Source: tcpsock_posix.go in package net

+build aix darwin dragonfly freebsd js,wasm linux netbsd openbsd solaris windows


package net

import (
	"context"
	"io"
	"os"
	"syscall"
)

func sockaddrToTCP(sa syscall.Sockaddr) Addr {
	switch sa := sa.(type) {
	case *syscall.SockaddrInet4:
		return &TCPAddr{IP: sa.Addr[0:], Port: sa.Port}
	case *syscall.SockaddrInet6:
		return &TCPAddr{IP: sa.Addr[0:], Port: sa.Port, Zone: zoneCache.name(int(sa.ZoneId))}
	}
	return nil
}

func (a *TCPAddr) family() int {
	if a == nil || len(a.IP) <= IPv4len {
		return syscall.AF_INET
	}
	if a.IP.To4() != nil {
		return syscall.AF_INET
	}
	return syscall.AF_INET6
}

func (a *TCPAddr) sockaddr(family int) (syscall.Sockaddr, error) {
	if a == nil {
		return nil, nil
	}
	return ipToSockaddr(family, a.IP, a.Port, a.Zone)
}

func (a *TCPAddr) toLocal(net string) sockaddr {
	return &TCPAddr{loopbackIP(net), a.Port, a.Zone}
}

func (c *TCPConn) readFrom(r io.Reader) (int64, error) {
	if n, err, handled := splice(c.fd, r); handled {
		return n, err
	}
	if n, err, handled := sendFile(c.fd, r); handled {
		return n, err
	}
	return genericReadFrom(c, r)
}

func (sd *sysDialer) dialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPConn, error) {
	if testHookDialTCP != nil {
		return testHookDialTCP(ctx, sd.network, laddr, raddr)
	}
	return sd.doDialTCP(ctx, laddr, raddr)
}

func (sd *sysDialer) doDialTCP(ctx context.Context, laddr, raddr *TCPAddr) (*TCPConn, error) {
	fd, err := internetSocket(ctx, sd.network, laddr, raddr, syscall.SOCK_STREAM, 0, "dial", sd.Dialer.Control)

TCP has a rarely used mechanism called a 'simultaneous connection' in which Dial("tcp", addr1, addr2) run on the machine at addr1 can connect to a simultaneous Dial("tcp", addr2, addr1) run on the machine at addr2, without either machine executing Listen. If laddr == nil, it means we want the kernel to pick an appropriate originating local address. Some Linux kernels cycle blindly through a fixed range of local ports, regardless of destination port. If a kernel happens to pick local port 50001 as the source for a Dial("tcp", "", "localhost:50001"), then the Dial will succeed, having simultaneously connected to itself. This can only happen when we are letting the kernel pick a port (laddr == nil) and when there is no listener for the destination address. It's hard to argue this is anything other than a kernel bug. If we see this happen, rather than expose the buggy effect to users, we close the fd and try again. If it happens twice more, we relent and use the result. See also: https://golang.org/issue/2690 https://stackoverflow.com/questions/4949858/ The opposite can also happen: if we ask the kernel to pick an appropriate originating local address, sometimes it picks one that is already in use. So if the error is EADDRNOTAVAIL, we have to try again too, just for a different reason. The kernel socket code is no doubt enjoying watching us squirm.

	for i := 0; i < 2 && (laddr == nil || laddr.Port == 0) && (selfConnect(fd, err) || spuriousENOTAVAIL(err)); i++ {
		if err == nil {
			fd.Close()
		}
		fd, err = internetSocket(ctx, sd.network, laddr, raddr, syscall.SOCK_STREAM, 0, "dial", sd.Dialer.Control)
	}

	if err != nil {
		return nil, err
	}
	return newTCPConn(fd), nil
}

If the connect failed, we clearly didn't connect to ourselves.

	if err != nil {
		return false
	}

The socket constructor can return an fd with raddr nil under certain unknown conditions. The errors in the calls there to Getpeername are discarded, but we can't catch the problem there because those calls are sometimes legally erroneous with a "socket not connected". Since this code (selfConnect) is already trying to work around a problem, we make sure if this happens we recognize trouble and ask the DialTCP routine to try again. TODO: try to understand what's really going on.

	if fd.laddr == nil || fd.raddr == nil {
		return true
	}
	l := fd.laddr.(*TCPAddr)
	r := fd.raddr.(*TCPAddr)
	return l.Port == r.Port && l.IP.Equal(r.IP)
}

func spuriousENOTAVAIL(err error) bool {
	if op, ok := err.(*OpError); ok {
		err = op.Err
	}
	if sys, ok := err.(*os.SyscallError); ok {
		err = sys.Err
	}
	return err == syscall.EADDRNOTAVAIL
}

func (ln *TCPListener) ok() bool { return ln != nil && ln.fd != nil }

func (ln *TCPListener) accept() (*TCPConn, error) {
	fd, err := ln.fd.accept()
	if err != nil {
		return nil, err
	}
	tc := newTCPConn(fd)
	if ln.lc.KeepAlive >= 0 {
		setKeepAlive(fd, true)
		ka := ln.lc.KeepAlive
		if ln.lc.KeepAlive == 0 {
			ka = defaultTCPKeepAlive
		}
		setKeepAlivePeriod(fd, ka)
	}
	return tc, nil
}

func (ln *TCPListener) close() error {
	return ln.fd.Close()
}

func (ln *TCPListener) file() (*os.File, error) {
	f, err := ln.fd.dup()
	if err != nil {
		return nil, err
	}
	return f, nil
}

func (sl *sysListener) listenTCP(ctx context.Context, laddr *TCPAddr) (*TCPListener, error) {
	fd, err := internetSocket(ctx, sl.network, laddr, nil, syscall.SOCK_STREAM, 0, "listen", sl.ListenConfig.Control)
	if err != nil {
		return nil, err
	}
	return &TCPListener{fd: fd, lc: sl.ListenConfig}, nil