cmd/compile/internal/inline: score call sites exposed by inlines

[gostls13.git] / src / net / dial.go

// 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/bytealg"
        "internal/godebug"
        "internal/nettrace"
        "syscall"
        "time"
)

const (
        // defaultTCPKeepAlive is a default constant value for TCPKeepAlive times
        // See go.dev/issue/31510
        defaultTCPKeepAlive = 15 * time.Second

        // For the moment, MultiPath TCP is not used by default
        // See go.dev/issue/56539
        defaultMPTCPEnabled = false
)

var multipathtcp = godebug.New("multipathtcp")

// mptcpStatus is a tristate for Multipath TCP, see go.dev/issue/56539
type mptcpStatus uint8

const (
        // The value 0 is the system default, linked to defaultMPTCPEnabled
        mptcpUseDefault mptcpStatus = iota
        mptcpEnabled
        mptcpDisabled
)

func (m *mptcpStatus) get() bool {
        switch *m {
        case mptcpEnabled:
                return true
        case mptcpDisabled:
                return false
        }

        // If MPTCP is forced via GODEBUG=multipathtcp=1
        if multipathtcp.Value() == "1" {
                multipathtcp.IncNonDefault()

                return true
        }

        return defaultMPTCPEnabled
}

func (m *mptcpStatus) set(use bool) {
        if use {
                *m = mptcpEnabled
        } else {
                *m = mptcpDisabled
        }
}

// 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.
//
// It is safe to call Dialer's methods concurrently.
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 previously enabled RFC 6555 Fast Fallback
        // support, also known as "Happy Eyeballs", in which IPv4 is
        // tried soon if IPv6 appears to be misconfigured and
        // hanging.
        //
        // Deprecated: Fast Fallback is enabled by default. To
        // disable, set FallbackDelay to a negative value.
        DualStack bool

        // FallbackDelay specifies the length of time to wait before
        // spawning a RFC 6555 Fast Fallback connection. That is, this
        // is the amount of time to wait for IPv6 to succeed before
        // assuming that IPv6 is misconfigured and falling back to
        // IPv4.
        //
        // If zero, a default delay of 300ms is used.
        // A negative value disables Fast Fallback support.
        FallbackDelay time.Duration

        // KeepAlive specifies the interval between keep-alive
        // probes for an active network connection.
        // If zero, keep-alive probes are sent with a default value
        // (currently 15 seconds), if supported by the protocol and operating
        // system. Network protocols or operating systems that do
        // not support keep-alives ignore this field.
        // If negative, keep-alive probes are disabled.
        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
        // cancellation.
        //
        // Deprecated: Use DialContext instead.
        Cancel <-chan struct{}

        // If Control is not nil, it is called after creating the network
        // connection but before actually dialing.
        //
        // Network and address parameters passed to Control function are not
        // necessarily the ones passed to Dial. For example, passing "tcp" to Dial
        // will cause the Control function to be called with "tcp4" or "tcp6".
        //
        // Control is ignored if ControlContext is not nil.
        Control func(network, address string, c syscall.RawConn) error

        // If ControlContext is not nil, it is called after creating the network
        // connection but before actually dialing.
        //
        // Network and address parameters passed to ControlContext function are not
        // necessarily the ones passed to Dial. For example, passing "tcp" to Dial
        // will cause the ControlContext function to be called with "tcp4" or "tcp6".
        //
        // If ControlContext is not nil, Control is ignored.
        ControlContext func(ctx context.Context, network, address string, c syscall.RawConn) error

        // If mptcpStatus is set to a value allowing Multipath TCP (MPTCP) to be
        // used, any call to Dial with "tcp(4|6)" as network will use MPTCP if
        // supported by the operating system.
        mptcpStatus mptcpStatus
}

func (d *Dialer) dualStack() bool { return d.FallbackDelay >= 0 }

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{}, 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 := bytealg.LastIndexByteString(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
}

// MultipathTCP reports whether MPTCP will be used.
//
// This method doesn't check if MPTCP is supported by the operating
// system or not.
func (d *Dialer) MultipathTCP() bool {
        return d.mptcpStatus.get()
}

// SetMultipathTCP directs the Dial methods to use, or not use, MPTCP,
// if supported by the operating system. This method overrides the
// system default and the GODEBUG=multipathtcp=... setting if any.
//
// If MPTCP is not available on the host or not supported by the server,
// the Dial methods will fall back to TCP.
func (d *Dialer) SetMultipathTCP(use bool) {
        d.mptcpStatus.set(use)
}

// 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)
}

// sysDialer contains a Dial's parameters and configuration.
type sysDialer struct {
        Dialer
        network, address string
        testHookDialTCP  func(ctx context.Context, net string, laddr, raddr *TCPAddr) (*TCPConn, error)
}

// Dial connects to the address on the named network.
//
// See func Dial for a description of the network and address
// parameters.
//
// Dial uses context.Background internally; to specify the context, use
// DialContext.
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() {
                testHookStepTime()
                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}
        }

        sd := &sysDialer{
                Dialer:  *d,
                network: network,
                address: address,
        }

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

        return sd.dialParallel(ctx, primaries, fallbacks)
}

// 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 (sd *sysDialer) dialParallel(ctx context.Context, primaries, fallbacks addrList) (Conn, error) {
        if len(fallbacks) == 0 {
                return sd.dialSerial(ctx, 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 := sd.dialSerial(ctx, 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(sd.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 (sd *sysDialer) dialSerial(ctx context.Context, 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: sd.network, Source: sd.LocalAddr, Addr: ra, Err: mapErr(ctx.Err())}
                default:
                }

                dialCtx := ctx
                if deadline, hasDeadline := ctx.Deadline(); hasDeadline {
                        partialDeadline, 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
                        }
                        if partialDeadline.Before(deadline) {
                                var cancel context.CancelFunc
                                dialCtx, cancel = context.WithDeadline(ctx, partialDeadline)
                                defer cancel()
                        }
                }

                c, err := sd.dialSingle(dialCtx, ra)
                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
}

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

// ListenConfig contains options for listening to an address.
type ListenConfig struct {
        // If Control is not nil, it is called after creating the network
        // connection but before binding it to the operating system.
        //
        // Network and address parameters passed to Control method are not
        // necessarily the ones passed to Listen. For example, passing "tcp" to
        // Listen will cause the Control function to be called with "tcp4" or "tcp6".
        Control func(network, address string, c syscall.RawConn) error

        // KeepAlive specifies the keep-alive period for network
        // connections accepted by this listener.
        // If zero, keep-alives are enabled if supported by the protocol
        // and operating system. Network protocols or operating systems
        // that do not support keep-alives ignore this field.
        // If negative, keep-alives are disabled.
        KeepAlive time.Duration

        // If mptcpStatus is set to a value allowing Multipath TCP (MPTCP) to be
        // used, any call to Listen with "tcp(4|6)" as network will use MPTCP if
        // supported by the operating system.
        mptcpStatus mptcpStatus
}

// MultipathTCP reports whether MPTCP will be used.
//
// This method doesn't check if MPTCP is supported by the operating
// system or not.
func (lc *ListenConfig) MultipathTCP() bool {
        return lc.mptcpStatus.get()
}

// SetMultipathTCP directs the Listen method to use, or not use, MPTCP,
// if supported by the operating system. This method overrides the
// system default and the GODEBUG=multipathtcp=... setting if any.
//
// If MPTCP is not available on the host or not supported by the client,
// the Listen method will fall back to TCP.
func (lc *ListenConfig) SetMultipathTCP(use bool) {
        lc.mptcpStatus.set(use)
}

// Listen announces on the local network address.
//
// See func Listen for a description of the network and address
// parameters.
func (lc *ListenConfig) Listen(ctx context.Context, network, address string) (Listener, error) {
        addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
        if err != nil {
                return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
        }
        sl := &sysListener{
                ListenConfig: *lc,
                network:      network,
                address:      address,
        }
        var l Listener
        la := addrs.first(isIPv4)
        switch la := la.(type) {
        case *TCPAddr:
                if sl.MultipathTCP() {
                        l, err = sl.listenMPTCP(ctx, la)
                } else {
                        l, err = sl.listenTCP(ctx, la)
                }
        case *UnixAddr:
                l, err = sl.listenUnix(ctx, la)
        default:
                return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
        }
        if err != nil {
                return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: err} // l is non-nil interface containing nil pointer
        }
        return l, nil
}

// ListenPacket announces on the local network address.
//
// See func ListenPacket for a description of the network and address
// parameters.
func (lc *ListenConfig) ListenPacket(ctx context.Context, network, address string) (PacketConn, error) {
        addrs, err := DefaultResolver.resolveAddrList(ctx, "listen", network, address, nil)
        if err != nil {
                return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: nil, Err: err}
        }
        sl := &sysListener{
                ListenConfig: *lc,
                network:      network,
                address:      address,
        }
        var c PacketConn
        la := addrs.first(isIPv4)
        switch la := la.(type) {
        case *UDPAddr:
                c, err = sl.listenUDP(ctx, la)
        case *IPAddr:
                c, err = sl.listenIP(ctx, la)
        case *UnixAddr:
                c, err = sl.listenUnixgram(ctx, la)
        default:
                return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: &AddrError{Err: "unexpected address type", Addr: address}}
        }
        if err != nil {
                return nil, &OpError{Op: "listen", Net: sl.network, Source: nil, Addr: la, Err: err} // c is non-nil interface containing nil pointer
        }
        return c, nil
}

// sysListener contains a Listen's parameters and configuration.
type sysListener struct {
        ListenConfig
        network, address string
}

// 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.
//
// Listen uses context.Background internally; to specify the context, use
// ListenConfig.Listen.
func Listen(network, address string) (Listener, error) {
        var lc ListenConfig
        return lc.Listen(context.Background(), network, address)
}

// 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.
//
// ListenPacket uses context.Background internally; to specify the context, use
// ListenConfig.ListenPacket.
func ListenPacket(network, address string) (PacketConn, error) {
        var lc ListenConfig
        return lc.ListenPacket(context.Background(), network, address)
}