libgo/go/net/dial.go - gofrontend - 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 (
 	"context"
 	"internal/nettrace"
 	"internal/poll"
 	"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 using TCP and dialing a host 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 host in the
 	// address parameter resolves to 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

 	// Resolver optionally specifies an alternate resolver to use.
 	Resolver *Resolver

 	// Cancel is an optional channel whose closure indicates that
 	// the dial should be canceled. Not all types of dials support
 	// cancelation.
 	//
 	// Deprecated: Use DialContext instead.
 	Cancel <-chan struct{}
 }

 func minNonzeroTime(a, b time.Time) time.Time {
 	if a.IsZero() {
 		return b
 	}
 	if b.IsZero() || a.Before(b) {
 		return a
 	}
 	return b
 }

 // deadline returns the earliest of:
 //   - now+Timeout
 //   - d.Deadline
 //   - the context's deadline
 // Or zero, if none of Timeout, Deadline, or context's deadline is set.
 func (d *Dialer) deadline(ctx context.Context, now time.Time) (earliest time.Time) {
 	if d.Timeout != 0 { // including negative, for historical reasons
 		earliest = now.Add(d.Timeout)
 	}
 	if d, ok := ctx.Deadline(); ok {
 		earliest = minNonzeroTime(earliest, d)
 	}
 	return minNonzeroTime(earliest, d.Deadline)
 }

 func (d *Dialer) resolver() *Resolver {
 	if d.Resolver != nil {
 		return d.Resolver
 	}
 	return DefaultResolver
 }

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

 // resolveAddrList resolves addr using hint and returns a list of
 // addresses. The result contains at least one address when error is
 // nil.
 func (r *Resolver) resolveAddrList(ctx context.Context, op, network, addr string, hint Addr) (addrList, error) {
 	afnet, _, err := parseNetwork(ctx, network, true)
 	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
 		}
 		if op == "dial" && hint != nil && addr.Network() != hint.Network() {
 			return nil, &AddrError{Err: "mismatched local address type", Addr: hint.String()}
 		}
 		return addrList{addr}, nil
 	}
 	addrs, err := r.internetAddrList(ctx, afnet, addr)
 	if err != nil || op != "dial" || hint == nil {
 		return addrs, err
 	}
 	var (
 		tcp      *TCPAddr
 		udp      *UDPAddr
 		ip       *IPAddr
 		wildcard bool
 	)
 	switch hint := hint.(type) {
 	case *TCPAddr:
 		tcp = hint
 		wildcard = tcp.isWildcard()
 	case *UDPAddr:
 		udp = hint
 		wildcard = udp.isWildcard()
 	case *IPAddr:
 		ip = hint
 		wildcard = ip.isWildcard()
 	}
 	naddrs := addrs[:0]
 	for _, addr := range addrs {
 		if addr.Network() != hint.Network() {
 			return nil, &AddrError{Err: "mismatched local address type", Addr: hint.String()}
 		}
 		switch addr := addr.(type) {
 		case *TCPAddr:
 			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(tcp.IP) {
 				continue
 			}
 			naddrs = append(naddrs, addr)
 		case *UDPAddr:
 			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(udp.IP) {
 				continue
 			}
 			naddrs = append(naddrs, addr)
 		case *IPAddr:
 			if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(ip.IP) {
 				continue
 			}
 			naddrs = append(naddrs, addr)
 		}
 	}
 	if len(naddrs) == 0 {
 		return nil, &AddrError{Err: errNoSuitableAddress.Error(), Addr: hint.String()}
 	}
 	return naddrs, nil
 }

 // 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, the address has the form "host:port".
 // The host must be a literal IP address, or a host name that can be
 // resolved to IP addresses.
 // The port must be a literal port number or a service name.
 // If the host is a literal IPv6 address it must be enclosed in square
 // brackets, as in "[2001:db8::1]:80" or "[fe80::1%zone]:80".
 // The zone specifies the scope of the literal IPv6 address as defined
 // in RFC 4007.
 // The functions JoinHostPort and SplitHostPort manipulate a pair of
 // host and port in this form.
 // When using TCP, and the host resolves to multiple IP addresses,
 // Dial will try each IP address in order until one succeeds.
 //
 // Examples:
 //	Dial("tcp", "golang.org:http")
 //	Dial("tcp", "192.0.2.1:http")
 //	Dial("tcp", "198.51.100.1:80")
 //	Dial("udp", "[2001:db8::1]:domain")
 //	Dial("udp", "[fe80::1%lo0]:53")
 //	Dial("tcp", ":80")
 //
 // For IP networks, the network must be "ip", "ip4" or "ip6" followed
 // by a colon and a literal protocol number or a protocol name, and
 // the address has the form "host". The host must be a literal IP
 // address or a literal IPv6 address with zone.
 // It depends on each operating system how the operating system
 // behaves with a non-well known protocol number such as "0" or "255".
 //
 // Examples:
 //	Dial("ip4:1", "192.0.2.1")
 //	Dial("ip6:ipv6-icmp", "2001:db8::1")
 //	Dial("ip6:58", "fe80::1%lo0")
 //
 // For TCP, UDP and IP networks, if the host is empty or a literal
 // unspecified IP address, as in ":80", "0.0.0.0:80" or "[::]:80" for
 // TCP and UDP, "", "0.0.0.0" or "::" for IP, the local system is
 // assumed.
 //
 // 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.
 // When using TCP, and the host in the address parameter resolves to
 // multiple IP addresses, the timeout is spread over each consecutive
 // dial, such that each is given an appropriate fraction of the time
 // to connect.
 //
 // See func Dial for a description of the network and address
 // parameters.
 func DialTimeout(network, address string, timeout time.Duration) (Conn, error) {
 	d := Dialer{Timeout: timeout}
 	return d.Dial(network, address)
 }

 // dialParam contains a Dial's parameters and configuration.
 type dialParam struct {
 	Dialer
 	network, address string
 }

 // 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) {
 	return d.DialContext(context.Background(), network, address)
 }

 // DialContext connects to the address on the named network using
 // the provided context.
 //
 // The provided Context must be non-nil. If the context expires before
 // the connection is complete, an error is returned. Once successfully
 // connected, any expiration of the context will not affect the
 // connection.
 //
 // When using TCP, and the host in the address parameter resolves to multiple
 // network addresses, any dial timeout (from d.Timeout or ctx) is spread
 // over each consecutive dial, such that each is given an appropriate
 // fraction of the time to connect.
 // For example, if a host has 4 IP addresses and the timeout is 1 minute,
 // the connect to each single address will be given 15 seconds to complete
 // before trying the next one.
 //
 // See func Dial for a description of the network and address
 // parameters.
 func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn, error) {
 	if ctx == nil {
 		panic("nil context")
 	}
 	deadline := d.deadline(ctx, time.Now())
 	if !deadline.IsZero() {
 		if d, ok := ctx.Deadline(); !ok || deadline.Before(d) {
 			subCtx, cancel := context.WithDeadline(ctx, deadline)
 			defer cancel()
 			ctx = subCtx
 		}
 	}
 	if 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 := ctx
 	if trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace); trace != nil {
 		shadow := *trace
 		shadow.ConnectStart = nil
 		shadow.ConnectDone = nil
 		resolveCtx = context.WithValue(resolveCtx, nettrace.TraceKey{}, &shadow)
 	}

 	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}
 	}

 	dp := &dialParam{
 		Dialer:  *d,
 		network: network,
 		address: address,
 	}

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

 	var c Conn
 	if len(fallbacks) > 0 {
 		c, err = dialParallel(ctx, dp, primaries, fallbacks)
 	} else {
 		c, err = dialSerial(ctx, dp, primaries)
 	}
 	if err != nil {
 		return nil, err
 	}

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

 // 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 context.Context, dp *dialParam, primaries, fallbacks addrList) (Conn, error) {
 	if len(fallbacks) == 0 {
 		return dialSerial(ctx, dp, primaries)
 	}

 	returned := make(chan struct{})
 	defer close(returned)

 	type dialResult struct {
 		Conn
 		error
 		primary bool
 		done    bool
 	}
 	results := make(chan dialResult) // unbuffered

 	startRacer := func(ctx context.Context, primary bool) {
 		ras := primaries
 		if !primary {
 			ras = fallbacks
 		}
 		c, err := dialSerial(ctx, dp, ras)
 		select {
 		case results <- dialResult{Conn: c, error: err, primary: primary, done: true}:
 		case <-returned:
 			if c != nil {
 				c.Close()
 			}
 		}
 	}

 	var primary, fallback dialResult

 	// Start the main racer.
 	primaryCtx, primaryCancel := context.WithCancel(ctx)
 	defer primaryCancel()
 	go startRacer(primaryCtx, true)

 	// Start the timer for the fallback racer.
 	fallbackTimer := time.NewTimer(dp.fallbackDelay())
 	defer fallbackTimer.Stop()

 	for {
 		select {
 		case <-fallbackTimer.C:
 			fallbackCtx, fallbackCancel := context.WithCancel(ctx)
 			defer fallbackCancel()
 			go startRacer(fallbackCtx, false)

 		case res := <-results:
 			if res.error == nil {
 				return res.Conn, nil
 			}
 			if res.primary {
 				primary = res
 			} else {
 				fallback = res
 			}
 			if primary.done && fallback.done {
 				return nil, primary.error
 			}
 			if res.primary && fallbackTimer.Stop() {
 				// If we were able to stop the timer, that means it
 				// was running (hadn't yet started the fallback), but
 				// we just got an error on the primary path, so start
 				// the fallback immediately (in 0 nanoseconds).
 				fallbackTimer.Reset(0)
 			}
 		}
 	}
 }

 // dialSerial connects to a list of addresses in sequence, returning
 // either the first successful connection, or the first error.
 func dialSerial(ctx context.Context, dp *dialParam, ras addrList) (Conn, error) {
 	var firstErr error // The error from the first address is most relevant.

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

 		deadline, _ := ctx.Deadline()
 		partialDeadline, err := partialDeadline(time.Now(), deadline, len(ras)-i)
 		if err != nil {
 			// Ran out of time.
 			if firstErr == nil {
 				firstErr = &OpError{Op: "dial", Net: dp.network, Source: dp.LocalAddr, Addr: ra, Err: err}
 			}
 			break
 		}
 		dialCtx := ctx
 		if partialDeadline.Before(deadline) {
 			var cancel context.CancelFunc
 			dialCtx, cancel = context.WithDeadline(ctx, partialDeadline)
 			defer cancel()
 		}

 		c, err := dialSingle(dialCtx, dp, ra)
 		if err == nil {
 			return c, nil
 		}
 		if firstErr == nil {
 			firstErr = err
 		}
 	}

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

 // dialSingle attempts to establish and returns a single connection to
 // the destination address.
 func dialSingle(ctx context.Context, dp *dialParam, ra Addr) (c Conn, err error) {
 	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
 	if trace != nil {
 		raStr := ra.String()
 		if trace.ConnectStart != nil {
 			trace.ConnectStart(dp.network, raStr)
 		}
 		if trace.ConnectDone != nil {
 			defer func() { trace.ConnectDone(dp.network, raStr, err) }()
 		}
 	}
 	la := dp.LocalAddr
 	switch ra := ra.(type) {
 	case *TCPAddr:
 		la, _ := la.(*TCPAddr)
 		c, err = dialTCP(ctx, dp.network, la, ra)
 	case *UDPAddr:
 		la, _ := la.(*UDPAddr)
 		c, err = dialUDP(ctx, dp.network, la, ra)
 	case *IPAddr:
 		la, _ := la.(*IPAddr)
 		c, err = dialIP(ctx, dp.network, la, ra)
 	case *UnixAddr:
 		la, _ := la.(*UnixAddr)
 		c, err = dialUnix(ctx, dp.network, la, ra)
 	default:
 		return nil, &OpError{Op: "dial", Net: dp.network, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: dp.address}}
 	}
 	if err != nil {
 		return nil, &OpError{Op: "dial", Net: dp.network, Source: la, Addr: ra, Err: err} // c is non-nil interface containing nil pointer
 	}
 	return c, nil
 }

 // Listen announces on the local network address.
 //
 // The network must be "tcp", "tcp4", "tcp6", "unix" or "unixpacket".
 //
 // For TCP networks, if the host in the address parameter is empty or
 // a literal unspecified IP address, Listen listens on all available
 // unicast and anycast IP addresses of the local system.
 // To only use IPv4, use network "tcp4".
 // The address can use a host name, but this is not recommended,
 // because it will create a listener for at most one of the host's IP
 // addresses.
 // If the port in the address parameter is empty or "0", as in
 // "127.0.0.1:" or "[::1]:0", a port number is automatically chosen.
 // The Addr method of Listener can be used to discover the chosen
 // port.
 //
 // See func Dial for a description of the network and address
 // parameters.
 func Listen(network, address string) (Listener, error) {
 	addrs, err := DefaultResolver.resolveAddrList(context.Background(), "listen", network, address, nil)
 	if err != nil {
 		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
 	}
 	var l Listener
 	switch la := addrs.first(isIPv4).(type) {
 	case *TCPAddr:
 		l, err = ListenTCP(network, la)
 	case *UnixAddr:
 		l, err = ListenUnix(network, la)
 	default:
 		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
 	}
 	if err != nil {
 		return nil, err // l is non-nil interface containing nil pointer
 	}
 	return l, nil
 }

 // ListenPacket announces on the local network address.
 //
 // The network must be "udp", "udp4", "udp6", "unixgram", or an IP
 // transport. The IP transports are "ip", "ip4", or "ip6" followed by
 // a colon and a literal protocol number or a protocol name, as in
 // "ip:1" or "ip:icmp".
 //
 // For UDP and IP networks, if the host in the address parameter is
 // empty or a literal unspecified IP address, ListenPacket listens on
 // all available IP addresses of the local system except multicast IP
 // addresses.
 // To only use IPv4, use network "udp4" or "ip4:proto".
 // The address can use a host name, but this is not recommended,
 // because it will create a listener for at most one of the host's IP
 // addresses.
 // If the port in the address parameter is empty or "0", as in
 // "127.0.0.1:" or "[::1]:0", a port number is automatically chosen.
 // The LocalAddr method of PacketConn can be used to discover the
 // chosen port.
 //
 // See func Dial for a description of the network and address
 // parameters.
 func ListenPacket(network, address string) (PacketConn, error) {
 	addrs, err := DefaultResolver.resolveAddrList(context.Background(), "listen", network, address, nil)
 	if err != nil {
 		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
 	}
 	var l PacketConn
 	switch la := addrs.first(isIPv4).(type) {
 	case *UDPAddr:
 		l, err = ListenUDP(network, la)
 	case *IPAddr:
 		l, err = ListenIP(network, la)
 	case *UnixAddr:
 		l, err = ListenUnixgram(network, la)
 	default:
 		return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
 	}
 	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 (
	"context"
	"internal/nettrace"
	"internal/poll"
	"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 using TCP and dialing a host 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 host in the
	// address parameter resolves to 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

	// Resolver optionally specifies an alternate resolver to use.
	Resolver *Resolver

	// Cancel is an optional channel whose closure indicates that
	// the dial should be canceled. Not all types of dials support
	// cancelation.
	//
	// Deprecated: Use DialContext instead.
	Cancel <-chan struct{}
	}

	func minNonzeroTime(a, b time.Time) time.Time {
	if a.IsZero() {
	return b
	}
	if b.IsZero() \|\| a.Before(b) {
	return a
	}
	return b
	}

	// deadline returns the earliest of:
	// - now+Timeout
	// - d.Deadline
	// - the context's deadline
	// Or zero, if none of Timeout, Deadline, or context's deadline is set.
	func (d *Dialer) deadline(ctx context.Context, now time.Time) (earliest time.Time) {
	if d.Timeout != 0 { // including negative, for historical reasons
	earliest = now.Add(d.Timeout)
	}
	if d, ok := ctx.Deadline(); ok {
	earliest = minNonzeroTime(earliest, d)
	}
	return minNonzeroTime(earliest, d.Deadline)
	}

	func (d Dialer) resolver() Resolver {
	if d.Resolver != nil {
	return d.Resolver
	}
	return DefaultResolver
	}

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

	// resolveAddrList resolves addr using hint and returns a list of
	// addresses. The result contains at least one address when error is
	// nil.
	func (r *Resolver) resolveAddrList(ctx context.Context, op, network, addr string, hint Addr) (addrList, error) {
	afnet, _, err := parseNetwork(ctx, network, true)
	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
	}
	if op == "dial" && hint != nil && addr.Network() != hint.Network() {
	return nil, &AddrError{Err: "mismatched local address type", Addr: hint.String()}
	}
	return addrList{addr}, nil
	}
	addrs, err := r.internetAddrList(ctx, afnet, addr)
	if err != nil \|\| op != "dial" \|\| hint == nil {
	return addrs, err
	}
	var (
	tcp *TCPAddr
	udp *UDPAddr
	ip *IPAddr
	wildcard bool
	)
	switch hint := hint.(type) {
	case *TCPAddr:
	tcp = hint
	wildcard = tcp.isWildcard()
	case *UDPAddr:
	udp = hint
	wildcard = udp.isWildcard()
	case *IPAddr:
	ip = hint
	wildcard = ip.isWildcard()
	}
	naddrs := addrs[:0]
	for _, addr := range addrs {
	if addr.Network() != hint.Network() {
	return nil, &AddrError{Err: "mismatched local address type", Addr: hint.String()}
	}
	switch addr := addr.(type) {
	case *TCPAddr:
	if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(tcp.IP) {
	continue
	}
	naddrs = append(naddrs, addr)
	case *UDPAddr:
	if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(udp.IP) {
	continue
	}
	naddrs = append(naddrs, addr)
	case *IPAddr:
	if !wildcard && !addr.isWildcard() && !addr.IP.matchAddrFamily(ip.IP) {
	continue
	}
	naddrs = append(naddrs, addr)
	}
	}
	if len(naddrs) == 0 {
	return nil, &AddrError{Err: errNoSuitableAddress.Error(), Addr: hint.String()}
	}
	return naddrs, nil
	}

	// 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, the address has the form "host:port".
	// The host must be a literal IP address, or a host name that can be
	// resolved to IP addresses.
	// The port must be a literal port number or a service name.
	// If the host is a literal IPv6 address it must be enclosed in square
	// brackets, as in "[2001:db8::1]:80" or "[fe80::1%zone]:80".
	// The zone specifies the scope of the literal IPv6 address as defined
	// in RFC 4007.
	// The functions JoinHostPort and SplitHostPort manipulate a pair of
	// host and port in this form.
	// When using TCP, and the host resolves to multiple IP addresses,
	// Dial will try each IP address in order until one succeeds.
	//
	// Examples:
	// Dial("tcp", "golang.org:http")
	// Dial("tcp", "192.0.2.1:http")
	// Dial("tcp", "198.51.100.1:80")
	// Dial("udp", "[2001:db8::1]:domain")
	// Dial("udp", "[fe80::1%lo0]:53")
	// Dial("tcp", ":80")
	//
	// For IP networks, the network must be "ip", "ip4" or "ip6" followed
	// by a colon and a literal protocol number or a protocol name, and
	// the address has the form "host". The host must be a literal IP
	// address or a literal IPv6 address with zone.
	// It depends on each operating system how the operating system
	// behaves with a non-well known protocol number such as "0" or "255".
	//
	// Examples:
	// Dial("ip4:1", "192.0.2.1")
	// Dial("ip6:ipv6-icmp", "2001:db8::1")
	// Dial("ip6:58", "fe80::1%lo0")
	//
	// For TCP, UDP and IP networks, if the host is empty or a literal
	// unspecified IP address, as in ":80", "0.0.0.0:80" or "[::]:80" for
	// TCP and UDP, "", "0.0.0.0" or "::" for IP, the local system is
	// assumed.
	//
	// 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.
	// When using TCP, and the host in the address parameter resolves to
	// multiple IP addresses, the timeout is spread over each consecutive
	// dial, such that each is given an appropriate fraction of the time
	// to connect.
	//
	// See func Dial for a description of the network and address
	// parameters.
	func DialTimeout(network, address string, timeout time.Duration) (Conn, error) {
	d := Dialer{Timeout: timeout}
	return d.Dial(network, address)
	}

	// dialParam contains a Dial's parameters and configuration.
	type dialParam struct {
	Dialer
	network, address string
	}

	// 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) {
	return d.DialContext(context.Background(), network, address)
	}

	// DialContext connects to the address on the named network using
	// the provided context.
	//
	// The provided Context must be non-nil. If the context expires before
	// the connection is complete, an error is returned. Once successfully
	// connected, any expiration of the context will not affect the
	// connection.
	//
	// When using TCP, and the host in the address parameter resolves to multiple
	// network addresses, any dial timeout (from d.Timeout or ctx) is spread
	// over each consecutive dial, such that each is given an appropriate
	// fraction of the time to connect.
	// For example, if a host has 4 IP addresses and the timeout is 1 minute,
	// the connect to each single address will be given 15 seconds to complete
	// before trying the next one.
	//
	// See func Dial for a description of the network and address
	// parameters.
	func (d *Dialer) DialContext(ctx context.Context, network, address string) (Conn, error) {
	if ctx == nil {
	panic("nil context")
	}
	deadline := d.deadline(ctx, time.Now())
	if !deadline.IsZero() {
	if d, ok := ctx.Deadline(); !ok \|\| deadline.Before(d) {
	subCtx, cancel := context.WithDeadline(ctx, deadline)
	defer cancel()
	ctx = subCtx
	}
	}
	if 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 := ctx
	if trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace); trace != nil {
	shadow := *trace
	shadow.ConnectStart = nil
	shadow.ConnectDone = nil
	resolveCtx = context.WithValue(resolveCtx, nettrace.TraceKey{}, &shadow)
	}

	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}
	}

	dp := &dialParam{
	Dialer: *d,
	network: network,
	address: address,
	}

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

	var c Conn
	if len(fallbacks) > 0 {
	c, err = dialParallel(ctx, dp, primaries, fallbacks)
	} else {
	c, err = dialSerial(ctx, dp, primaries)
	}
	if err != nil {
	return nil, err
	}

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

	// 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 context.Context, dp *dialParam, primaries, fallbacks addrList) (Conn, error) {
	if len(fallbacks) == 0 {
	return dialSerial(ctx, dp, primaries)
	}

	returned := make(chan struct{})
	defer close(returned)

	type dialResult struct {
	Conn
	error
	primary bool
	done bool
	}
	results := make(chan dialResult) // unbuffered

	startRacer := func(ctx context.Context, primary bool) {
	ras := primaries
	if !primary {
	ras = fallbacks
	}
	c, err := dialSerial(ctx, dp, ras)
	select {
	case results <- dialResult{Conn: c, error: err, primary: primary, done: true}:
	case <-returned:
	if c != nil {
	c.Close()
	}
	}
	}

	var primary, fallback dialResult

	// Start the main racer.
	primaryCtx, primaryCancel := context.WithCancel(ctx)
	defer primaryCancel()
	go startRacer(primaryCtx, true)

	// Start the timer for the fallback racer.
	fallbackTimer := time.NewTimer(dp.fallbackDelay())
	defer fallbackTimer.Stop()

	for {
	select {
	case <-fallbackTimer.C:
	fallbackCtx, fallbackCancel := context.WithCancel(ctx)
	defer fallbackCancel()
	go startRacer(fallbackCtx, false)

	case res := <-results:
	if res.error == nil {
	return res.Conn, nil
	}
	if res.primary {
	primary = res
	} else {
	fallback = res
	}
	if primary.done && fallback.done {
	return nil, primary.error
	}
	if res.primary && fallbackTimer.Stop() {
	// If we were able to stop the timer, that means it
	// was running (hadn't yet started the fallback), but
	// we just got an error on the primary path, so start
	// the fallback immediately (in 0 nanoseconds).
	fallbackTimer.Reset(0)
	}
	}
	}
	}

	// dialSerial connects to a list of addresses in sequence, returning
	// either the first successful connection, or the first error.
	func dialSerial(ctx context.Context, dp *dialParam, ras addrList) (Conn, error) {
	var firstErr error // The error from the first address is most relevant.

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

	deadline, _ := ctx.Deadline()
	partialDeadline, err := partialDeadline(time.Now(), deadline, len(ras)-i)
	if err != nil {
	// Ran out of time.
	if firstErr == nil {
	firstErr = &OpError{Op: "dial", Net: dp.network, Source: dp.LocalAddr, Addr: ra, Err: err}
	}
	break
	}
	dialCtx := ctx
	if partialDeadline.Before(deadline) {
	var cancel context.CancelFunc
	dialCtx, cancel = context.WithDeadline(ctx, partialDeadline)
	defer cancel()
	}

	c, err := dialSingle(dialCtx, dp, ra)
	if err == nil {
	return c, nil
	}
	if firstErr == nil {
	firstErr = err
	}
	}

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

	// dialSingle attempts to establish and returns a single connection to
	// the destination address.
	func dialSingle(ctx context.Context, dp *dialParam, ra Addr) (c Conn, err error) {
	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
	if trace != nil {
	raStr := ra.String()
	if trace.ConnectStart != nil {
	trace.ConnectStart(dp.network, raStr)
	}
	if trace.ConnectDone != nil {
	defer func() { trace.ConnectDone(dp.network, raStr, err) }()
	}
	}
	la := dp.LocalAddr
	switch ra := ra.(type) {
	case *TCPAddr:
	la, _ := la.(*TCPAddr)
	c, err = dialTCP(ctx, dp.network, la, ra)
	case *UDPAddr:
	la, _ := la.(*UDPAddr)
	c, err = dialUDP(ctx, dp.network, la, ra)
	case *IPAddr:
	la, _ := la.(*IPAddr)
	c, err = dialIP(ctx, dp.network, la, ra)
	case *UnixAddr:
	la, _ := la.(*UnixAddr)
	c, err = dialUnix(ctx, dp.network, la, ra)
	default:
	return nil, &OpError{Op: "dial", Net: dp.network, Source: la, Addr: ra, Err: &AddrError{Err: "unexpected address type", Addr: dp.address}}
	}
	if err != nil {
	return nil, &OpError{Op: "dial", Net: dp.network, Source: la, Addr: ra, Err: err} // c is non-nil interface containing nil pointer
	}
	return c, nil
	}

	// Listen announces on the local network address.
	//
	// The network must be "tcp", "tcp4", "tcp6", "unix" or "unixpacket".
	//
	// For TCP networks, if the host in the address parameter is empty or
	// a literal unspecified IP address, Listen listens on all available
	// unicast and anycast IP addresses of the local system.
	// To only use IPv4, use network "tcp4".
	// The address can use a host name, but this is not recommended,
	// because it will create a listener for at most one of the host's IP
	// addresses.
	// If the port in the address parameter is empty or "0", as in
	// "127.0.0.1:" or "[::1]:0", a port number is automatically chosen.
	// The Addr method of Listener can be used to discover the chosen
	// port.
	//
	// See func Dial for a description of the network and address
	// parameters.
	func Listen(network, address string) (Listener, error) {
	addrs, err := DefaultResolver.resolveAddrList(context.Background(), "listen", network, address, nil)
	if err != nil {
	return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
	}
	var l Listener
	switch la := addrs.first(isIPv4).(type) {
	case *TCPAddr:
	l, err = ListenTCP(network, la)
	case *UnixAddr:
	l, err = ListenUnix(network, la)
	default:
	return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
	}
	if err != nil {
	return nil, err // l is non-nil interface containing nil pointer
	}
	return l, nil
	}

	// ListenPacket announces on the local network address.
	//
	// The network must be "udp", "udp4", "udp6", "unixgram", or an IP
	// transport. The IP transports are "ip", "ip4", or "ip6" followed by
	// a colon and a literal protocol number or a protocol name, as in
	// "ip:1" or "ip:icmp".
	//
	// For UDP and IP networks, if the host in the address parameter is
	// empty or a literal unspecified IP address, ListenPacket listens on
	// all available IP addresses of the local system except multicast IP
	// addresses.
	// To only use IPv4, use network "udp4" or "ip4:proto".
	// The address can use a host name, but this is not recommended,
	// because it will create a listener for at most one of the host's IP
	// addresses.
	// If the port in the address parameter is empty or "0", as in
	// "127.0.0.1:" or "[::1]:0", a port number is automatically chosen.
	// The LocalAddr method of PacketConn can be used to discover the
	// chosen port.
	//
	// See func Dial for a description of the network and address
	// parameters.
	func ListenPacket(network, address string) (PacketConn, error) {
	addrs, err := DefaultResolver.resolveAddrList(context.Background(), "listen", network, address, nil)
	if err != nil {
	return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
	}
	var l PacketConn
	switch la := addrs.first(isIPv4).(type) {
	case *UDPAddr:
	l, err = ListenUDP(network, la)
	case *IPAddr:
	l, err = ListenIP(network, la)
	case *UnixAddr:
	l, err = ListenUnixgram(network, la)
	default:
	return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
	}
	if err != nil {
	return nil, err // l is non-nil interface containing nil pointer
	}
	return l, nil
	}