ncurses/tty/lib_tstp.c

/****************************************************************************
 * Copyright 2020-2021,2022 Thomas E. Dickey                                *
 * Copyright 1998-2014,2017 Free Software Foundation, Inc.                  *
 *                                                                          *
 * Permission is hereby granted, free of charge, to any person obtaining a  *
 * copy of this software and associated documentation files (the            *
 * "Software"), to deal in the Software without restriction, including      *
 * without limitation the rights to use, copy, modify, merge, publish,      *
 * distribute, distribute with modifications, sublicense, and/or sell       *
 * copies of the Software, and to permit persons to whom the Software is    *
 * furnished to do so, subject to the following conditions:                 *
 *                                                                          *
 * The above copyright notice and this permission notice shall be included  *
 * in all copies or substantial portions of the Software.                   *
 *                                                                          *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS  *
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF               *
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.   *
 * IN NO EVENT SHALL THE ABOVE COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,   *
 * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR    *
 * OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR    *
 * THE USE OR OTHER DEALINGS IN THE SOFTWARE.                               *
 *                                                                          *
 * Except as contained in this notice, the name(s) of the above copyright   *
 * holders shall not be used in advertising or otherwise to promote the     *
 * sale, use or other dealings in this Software without prior written       *
 * authorization.                                                           *
 ****************************************************************************/

/****************************************************************************
 *  Author: Zeyd M. Ben-Halim <zmbenhal@netcom.com> 1992,1995               *
 *     and: Eric S. Raymond <esr@snark.thyrsus.com>                         *
 *     and: Thomas E. Dickey                        1995-on                 *
 ****************************************************************************/

/*
**	lib_tstp.c
**
**	The routine _nc_signal_handler().
**
*/
#include <curses.priv.h>

#include <SigAction.h>

MODULE_ID("$Id: lib_tstp.c,v 1.54 2022/12/24 22:22:10 tom Exp $")

#if defined(SIGTSTP) && (HAVE_SIGACTION || HAVE_SIGVEC)
#define USE_SIGTSTP 1
#else
#define USE_SIGTSTP 0
#endif

#ifdef TRACE
static const char *
signal_name(int sig)
{
    switch (sig) {
#ifdef SIGALRM
    case SIGALRM:
	return "SIGALRM";
#endif
#ifdef SIGCONT
    case SIGCONT:
	return "SIGCONT";
#endif
    case SIGINT:
	return "SIGINT";
#ifdef SIGQUIT
    case SIGQUIT:
	return "SIGQUIT";
#endif
    case SIGTERM:
	return "SIGTERM";
#ifdef SIGTSTP
    case SIGTSTP:
	return "SIGTSTP";
#endif
#ifdef SIGTTOU
    case SIGTTOU:
	return "SIGTTOU";
#endif
#ifdef SIGWINCH
    case SIGWINCH:
	return "SIGWINCH";
#endif
    default:
	return "unknown signal";
    }
}
#endif

/*
 * Note: This code is fragile!  Its problem is that different OSs
 * handle restart of system calls interrupted by signals differently.
 * The ncurses code needs signal-call restart to happen -- otherwise,
 * interrupted wgetch() calls will return FAIL, probably making the
 * application think the input stream has ended and it should
 * terminate.  In particular, you know you have this problem if, when
 * you suspend an ncurses-using lynx with ^Z and resume, it dies
 * immediately.
 *
 * Default behavior of POSIX sigaction(2) is not to restart
 * interrupted system calls, but Linux's sigaction does it anyway (at
 * least, on and after the 1.1.47 I (esr) use).  Thus this code works
 * OK under Linux.  The 4.4BSD sigaction(2) supports a (non-portable)
 * SA_RESTART flag that forces the right behavior.  Thus, this code
 * should work OK under BSD/OS, NetBSD, and FreeBSD (let us know if it
 * does not).
 *
 * Stock System Vs (and anything else using a strict-POSIX
 * sigaction(2) without SA_RESTART) may have a problem.  Possible
 * solutions:
 *
 *    sigvec      restarts by default (SV_INTERRUPT flag to not restart)
 *    signal      restarts by default in SVr4 (assuming you link with -lucb)
 *                and BSD, but not SVr3.
 *    sigset      restarts, but is only available under SVr4/Solaris.
 *
 * The signal(3) call is mandated by the ANSI standard, and its
 * interaction with sigaction(2) is described in the POSIX standard
 * (3.3.4.2, page 72,line 934).  According to section 8.1, page 191,
 * however, signal(3) itself is not required by POSIX.1.  And POSIX is
 * silent on whether it is required to restart signals.
 *
 * So.  The present situation is, we use sigaction(2) with no
 * guarantee of restart anywhere but on Linux and BSD.  We could
 * switch to signal(3) and collar Linux, BSD, and SVr4.  Any way
 * we slice it, System V UNIXes older than SVr4 will probably lose
 * (this may include XENIX).
 *
 * This implementation will probably be changed to use signal(3) in
 * the future.  If nothing else, it is simpler...
 */

#if USE_SIGTSTP
static void
handle_SIGTSTP(int dummy GCC_UNUSED)
{
    SCREEN *sp = CURRENT_SCREEN;
    sigset_t mask, omask;
    sigaction_t act, oact;

#ifdef SIGTTOU
    int sigttou_blocked;
#endif

    _nc_globals.have_sigtstp = 1;
    T(("handle_SIGTSTP() called"));

    /*
     * The user may have changed the prog_mode tty bits, so save them.
     *
     * But first try to detect whether we still are in the foreground
     * process group - if not, an interactive shell may already have
     * taken ownership of the tty and modified the settings when our
     * parent was stopped before us, and we would likely pick up the
     * settings already modified by the shell.
     *
     * Don't do this if we're not in curses -
     */
    if (sp != 0 && (sp->_endwin == ewRunning))
#if HAVE_TCGETPGRP
	if (tcgetpgrp(STDIN_FILENO) == getpgrp())
#endif
	    NCURSES_SP_NAME(def_prog_mode) (NCURSES_SP_ARG);

    /*
     * Block window change and timer signals.  The latter
     * is because applications use timers to decide when
     * to repaint the screen.
     */
    (void) sigemptyset(&mask);
#ifdef SIGALRM
    (void) sigaddset(&mask, SIGALRM);
#endif
#if USE_SIGWINCH
    (void) sigaddset(&mask, SIGWINCH);
#endif
    (void) sigprocmask(SIG_BLOCK, &mask, &omask);

#ifdef SIGTTOU
    sigttou_blocked = sigismember(&omask, SIGTTOU);
    if (!sigttou_blocked) {
	(void) sigemptyset(&mask);
	(void) sigaddset(&mask, SIGTTOU);
	(void) sigprocmask(SIG_BLOCK, &mask, NULL);
    }
#endif

    /*
     * End window mode, which also resets the terminal state to the
     * original (pre-curses) modes.
     */
    NCURSES_SP_NAME(endwin) (NCURSES_SP_ARG);

    /* Unblock SIGTSTP. */
    (void) sigemptyset(&mask);
    (void) sigaddset(&mask, SIGTSTP);
#ifdef SIGTTOU
    if (!sigttou_blocked) {
	/* Unblock this too if it wasn't blocked on entry */
	(void) sigaddset(&mask, SIGTTOU);
    }
#endif
    (void) sigprocmask(SIG_UNBLOCK, &mask, NULL);

    /* Now we want to resend SIGSTP to this process and suspend it */
    act.sa_handler = SIG_DFL;
    sigemptyset(&act.sa_mask);
    act.sa_flags = 0;
#ifdef SA_RESTART
    act.sa_flags |= SA_RESTART;
#endif /* SA_RESTART */
    sigaction(SIGTSTP, &act, &oact);
    kill(getpid(), SIGTSTP);

    /* Process gets suspended...time passes...process resumes */

    T(("SIGCONT received"));
    sigaction(SIGTSTP, &oact, NULL);
    NCURSES_SP_NAME(flushinp) (NCURSES_SP_ARG);

    /*
     * If the user modified the tty state while suspended, he wants
     * those changes to stick.  So save the new "default" terminal state.
     */
    NCURSES_SP_NAME(def_shell_mode) (NCURSES_SP_ARG);

    /*
     * This relies on the fact that doupdate() will restore the
     * program-mode tty state, and issue enter_ca_mode if need be.
     */
    NCURSES_SP_NAME(doupdate) (NCURSES_SP_ARG);

    /* Reset the signals. */
    (void) sigprocmask(SIG_SETMASK, &omask, NULL);
}
#endif /* USE_SIGTSTP */

static void
handle_SIGINT(int sig)
{
    SCREEN *sp = CURRENT_SCREEN;

    /*
     * Much of this is unsafe from a signal handler.  But we'll _try_ to clean
     * up the screen and terminal settings on the way out.
     *
     * There are at least the following problems:
     * 1) Walking the SCREEN list is unsafe, since all list management
     *    is done without any signal blocking.
     * 2) On systems which have REENTRANT turned on, set_term() uses
     *    _nc_lock_global() which could deadlock or misbehave in other ways.
     * 3) endwin() calls all sorts of stuff, many of which use stdio or
     *    other library functions which are clearly unsafe.
     */
    if (!_nc_globals.cleanup_nested++
	&& (sig == SIGINT || sig == SIGTERM)) {
#if HAVE_SIGACTION || HAVE_SIGVEC
	sigaction_t act;
	sigemptyset(&act.sa_mask);
	act.sa_flags = 0;
	act.sa_handler = SIG_IGN;
	if (sigaction(sig, &act, NULL) == 0)
#else
	if (signal(sig, SIG_IGN) != SIG_ERR)
#endif
	{
	    SCREEN *scan;
	    for (each_screen(scan)) {
		if (scan->_ofp != 0
		    && NC_ISATTY(fileno(scan->_ofp))) {
		    scan->_outch = NCURSES_SP_NAME(_nc_outch);
		}
		set_term(scan);
		NCURSES_SP_NAME(endwin) (NCURSES_SP_ARG);
		if (sp)
		    sp->_endwin = ewInitial;	/* in case of reuse */
	    }
	}
    }
    _exit(EXIT_FAILURE);
}

# ifndef _nc_set_read_thread
NCURSES_EXPORT(void)
_nc_set_read_thread(bool enable)
{
    _nc_lock_global(curses);
    if (enable) {
#  if USE_WEAK_SYMBOLS
	if ((pthread_self) && (pthread_kill) && (pthread_equal))
#  endif
	    _nc_globals.read_thread = pthread_self();
    } else {
	_nc_globals.read_thread = 0;
    }
    _nc_unlock_global(curses);
}
# endif

#if USE_SIGWINCH

static void
handle_SIGWINCH(int sig GCC_UNUSED)
{
    _nc_globals.have_sigwinch = 1;
# if USE_PTHREADS_EINTR
    if (_nc_globals.read_thread) {
	if (!pthread_equal(pthread_self(), _nc_globals.read_thread))
	    pthread_kill(_nc_globals.read_thread, SIGWINCH);
	_nc_globals.read_thread = 0;
    }
# endif
}
#endif /* USE_SIGWINCH */

/*
 * If the given signal is still in its default state, set it to the given
 * handler.
 */
static int
CatchIfDefault(int sig, void (*handler) (int))
{
    int result;
#if HAVE_SIGACTION || HAVE_SIGVEC
    sigaction_t old_act;
    sigaction_t new_act;

    memset(&new_act, 0, sizeof(new_act));
    sigemptyset(&new_act.sa_mask);
#ifdef SA_RESTART
#ifdef SIGWINCH
    if (sig != SIGWINCH)
#endif
	new_act.sa_flags |= SA_RESTART;
#endif /* SA_RESTART */
    new_act.sa_handler = handler;

    if (sigaction(sig, NULL, &old_act) == 0
	&& (old_act.sa_handler == SIG_DFL
	    || old_act.sa_handler == handler
#if USE_SIGWINCH
	    || (sig == SIGWINCH && old_act.sa_handler == SIG_IGN)
#endif
	)) {
	(void) sigaction(sig, &new_act, NULL);
	result = TRUE;
    } else {
	result = FALSE;
    }
#else /* !HAVE_SIGACTION */
    void (*ohandler) (int);

    ohandler = signal(sig, SIG_IGN);
    if (ohandler == SIG_DFL
	|| ohandler == handler
#if USE_SIGWINCH
	|| (sig == SIGWINCH && ohandler == SIG_IGN)
#endif
	) {
	signal(sig, handler);
	result = TRUE;
    } else {
	signal(sig, ohandler);
	result = FALSE;
    }
#endif
    T(("CatchIfDefault - will %scatch %s",
       result ? "" : "not ", signal_name(sig)));
    return result;
}

/*
 * This is invoked once at the beginning (e.g., from 'initscr()'), to
 * initialize the signal catchers, and thereafter when spawning a shell (and
 * returning) to disable/enable the SIGTSTP (i.e., ^Z) catcher.
 *
 * If the application has already set one of the signals, we'll not modify it
 * (during initialization).
 *
 * The XSI document implies that we shouldn't keep the SIGTSTP handler if
 * the caller later changes its mind, but that doesn't seem correct.
 */
NCURSES_EXPORT(void)
_nc_signal_handler(int enable)
{
    T((T_CALLED("_nc_signal_handler(%d)"), enable));
#if USE_SIGTSTP			/* Xenix 2.x doesn't have SIGTSTP, for example */
    {
	static bool ignore_tstp = FALSE;

	if (!ignore_tstp) {
	    static sigaction_t new_sigaction, old_sigaction;

	    if (!enable) {
		new_sigaction.sa_handler = SIG_IGN;
		sigaction(SIGTSTP, &new_sigaction, &old_sigaction);
	    } else if (new_sigaction.sa_handler != SIG_DFL) {
		sigaction(SIGTSTP, &old_sigaction, NULL);
	    } else if (sigaction(SIGTSTP, NULL, &old_sigaction) == 0
		       && (old_sigaction.sa_handler == SIG_DFL)) {
		sigemptyset(&new_sigaction.sa_mask);
#ifdef SA_RESTART
		new_sigaction.sa_flags |= SA_RESTART;
#endif /* SA_RESTART */
		new_sigaction.sa_handler = handle_SIGTSTP;
		(void) sigaction(SIGTSTP, &new_sigaction, NULL);
	    } else {
		ignore_tstp = TRUE;
	    }
	}
    }
#endif /* !USE_SIGTSTP */

    if (!_nc_globals.init_signals) {
	if (enable) {
	    CatchIfDefault(SIGINT, handle_SIGINT);
	    CatchIfDefault(SIGTERM, handle_SIGINT);
#if USE_SIGWINCH
	    CatchIfDefault(SIGWINCH, handle_SIGWINCH);
#endif
	    _nc_globals.init_signals = TRUE;
	}
    }
    returnVoid;
}