xref: /aosp_15_r20/prebuilts/go/linux-x86/src/net/net.go

// Copyright 2009 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 provides a portable interface for network I/O, including
TCP/IP, UDP, domain name resolution, and Unix domain sockets.

Although the package provides access to low-level networking
primitives, most clients will need only the basic interface provided
by the [Dial], [Listen], and Accept functions and the associated
[Conn] and [Listener] interfaces. The crypto/tls package uses
the same interfaces and similar Dial and Listen functions.

The Dial function connects to a server:

	conn, err := net.Dial("tcp", "golang.org:80")
	if err != nil {
		// handle error
	}
	fmt.Fprintf(conn, "GET / HTTP/1.0\r\n\r\n")
	status, err := bufio.NewReader(conn).ReadString('\n')
	// ...

The Listen function creates servers:

	ln, err := net.Listen("tcp", ":8080")
	if err != nil {
		// handle error
	}
	for {
		conn, err := ln.Accept()
		if err != nil {
			// handle error
		}
		go handleConnection(conn)
	}

# Name Resolution

The method for resolving domain names, whether indirectly with functions like Dial
or directly with functions like [LookupHost] and [LookupAddr], varies by operating system.

On Unix systems, the resolver has two options for resolving names.
It can use a pure Go resolver that sends DNS requests directly to the servers
listed in /etc/resolv.conf, or it can use a cgo-based resolver that calls C
library routines such as getaddrinfo and getnameinfo.

On Unix the pure Go resolver is preferred over the cgo resolver, because a blocked DNS
request consumes only a goroutine, while a blocked C call consumes an operating system thread.
When cgo is available, the cgo-based resolver is used instead under a variety of
conditions: on systems that do not let programs make direct DNS requests (OS X),
when the LOCALDOMAIN environment variable is present (even if empty),
when the RES_OPTIONS or HOSTALIASES environment variable is non-empty,
when the ASR_CONFIG environment variable is non-empty (OpenBSD only),
when /etc/resolv.conf or /etc/nsswitch.conf specify the use of features that the
Go resolver does not implement.

On all systems (except Plan 9), when the cgo resolver is being used
this package applies a concurrent cgo lookup limit to prevent the system
from running out of system threads. Currently, it is limited to 500 concurrent lookups.

The resolver decision can be overridden by setting the netdns value of the
GODEBUG environment variable (see package runtime) to go or cgo, as in:

	export GODEBUG=netdns=go    # force pure Go resolver
	export GODEBUG=netdns=cgo   # force native resolver (cgo, win32)

The decision can also be forced while building the Go source tree
by setting the netgo or netcgo build tag.

A numeric netdns setting, as in GODEBUG=netdns=1, causes the resolver
to print debugging information about its decisions.
To force a particular resolver while also printing debugging information,
join the two settings by a plus sign, as in GODEBUG=netdns=go+1.

The Go resolver will send an EDNS0 additional header with a DNS request,
to signal a willingness to accept a larger DNS packet size.
This can reportedly cause sporadic failures with the DNS server run
by some modems and routers. Setting GODEBUG=netedns0=0 will disable
sending the additional header.

On macOS, if Go code that uses the net package is built with
-buildmode=c-archive, linking the resulting archive into a C program
requires passing -lresolv when linking the C code.

On Plan 9, the resolver always accesses /net/cs and /net/dns.

On Windows, in Go 1.18.x and earlier, the resolver always used C
library functions, such as GetAddrInfo and DnsQuery.
*/
package net

import (
	"context"
	"errors"
	"internal/poll"
	"io"
	"os"
	"sync"
	"syscall"
	"time"
	_ "unsafe" // for linkname
)

// Addr represents a network end point address.
//
// The two methods [Addr.Network] and [Addr.String] conventionally return strings
// that can be passed as the arguments to [Dial], but the exact form
// and meaning of the strings is up to the implementation.
type Addr interface {
	Network() string // name of the network (for example, "tcp", "udp")
	String() string  // string form of address (for example, "192.0.2.1:25", "[2001:db8::1]:80")
}

// Conn is a generic stream-oriented network connection.
//
// Multiple goroutines may invoke methods on a Conn simultaneously.
type Conn interface {
	// Read reads data from the connection.
	// Read can be made to time out and return an error after a fixed
	// time limit; see SetDeadline and SetReadDeadline.
	Read(b []byte) (n int, err error)

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

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

	// LocalAddr returns the local network address, if known.
	LocalAddr() Addr

	// RemoteAddr returns the remote network address, if known.
	RemoteAddr() Addr

	// SetDeadline sets the read and write deadlines associated
	// with the connection. It is equivalent to calling both
	// SetReadDeadline and SetWriteDeadline.
	//
	// A deadline is an absolute time after which I/O operations
	// fail instead of blocking. The deadline applies to all future
	// and pending I/O, not just the immediately following call to
	// Read or Write. After a deadline has been exceeded, the
	// connection can be refreshed by setting a deadline in the future.
	//
	// If the deadline is exceeded a call to Read or Write or to other
	// I/O methods will return an error that wraps os.ErrDeadlineExceeded.
	// This can be tested using errors.Is(err, os.ErrDeadlineExceeded).
	// The error's Timeout method will return true, but note that there
	// are other possible errors for which the Timeout method will
	// return true even if the deadline has not been exceeded.
	//
	// An idle timeout can be implemented by repeatedly extending
	// the deadline after successful Read or Write calls.
	//
	// A zero value for t means I/O operations will not time out.
	SetDeadline(t time.Time) error

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

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

type conn struct {
	fd *netFD
}

func (c *conn) ok() bool { return c != nil && c.fd != nil }

// Implementation of the Conn interface.

// Read implements the Conn Read method.
func (c *conn) Read(b []byte) (int, error) {
	if !c.ok() {
		return 0, syscall.EINVAL
	}
	n, err := c.fd.Read(b)
	if err != nil && err != io.EOF {
		err = &OpError{Op: "read", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return n, err
}

// Write implements the Conn Write method.
func (c *conn) Write(b []byte) (int, error) {
	if !c.ok() {
		return 0, syscall.EINVAL
	}
	n, err := c.fd.Write(b)
	if err != nil {
		err = &OpError{Op: "write", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return n, err
}

// Close closes the connection.
func (c *conn) Close() error {
	if !c.ok() {
		return syscall.EINVAL
	}
	err := c.fd.Close()
	if err != nil {
		err = &OpError{Op: "close", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return err
}

// LocalAddr returns the local network address.
// The Addr returned is shared by all invocations of LocalAddr, so
// do not modify it.
func (c *conn) LocalAddr() Addr {
	if !c.ok() {
		return nil
	}
	return c.fd.laddr
}

// RemoteAddr returns the remote network address.
// The Addr returned is shared by all invocations of RemoteAddr, so
// do not modify it.
func (c *conn) RemoteAddr() Addr {
	if !c.ok() {
		return nil
	}
	return c.fd.raddr
}

// SetDeadline implements the Conn SetDeadline method.
func (c *conn) SetDeadline(t time.Time) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := c.fd.SetDeadline(t); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// SetReadDeadline implements the Conn SetReadDeadline method.
func (c *conn) SetReadDeadline(t time.Time) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := c.fd.SetReadDeadline(t); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// SetWriteDeadline implements the Conn SetWriteDeadline method.
func (c *conn) SetWriteDeadline(t time.Time) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := c.fd.SetWriteDeadline(t); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// SetReadBuffer sets the size of the operating system's
// receive buffer associated with the connection.
func (c *conn) SetReadBuffer(bytes int) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := setReadBuffer(c.fd, bytes); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// SetWriteBuffer sets the size of the operating system's
// transmit buffer associated with the connection.
func (c *conn) SetWriteBuffer(bytes int) error {
	if !c.ok() {
		return syscall.EINVAL
	}
	if err := setWriteBuffer(c.fd, bytes); err != nil {
		return &OpError{Op: "set", Net: c.fd.net, Source: nil, Addr: c.fd.laddr, Err: err}
	}
	return nil
}

// File returns a copy of the underlying [os.File].
// It is the caller's responsibility to close f when finished.
// Closing c does not affect f, and closing f does not affect c.
//
// The returned os.File's file descriptor is different from the connection's.
// Attempting to change properties of the original using this duplicate
// may or may not have the desired effect.
func (c *conn) File() (f *os.File, err error) {
	f, err = c.fd.dup()
	if err != nil {
		err = &OpError{Op: "file", Net: c.fd.net, Source: c.fd.laddr, Addr: c.fd.raddr, Err: err}
	}
	return
}

// PacketConn is a generic packet-oriented network connection.
//
// Multiple goroutines may invoke methods on a PacketConn simultaneously.
type PacketConn interface {
	// ReadFrom reads a packet from the connection,
	// copying the payload into p. It returns the number of
	// bytes copied into p and the return address that
	// was on the packet.
	// It returns the number of bytes read (0 <= n <= len(p))
	// and any error encountered. Callers should always process
	// the n > 0 bytes returned before considering the error err.
	// ReadFrom can be made to time out and return an error after a
	// fixed time limit; see SetDeadline and SetReadDeadline.
	ReadFrom(p []byte) (n int, addr Addr, err error)

	// WriteTo writes a packet with payload p to addr.
	// WriteTo can be made to time out and return an Error after a
	// fixed time limit; see SetDeadline and SetWriteDeadline.
	// On packet-oriented connections, write timeouts are rare.
	WriteTo(p []byte, addr Addr) (n int, err error)

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

	// LocalAddr returns the local network address, if known.
	LocalAddr() Addr

	// SetDeadline sets the read and write deadlines associated
	// with the connection. It is equivalent to calling both
	// SetReadDeadline and SetWriteDeadline.
	//
	// A deadline is an absolute time after which I/O operations
	// fail instead of blocking. The deadline applies to all future
	// and pending I/O, not just the immediately following call to
	// Read or Write. After a deadline has been exceeded, the
	// connection can be refreshed by setting a deadline in the future.
	//
	// If the deadline is exceeded a call to Read or Write or to other
	// I/O methods will return an error that wraps os.ErrDeadlineExceeded.
	// This can be tested using errors.Is(err, os.ErrDeadlineExceeded).
	// The error's Timeout method will return true, but note that there
	// are other possible errors for which the Timeout method will
	// return true even if the deadline has not been exceeded.
	//
	// An idle timeout can be implemented by repeatedly extending
	// the deadline after successful ReadFrom or WriteTo calls.
	//
	// A zero value for t means I/O operations will not time out.
	SetDeadline(t time.Time) error

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

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

var listenerBacklogCache struct {
	sync.Once
	val int
}

// listenerBacklog is a caching wrapper around maxListenerBacklog.
//
// listenerBacklog should be an internal detail,
// but widely used packages access it using linkname.
// Notable members of the hall of shame include:
//   - github.com/database64128/tfo-go/v2
//   - github.com/metacubex/tfo-go
//   - github.com/sagernet/tfo-go
//
// Do not remove or change the type signature.
// See go.dev/issue/67401.
//
//go:linkname listenerBacklog
func listenerBacklog() int {
	listenerBacklogCache.Do(func() { listenerBacklogCache.val = maxListenerBacklog() })
	return listenerBacklogCache.val
}

// A Listener is a generic network listener for stream-oriented protocols.
//
// Multiple goroutines may invoke methods on a Listener simultaneously.
type Listener interface {
	// Accept waits for and returns the next connection to the listener.
	Accept() (Conn, error)

	// Close closes the listener.
	// Any blocked Accept operations will be unblocked and return errors.
	Close() error

	// Addr returns the listener's network address.
	Addr() Addr
}

// An Error represents a network error.
type Error interface {
	error
	Timeout() bool // Is the error a timeout?

	// Deprecated: Temporary errors are not well-defined.
	// Most "temporary" errors are timeouts, and the few exceptions are surprising.
	// Do not use this method.
	Temporary() bool
}

// Various errors contained in OpError.
var (
	// For connection setup operations.
	errNoSuitableAddress = errors.New("no suitable address found")

	// For connection setup and write operations.
	errMissingAddress = errors.New("missing address")

	// For both read and write operations.
	errCanceled         = canceledError{}
	ErrWriteToConnected = errors.New("use of WriteTo with pre-connected connection")
)

// canceledError lets us return the same error string we have always
// returned, while still being Is context.Canceled.
type canceledError struct{}

func (canceledError) Error() string { return "operation was canceled" }

func (canceledError) Is(err error) bool { return err == context.Canceled }

// mapErr maps from the context errors to the historical internal net
// error values.
func mapErr(err error) error {
	switch err {
	case context.Canceled:
		return errCanceled
	case context.DeadlineExceeded:
		return errTimeout
	default:
		return err
	}
}

// OpError is the error type usually returned by functions in the net
// package. It describes the operation, network type, and address of
// an error.
type OpError struct {
	// Op is the operation which caused the error, such as
	// "read" or "write".
	Op string

	// Net is the network type on which this error occurred,
	// such as "tcp" or "udp6".
	Net string

	// For operations involving a remote network connection, like
	// Dial, Read, or Write, Source is the corresponding local
	// network address.
	Source Addr

	// Addr is the network address for which this error occurred.
	// For local operations, like Listen or SetDeadline, Addr is
	// the address of the local endpoint being manipulated.
	// For operations involving a remote network connection, like
	// Dial, Read, or Write, Addr is the remote address of that
	// connection.
	Addr Addr

	// Err is the error that occurred during the operation.
	// The Error method panics if the error is nil.
	Err error
}

func (e *OpError) Unwrap() error { return e.Err }

func (e *OpError) Error() string {
	if e == nil {
		return "<nil>"
	}
	s := e.Op
	if e.Net != "" {
		s += " " + e.Net
	}
	if e.Source != nil {
		s += " " + e.Source.String()
	}
	if e.Addr != nil {
		if e.Source != nil {
			s += "->"
		} else {
			s += " "
		}
		s += e.Addr.String()
	}
	s += ": " + e.Err.Error()
	return s
}

var (
	// aLongTimeAgo is a non-zero time, far in the past, used for
	// immediate cancellation of dials.
	aLongTimeAgo = time.Unix(1, 0)

	// noDeadline and noCancel are just zero values for
	// readability with functions taking too many parameters.
	noDeadline = time.Time{}
	noCancel   = (chan struct{})(nil)
)

type timeout interface {
	Timeout() bool
}

func (e *OpError) Timeout() bool {
	if ne, ok := e.Err.(*os.SyscallError); ok {
		t, ok := ne.Err.(timeout)
		return ok && t.Timeout()
	}
	t, ok := e.Err.(timeout)
	return ok && t.Timeout()
}

type temporary interface {
	Temporary() bool
}

func (e *OpError) Temporary() bool {
	// Treat ECONNRESET and ECONNABORTED as temporary errors when
	// they come from calling accept. See issue 6163.
	if e.Op == "accept" && isConnError(e.Err) {
		return true
	}

	if ne, ok := e.Err.(*os.SyscallError); ok {
		t, ok := ne.Err.(temporary)
		return ok && t.Temporary()
	}
	t, ok := e.Err.(temporary)
	return ok && t.Temporary()
}

// A ParseError is the error type of literal network address parsers.
type ParseError struct {
	// Type is the type of string that was expected, such as
	// "IP address", "CIDR address".
	Type string

	// Text is the malformed text string.
	Text string
}

func (e *ParseError) Error() string { return "invalid " + e.Type + ": " + e.Text }

func (e *ParseError) Timeout() bool   { return false }
func (e *ParseError) Temporary() bool { return false }

type AddrError struct {
	Err  string
	Addr string
}

func (e *AddrError) Error() string {
	if e == nil {
		return "<nil>"
	}
	s := e.Err
	if e.Addr != "" {
		s = "address " + e.Addr + ": " + s
	}
	return s
}

func (e *AddrError) Timeout() bool   { return false }
func (e *AddrError) Temporary() bool { return false }

type UnknownNetworkError string

func (e UnknownNetworkError) Error() string   { return "unknown network " + string(e) }
func (e UnknownNetworkError) Timeout() bool   { return false }
func (e UnknownNetworkError) Temporary() bool { return false }

type InvalidAddrError string

func (e InvalidAddrError) Error() string   { return string(e) }
func (e InvalidAddrError) Timeout() bool   { return false }
func (e InvalidAddrError) Temporary() bool { return false }

// errTimeout exists to return the historical "i/o timeout" string
// for context.DeadlineExceeded. See mapErr.
// It is also used when Dialer.Deadline is exceeded.
// error.Is(errTimeout, context.DeadlineExceeded) returns true.
//
// TODO(iant): We could consider changing this to os.ErrDeadlineExceeded
// in the future, if we make
//
//	errors.Is(os.ErrDeadlineExceeded, context.DeadlineExceeded)
//
// return true.
var errTimeout error = &timeoutError{}

type timeoutError struct{}

func (e *timeoutError) Error() string   { return "i/o timeout" }
func (e *timeoutError) Timeout() bool   { return true }
func (e *timeoutError) Temporary() bool { return true }

func (e *timeoutError) Is(err error) bool {
	return err == context.DeadlineExceeded
}

// DNSConfigError represents an error reading the machine's DNS configuration.
// (No longer used; kept for compatibility.)
type DNSConfigError struct {
	Err error
}

func (e *DNSConfigError) Unwrap() error   { return e.Err }
func (e *DNSConfigError) Error() string   { return "error reading DNS config: " + e.Err.Error() }
func (e *DNSConfigError) Timeout() bool   { return false }
func (e *DNSConfigError) Temporary() bool { return false }

// Various errors contained in DNSError.
var (
	errNoSuchHost  = &notFoundError{"no such host"}
	errUnknownPort = &notFoundError{"unknown port"}
)

// notFoundError is a special error understood by the newDNSError function,
// which causes a creation of a DNSError with IsNotFound field set to true.
type notFoundError struct{ s string }

func (e *notFoundError) Error() string { return e.s }

// temporaryError is an error type that implements the [Error] interface.
// It returns true from the Temporary method.
type temporaryError struct{ s string }

func (e *temporaryError) Error() string   { return e.s }
func (e *temporaryError) Temporary() bool { return true }
func (e *temporaryError) Timeout() bool   { return false }

// DNSError represents a DNS lookup error.
type DNSError struct {
	UnwrapErr   error  // error returned by the [DNSError.Unwrap] method, might be nil
	Err         string // description of the error
	Name        string // name looked for
	Server      string // server used
	IsTimeout   bool   // if true, timed out; not all timeouts set this
	IsTemporary bool   // if true, error is temporary; not all errors set this

	// IsNotFound is set to true when the requested name does not
	// contain any records of the requested type (data not found),
	// or the name itself was not found (NXDOMAIN).
	IsNotFound bool
}

// newDNSError creates a new *DNSError.
// Based on the err, it sets the UnwrapErr, IsTimeout, IsTemporary, IsNotFound fields.
func newDNSError(err error, name, server string) *DNSError {
	var (
		isTimeout   bool
		isTemporary bool
		unwrapErr   error
	)

	if err, ok := err.(Error); ok {
		isTimeout = err.Timeout()
		isTemporary = err.Temporary()
	}

	// At this time, the only errors we wrap are context errors, to allow
	// users to check for canceled/timed out requests.
	if errors.Is(err, context.DeadlineExceeded) || errors.Is(err, context.Canceled) {
		unwrapErr = err
	}

	_, isNotFound := err.(*notFoundError)
	return &DNSError{
		UnwrapErr:   unwrapErr,
		Err:         err.Error(),
		Name:        name,
		Server:      server,
		IsTimeout:   isTimeout,
		IsTemporary: isTemporary,
		IsNotFound:  isNotFound,
	}
}

// Unwrap returns e.UnwrapErr.
func (e *DNSError) Unwrap() error { return e.UnwrapErr }

func (e *DNSError) Error() string {
	if e == nil {
		return "<nil>"
	}
	s := "lookup " + e.Name
	if e.Server != "" {
		s += " on " + e.Server
	}
	s += ": " + e.Err
	return s
}

// Timeout reports whether the DNS lookup is known to have timed out.
// This is not always known; a DNS lookup may fail due to a timeout
// and return a [DNSError] for which Timeout returns false.
func (e *DNSError) Timeout() bool { return e.IsTimeout }

// Temporary reports whether the DNS error is known to be temporary.
// This is not always known; a DNS lookup may fail due to a temporary
// error and return a [DNSError] for which Temporary returns false.
func (e *DNSError) Temporary() bool { return e.IsTimeout || e.IsTemporary }

// errClosed exists just so that the docs for ErrClosed don't mention
// the internal package poll.
var errClosed = poll.ErrNetClosing

// ErrClosed is the error returned by an I/O call on a network
// connection that has already been closed, or that is closed by
// another goroutine before the I/O is completed. This may be wrapped
// in another error, and should normally be tested using
// errors.Is(err, net.ErrClosed).
var ErrClosed error = errClosed

// noReadFrom can be embedded alongside another type to
// hide the ReadFrom method of that other type.
type noReadFrom struct{}

// ReadFrom hides another ReadFrom method.
// It should never be called.
func (noReadFrom) ReadFrom(io.Reader) (int64, error) {
	panic("can't happen")
}

// tcpConnWithoutReadFrom implements all the methods of *TCPConn other
// than ReadFrom. This is used to permit ReadFrom to call io.Copy
// without leading to a recursive call to ReadFrom.
type tcpConnWithoutReadFrom struct {
	noReadFrom
	*TCPConn
}

// Fallback implementation of io.ReaderFrom's ReadFrom, when sendfile isn't
// applicable.
func genericReadFrom(c *TCPConn, r io.Reader) (n int64, err error) {
	// Use wrapper to hide existing r.ReadFrom from io.Copy.
	return io.Copy(tcpConnWithoutReadFrom{TCPConn: c}, r)
}

// noWriteTo can be embedded alongside another type to
// hide the WriteTo method of that other type.
type noWriteTo struct{}

// WriteTo hides another WriteTo method.
// It should never be called.
func (noWriteTo) WriteTo(io.Writer) (int64, error) {
	panic("can't happen")
}

// tcpConnWithoutWriteTo implements all the methods of *TCPConn other
// than WriteTo. This is used to permit WriteTo to call io.Copy
// without leading to a recursive call to WriteTo.
type tcpConnWithoutWriteTo struct {
	noWriteTo
	*TCPConn
}

// Fallback implementation of io.WriterTo's WriteTo, when zero-copy isn't applicable.
func genericWriteTo(c *TCPConn, w io.Writer) (n int64, err error) {
	// Use wrapper to hide existing w.WriteTo from io.Copy.
	return io.Copy(w, tcpConnWithoutWriteTo{TCPConn: c})
}

// Limit the number of concurrent cgo-using goroutines, because
// each will block an entire operating system thread. The usual culprit
// is resolving many DNS names in separate goroutines but the DNS
// server is not responding. Then the many lookups each use a different
// thread, and the system or the program runs out of threads.

var threadLimit chan struct{}

var threadOnce sync.Once

func acquireThread(ctx context.Context) error {
	threadOnce.Do(func() {
		threadLimit = make(chan struct{}, concurrentThreadsLimit())
	})
	select {
	case threadLimit <- struct{}{}:
		return nil
	case <-ctx.Done():
		return ctx.Err()
	}
}

func releaseThread() {
	<-threadLimit
}

// buffersWriter is the interface implemented by Conns that support a
// "writev"-like batch write optimization.
// writeBuffers should fully consume and write all chunks from the
// provided Buffers, else it should report a non-nil error.
type buffersWriter interface {
	writeBuffers(*Buffers) (int64, error)
}

// Buffers contains zero or more runs of bytes to write.
//
// On certain machines, for certain types of connections, this is
// optimized into an OS-specific batch write operation (such as
// "writev").
type Buffers [][]byte

var (
	_ io.WriterTo = (*Buffers)(nil)
	_ io.Reader   = (*Buffers)(nil)
)

// WriteTo writes contents of the buffers to w.
//
// WriteTo implements [io.WriterTo] for [Buffers].
//
// WriteTo modifies the slice v as well as v[i] for 0 <= i < len(v),
// but does not modify v[i][j] for any i, j.
func (v *Buffers) WriteTo(w io.Writer) (n int64, err error) {
	if wv, ok := w.(buffersWriter); ok {
		return wv.writeBuffers(v)
	}
	for _, b := range *v {
		nb, err := w.Write(b)
		n += int64(nb)
		if err != nil {
			v.consume(n)
			return n, err
		}
	}
	v.consume(n)
	return n, nil
}

// Read from the buffers.
//
// Read implements [io.Reader] for [Buffers].
//
// Read modifies the slice v as well as v[i] for 0 <= i < len(v),
// but does not modify v[i][j] for any i, j.
func (v *Buffers) Read(p []byte) (n int, err error) {
	for len(p) > 0 && len(*v) > 0 {
		n0 := copy(p, (*v)[0])
		v.consume(int64(n0))
		p = p[n0:]
		n += n0
	}
	if len(*v) == 0 {
		err = io.EOF
	}
	return
}

func (v *Buffers) consume(n int64) {
	for len(*v) > 0 {
		ln0 := int64(len((*v)[0]))
		if ln0 > n {
			(*v)[0] = (*v)[0][n:]
			return
		}
		n -= ln0
		(*v)[0] = nil
		*v = (*v)[1:]
	}
}