accept an empty fallbacks list,
	// but this shortcut avoids the goroutine/channel overhead.
	c, err = dialSerial(ctx, primaries, nil)
	} else {
	c, err = dialParallel(ctx, primaries, fallbacks)
	}

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

	// dialParallel races two copies of dialSerial, giving the first a
	// head start. It returns the first established connection and
	// closes the others. Otherwise it returns an error from the first
	// primary address.
	func dialParallel(ctx *dialContext, primaries, fallbacks addrList) (Conn, error) {
	results := make(chan dialResult) // unbuffered, so dialSerialAsync can detect race loss & cleanup
	cancel := make(chan struct{})
	defer close(cancel)

	// Spawn the primary racer.
	go dialSerialAsync(ctx, primaries, nil, cancel, results)

	// Spawn the fallback racer.
	fallbackTimer := time.NewTimer(ctx.fallbackDelay())
	go dialSerialAsync(ctx, fallbacks, fallbackTimer, cancel, results)

	var primaryErr error
	for nracers := 2; nracers > 0; nracers-- {
	res := <-results
	// If we're still waiting for a connection, then hasten the delay.
	// Otherwise, disable the Timer and let cancel take over.
	if fallbackTimer.Stop() && res.error != nil {
	fallbackTimer.Reset(0)
	}
	if res.error == nil {
	return res.Conn, nil
	}
	if res.primary {
	primaryErr = res.error
	}
	}
	return nil, primaryErr
	}

	type dialResult struct {
	Conn
	error
	primary bool
	}

	// dialSerialAsync runs dialSerial after some delay, and returns the
	// resulting connection through a channel. When racing two connections,
	// the primary goroutine uses a nil timer to omit the delay.
	func dialSerialAsync(ctx dialContext, ras addrList, timer time.Timer, cancel <-chan struct{}, results chan<- dialResult) {
	if timer != nil {
	// We're in the fallback goroutine; sleep before connecting.
	select {
	case <-timer.C:
	case <-cancel:
	return
	}
	}
	c, err := dialSerial(ctx, ras, cancel)
	select {
	case results <- dialResult{c, err, timer == nil}:
	// We won the race.
	case <-cancel:
	// The other goroutine won the race.
	if c != nil {
	c.Close()
	}
	}
	}

	// dialSerial connects to a list of addresses in sequence, returning
	// either the first successful connection, or the first error.
	func dialSerial(ctx *dialContext, ras addrList, cancel <-chan struct{}) (Conn, error) {
	var firstErr error // The error from the first address is most relevant.

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

	partialDeadline, err := partialDeadline(time.Now(), ctx.finalDeadline, len(ras)-i)
	if err != nil {
	// Ran out of time.
	if firstErr == nil {
	firstErr = &OpError{Op: "dial", Net: ctx.network, Source: ctx.LocalAddr, Addr: ra, Err: err}
	}
	break
	}

	// dialTCP does not support cancelation (see golang.org/issue/11225),
	// so if cancel fires, we'll continue trying to connect until the next
	// timeout, or return a spurious connection for the caller to close.
	dialer := func(d time.Time) (Conn, error) {
	return dialSingle(ctx, ra, d)
	}
	c, err := dial(ctx.network, ra, dialer, partialDeadline)
	if err == nil {
	return c, nil
	}
	if firstErr == nil {
	firstErr = err
	}
	}

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

	// dialSingle attempts to establish and returns a single connection to
	// the destination address. This must be called through the OS-specific
	// dial function, because some OSes don't implement the deadline feature.
	func dialSingle(ctx *dialContext, ra Addr, deadline time.Time) (c Conn, err error) {
	la := ctx.LocalAddr
	if la != nil && la.Network() != ra.Network() {
	return nil, &OpError{Op: "dial", Net: ctx.network, Source: la, Addr: ra, Err: errors.New("mismatched local address type " + la.Network())}
	}
	switch ra := ra.(type) {
	case *TCPAddr:
	la, _ := la.(*TCPAddr)
	c, err = testHookDialTCP(ctx.network, la, ra, deadline)
	case *UDPAddr:
	la, _ := la.(*UDPAddr)
	c, err = dialUDP(ctx.network, la, ra, deadline)
	case *IPAddr:
	la, _ := la.(*IPAddr)
	c, err = dialIP(ctx.network, la, ra, deadline)
	case *UnixAddr:
	la, _ := la.(*UnixAddr)
	c, err = dialUnix(ctx.network, la, ra, deadline)
	default:
	return nil, &OpError{Op: "dial", Net: ctx.network, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: ctx.address}}
	}
	if err != nil {
	return nil, err // c is non-nil interface containing nil pointer
	}
	return c, nil
	}

	// Listen announces on the local network address laddr.
	// The network net must be a stream-oriented network: "tcp", "tcp4",
	// "tcp6", "unix" or "unixpacket".
	// See Dial for the syntax of laddr.
	func Listen(net, laddr string) (Listener, error) {
	addrs, err := resolveAddrList("listen", net, laddr, noDeadline)
	if err != nil {
	return nil, &OpError{Op: "listen", Net: net, Source: nil, Addr: nil, Err: err}
	}
	var l Listener
	switch la := addrs.first(isIPv4).(type) {
	case *TCPAddr:
	l, err = ListenTCP(net, la)
	case *UnixAddr:
	l, err = ListenUnix(net, la)
	default:
	return nil, &OpError{Op: "listen", Net: net, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: laddr}}
	}
	if err != nil {
	return nil, err // l is non-nil interface containing nil pointer
	}
	return l, nil
	}

	// ListenPacket announces on the local network address laddr.
	// The network net must be a packet-oriented network: "udp", "udp4",
	// "udp6", "ip", "ip4", "ip6" or "unixgram".
	// See Dial for the syntax of laddr.
	func ListenPacket(net, laddr string) (PacketConn, error) {
	addrs, err := resolveAddrList("listen", net, laddr, noDeadline)
	if err != nil {
	return nil, &OpError{Op: "listen", Net: net, Source: nil, Addr: nil, Err: err}
	}
	var l PacketConn
	switch la := addrs.first(isIPv4).(type) {
	case *UDPAddr:
	l, err = ListenUDP(net, la)
	case *IPAddr:
	l, err = ListenIP(net, la)
	case *UnixAddr:
	l, err = ListenUnixgram(net, la)
	default:
	return nil, &OpError{Op: "listen", Net: net, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: laddr}}
	}
	if err != nil {
	return nil, err // l is non-nil interface containing nil pointer
	}
	return l, nil
	}