rs2322tcp/internal/client/pty_linux.go

//go:build linux

/*
 * ============================================================================
 * Projekt.....: rs2322tcp
 * Datei.......: internal/client/pty_linux.go
 * Copyright (C) 2026 Dieter Lang
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 * Beschreibung:
 * Linux-spezifische Implementierung der virtuellen seriellen Schnittstelle
 * über ein PTY-Paar.
 *
 * Der PTY-Master wird intern vom Client verwendet. Der zugehörige PTY-Slave
 * wird über den virtuellen Port als serielle Schnittstelle bereitgestellt.
 * ============================================================================
 */

package client

import (
	"fmt"
	"sync/atomic"

	"golang.org/x/sys/unix"
)

///////////////////////////////////////////////////////////////////////////////
// ptySerial
///////////////////////////////////////////////////////////////////////////////

// ptySerial implements VirtualSerial using a Linux PTY pair.
//
// The file descriptors are stored atomically because Read/Write may execute
// concurrently with Close. Close must be able to invalidate the descriptors
// without racing with a concurrent Read or Write.
type ptySerial struct {
	masterFD atomic.Int64
	slaveFD  atomic.Int64
	path     string
}

///////////////////////////////////////////////////////////////////////////////
// Constructor
///////////////////////////////////////////////////////////////////////////////

// newVirtualSerial creates a new Linux PTY pair.
func newVirtualSerial() (VirtualSerial, error) {
	masterFD, err := unix.Open(
		"/dev/ptmx",
		unix.O_RDWR|unix.O_NOCTTY,
		0,
	)
	if err != nil {
		return nil, fmt.Errorf("open PTY master: %w", err)
	}

	// Wenn die weitere Initialisierung fehlschlägt,
	// darf der Master nicht offen bleiben.
	closeMaster := true
	defer func() {
		if closeMaster {
			_ = unix.Close(masterFD)
		}
	}()

	// PTY-Slave entsperren.
	if err := unix.IoctlSetPointerInt(
		masterFD,
		unix.TIOCSPTLCK,
		0,
	); err != nil {
		return nil, fmt.Errorf("unlock PTY slave: %w", err)
	}

	// PTY-Nummer ermitteln.
	ptsNumber, err := unix.IoctlGetInt(
		masterFD,
		unix.TIOCGPTN,
	)
	if err != nil {
		return nil, fmt.Errorf("get PTY number: %w", err)
	}

	path := fmt.Sprintf("/dev/pts/%d", ptsNumber)

	// Slave öffnen, um die Terminalparameter konfigurieren zu können.
	slaveFD, err := unix.Open(
		path,
		unix.O_RDWR|unix.O_NOCTTY,
		0,
	)
	if err != nil {
		return nil, fmt.Errorf(
			"open PTY slave %s: %w",
			path,
			err,
		)
	}

	closeSlave := true
	defer func() {
		if closeSlave {
			_ = unix.Close(slaveFD)
		}
	}()

	if err := configurePTY(slaveFD); err != nil {
		return nil, err
	}

	serial := &ptySerial{
		path: path,
	}

	serial.masterFD.Store(int64(masterFD))
	serial.slaveFD.Store(int64(slaveFD))

	closeMaster = false
	closeSlave = false

	return serial, nil
}

///////////////////////////////////////////////////////////////////////////////
// PTY configuration
///////////////////////////////////////////////////////////////////////////////

func configurePTY(fd int) error {
	termios, err := unix.IoctlGetTermios(
		fd,
		unix.TCGETS,
	)
	if err != nil {
		return fmt.Errorf("get PTY termios: %w", err)
	}

	// Raw Mode:
	// keine Canonical-Verarbeitung,
	// kein Echo,
	// keine Signalinterpretation,
	// keine CR/LF-Konvertierung,
	// keine XON/XOFF-Verarbeitung.
	termios.Iflag &^= unix.IGNBRK |
		unix.BRKINT |
		unix.PARMRK |
		unix.ISTRIP |
		unix.INLCR |
		unix.IGNCR |
		unix.ICRNL |
		unix.IXON

	termios.Oflag &^= unix.OPOST

	termios.Lflag &^= unix.ECHO |
		unix.ECHONL |
		unix.ICANON |
		unix.ISIG |
		unix.IEXTEN

	termios.Cflag &^= unix.CSIZE | unix.PARENB
	termios.Cflag |= unix.CS8

	// Byte-orientiertes, blockierendes Lesen.
	termios.Cc[unix.VMIN] = 1
	termios.Cc[unix.VTIME] = 0

	if err := unix.IoctlSetTermios(
		fd,
		unix.TCSETS,
		termios,
	); err != nil {
		return fmt.Errorf("set PTY raw mode: %w", err)
	}

	return nil
}

///////////////////////////////////////////////////////////////////////////////
// VirtualSerial implementation
///////////////////////////////////////////////////////////////////////////////

// Path returns the PTY slave path.
func (p *ptySerial) Path() string {
	return p.path
}

// Read reads raw bytes from the PTY master.
func (p *ptySerial) Read(b []byte) (int, error) {
	fd := int(p.masterFD.Load())

	if fd < 0 {
		return 0, fmt.Errorf("PTY master is closed")
	}

	return unix.Read(fd, b)
}

// Write writes raw bytes to the PTY master.
func (p *ptySerial) Write(b []byte) (int, error) {
	fd := int(p.masterFD.Load())

	if fd < 0 {
		return 0, fmt.Errorf("PTY master is closed")
	}

	return unix.Write(fd, b)
}

// Close closes the PTY master and slave.
//
// The descriptors are invalidated atomically before closing them so that
// concurrent Read or Write calls do not access the descriptor fields
// concurrently with Close.
func (p *ptySerial) Close() error {
	var firstErr error

	masterFD := int(p.masterFD.Swap(-1))
	if masterFD >= 0 {
		if err := unix.Close(masterFD); err != nil {
			firstErr = err
		}
	}

	slaveFD := int(p.slaveFD.Swap(-1))
	if slaveFD >= 0 {
		if err := unix.Close(slaveFD); err != nil && firstErr == nil {
			firstErr = err
		}
	}

	return firstErr
}