tcpecho.go

     1  // Copyright 2023 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package main
     6  
     7  import (
     8  	"errors"
     9  	"net"
    10  	"os"
    11  	"syscall"
    12  )
    13  
    14  func main() {
    15  	if err := run(); err != nil {
    16  		println(err)
    17  		os.Exit(1)
    18  	}
    19  }
    20  
    21  func run() error {
    22  	l, err := findListener()
    23  	if err != nil {
    24  		return err
    25  	}
    26  	if l == nil {
    27  		return errors.New("no pre-opened sockets available")
    28  	}
    29  	defer l.Close()
    30  
    31  	c, err := l.Accept()
    32  	if err != nil {
    33  		return err
    34  	}
    35  	return handleConn(c)
    36  }
    37  
    38  func handleConn(c net.Conn) error {
    39  	defer c.Close()
    40  
    41  	var buf [128]byte
    42  	n, err := c.Read(buf[:])
    43  	if err != nil {
    44  		return err
    45  	}
    46  	if _, err := c.Write(buf[:n]); err != nil {
    47  		return err
    48  	}
    49  	if err := c.(*net.TCPConn).CloseWrite(); err != nil {
    50  		return err
    51  	}
    52  	return c.Close()
    53  }
    54  
    55  func findListener() (net.Listener, error) {
    56  	// We start looking for pre-opened sockets at fd=3 because 0, 1, and 2
    57  	// are reserved for stdio. Pre-opened directors also start at fd=3, so
    58  	// we skip fds that aren't sockets. Once we reach EBADF we know there
    59  	// are no more pre-opens.
    60  	for preopenFd := uintptr(3); ; preopenFd++ {
    61  		f := os.NewFile(preopenFd, "")
    62  		l, err := net.FileListener(f)
    63  		f.Close()
    64  
    65  		var se syscall.Errno
    66  		switch errors.As(err, &se); se {
    67  		case syscall.ENOTSOCK:
    68  			continue
    69  		case syscall.EBADF:
    70  			err = nil
    71  		}
    72  		return l, err
    73  	}
    74  }
    75
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