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// Copyright 2023 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 main
import (
"errors"
"net"
"os"
"syscall"
)
func main() {
if err := run(); err != nil {
println(err)
os.Exit(1)
}
}
func run() error {
l, err := findListener()
if err != nil {
return err
}
if l == nil {
return errors.New("no pre-opened sockets available")
}
defer l.Close()
c, err := l.Accept()
if err != nil {
return err
}
return handleConn(c)
}
func handleConn(c net.Conn) error {
defer c.Close()
var buf [128]byte
n, err := c.Read(buf[:])
if err != nil {
return err
}
if _, err := c.Write(buf[:n]); err != nil {
return err
}
if err := c.(*net.TCPConn).CloseWrite(); err != nil {
return err
}
return c.Close()
}
func findListener() (net.Listener, error) {
// We start looking for pre-opened sockets at fd=3 because 0, 1, and 2
// are reserved for stdio. Pre-opened directors also start at fd=3, so
// we skip fds that aren't sockets. Once we reach EBADF we know there
// are no more pre-opens.
for preopenFd := uintptr(3); ; preopenFd++ {
f := os.NewFile(preopenFd, "")
l, err := net.FileListener(f)
f.Close()
var se syscall.Errno
switch errors.As(err, &se); se {
case syscall.ENOTSOCK:
continue
case syscall.EBADF:
err = nil
}
return l, err
}
}