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package main
import (
"errors"
"flag"
"fmt"
"io"
"net"
"os"
"os/exec"
"path/filepath"
"strconv"
"syscall"
multierror "github.com/hashicorp/go-multierror"
"github.com/sirupsen/logrus"
)
// This is similar to netcat's listen mode, except it logs which port it's
// assigned if it's told to attempt to bind to port 0. Or it's similar to
// inetd, if it wasn't a daemon, and it wasn't as well-written.
func main() {
pidFile := ""
portFile := ""
detach := false
flag.BoolVar(&detach, "detach", false, "detach from terminal")
flag.StringVar(&portFile, "port-file", "", "file to write listening port number")
flag.StringVar(&pidFile, "pid-file", "", "file to write process ID to")
flag.Parse()
args := flag.Args()
if len(args) < 1 {
fmt.Printf("Usage: %s [-port-file filename] [-pid-file filename] command ...\n", filepath.Base(os.Args[0]))
os.Exit(1)
}
// Start listening without specifying a port number.
ln, err := net.ListenTCP("tcp", &net.TCPAddr{})
if err != nil {
logrus.Fatalf("listening: %v", err)
}
// Retrieve the address we ended up bound to and write the port number
// part to the specified file, if one was specified.
addrString := ln.Addr().String()
_, portString, err := net.SplitHostPort(addrString)
if err != nil {
logrus.Fatalf("finding the port number in %q: %v", addrString, err)
}
if portFile != "" {
if err := os.WriteFile(portFile, []byte(portString), 0o644); err != nil {
logrus.Fatalf("writing listening port to %q: %v", portFile, err)
}
defer os.Remove(portFile)
}
// Write our process ID to the specified file, if one was specified.
if pidFile != "" {
pid := strconv.Itoa(os.Getpid())
if err := os.WriteFile(pidFile, []byte(pid), 0o644); err != nil {
logrus.Fatalf("writing pid %d to %q: %v", os.Getpid(), pidFile, err)
}
defer os.Remove(pidFile)
}
// Now we can log which port we're listening on.
fmt.Printf("process %d listening on port %s\n", os.Getpid(), portString)
closeCloser := func(closer io.Closer) {
if err := closer.Close(); err != nil {
logrus.Errorf("closing: %v", err)
}
}
// Helper function to shuttle data between a reader and a writer.
relay := func(reader io.Reader, writer io.Writer) error {
buffer := make([]byte, 1024)
for {
nr, err := reader.Read(buffer)
if err != nil {
if errors.Is(err, io.EOF) {
return nil
}
return err
}
if nr == 0 {
return nil
}
if nr < 0 {
// no error?
break
}
nw, err := writer.Write(buffer[:nr])
if err != nil {
return nil
}
if nw != nr {
return fmt.Errorf("short write: %d != %d", nw, nr)
}
}
return nil
}
for {
// Accept the next incoming connection.
conn, err := ln.AcceptTCP()
if err != nil {
logrus.Errorf("accepting new connection: %v", err)
continue
}
if conn == nil {
logrus.Error("no new connection?")
continue
}
go func() {
defer closeCloser(conn)
rawConn, err := conn.SyscallConn()
if err != nil {
logrus.Errorf("getting underlying connection: %v", err)
return
}
var setNonblockError error
if err := rawConn.Control(func(fd uintptr) {
setNonblockError = syscall.SetNonblock(int(fd), true)
}); err != nil {
logrus.Errorf("marking connection nonblocking (outer): %v", err)
return
}
if setNonblockError != nil {
logrus.Errorf("marking connection nonblocking (inner): %v", setNonblockError)
return
}
// Create pipes for the subprocess's stdio.
stdinReader, stdinWriter, err := os.Pipe()
if err != nil {
logrus.Errorf("opening pipe for stdin: %v", err)
return
}
defer closeCloser(stdinWriter)
stdoutReader, stdoutWriter, err := os.Pipe()
if err != nil {
logrus.Errorf("opening pipe for stdout: %v", err)
closeCloser(stdinReader)
return
}
defer closeCloser(stdoutReader)
if err := syscall.SetNonblock(int(stdoutReader.Fd()), true); err != nil {
logrus.Errorf("marking stdout reader nonblocking: %v", err)
closeCloser(stdinReader)
closeCloser(stdoutWriter)
return
}
// Start the subprocess.
cmd := exec.Command(args[0], args[1:]...)
cmd.Stdin = stdinReader
cmd.Stdout = stdoutWriter
cmd.Stderr = os.Stderr
if err := cmd.Start(); err != nil {
logrus.Errorf("starting %v: %v", args, err)
closeCloser(stdinReader)
closeCloser(stdoutWriter)
return
}
// Process the subprocess's stdio and wait for it to exit,
// presumably when it runs out of data.
var relayGroup multierror.Group
relayGroup.Go(func() error {
err := relay(conn, stdinWriter)
closeCloser(stdinWriter)
return err
})
relayGroup.Go(func() error {
err := relay(stdoutReader, conn)
closeCloser(stdoutReader)
return err
})
relayGroup.Go(func() error {
err := cmd.Wait()
closeCloser(conn)
return err
})
merr := relayGroup.Wait()
if merr != nil && merr.ErrorOrNil() != nil {
logrus.Errorf("%v\n", merr)
}
}()
}
}
|
