/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 2022 Jessica Clarke <jrtc27@FreeBSD.org>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/param.h>
#include <sys/errno.h>
#include <sys/procctl.h>
#include <sys/queue.h>
#include <sys/resource.h>
#include <sys/sysctl.h>
#include <sys/wait.h>

#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <stdarg.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sysexits.h>
#include <termios.h>
#include <ttyent.h>
#include <unistd.h>

#include "common.h"
#include "child.h"

/* -1: not started, 0: reaped */
static volatile pid_t grandchild_pid = -1;
static volatile int grandchild_status;

static struct pipe_barrier wait_grandchild_barrier;
static struct pipe_barrier wait_all_descendants_barrier;

static void
kill_descendants(int sig)
{
        struct procctl_reaper_kill rk;

        rk.rk_sig = sig;
        rk.rk_flags = 0;
        procctl(P_PID, getpid(), PROC_REAP_KILL, &rk);
}

static void
sigalrm_handler(int sig __unused)
{
        int saved_errno;

        saved_errno = errno;
        kill_descendants(SIGKILL);
        errno = saved_errno;
}

static void
wait_all_descendants(void)
{
        sigset_t set, oset;

        err_set_exit(NULL);

        /*
         * We may be run in a context where SIGALRM is blocked; temporarily
         * unblock so we can SIGKILL. Similarly, SIGCHLD may be blocked, but if
         * we're waiting on the pipe we need to make sure it's not.
         */
        sigemptyset(&set);
        sigaddset(&set, SIGALRM);
        sigaddset(&set, SIGCHLD);
        sigprocmask(SIG_UNBLOCK, &set, &oset);
        alarm(KILL_TIMEOUT);
        pipe_barrier_wait(&wait_all_descendants_barrier);
        alarm(0);
        sigprocmask(SIG_SETMASK, &oset, NULL);
}

static void
sigchld_handler(int sig __unused)
{
        int status, saved_errno;
        pid_t pid;

        saved_errno = errno;

        while ((void)(pid = waitpid(-1, &status, WNOHANG)),
            pid != -1 && pid != 0) {
                /* NB: No need to check grandchild_pid due to the pid checks */
                if (pid == grandchild_pid) {
                        grandchild_status = status;
                        grandchild_pid = 0;
                        pipe_barrier_ready(&wait_grandchild_barrier);
                }
        }

        /*
         * Another process calling kill(..., SIGCHLD) could cause us to get
         * here before we've spawned the grandchild; only ready when we have no
         * children if the grandchild has been reaped.
         */
        if (pid == -1 && errno == ECHILD && grandchild_pid == 0)
                pipe_barrier_ready(&wait_all_descendants_barrier);

        errno = saved_errno;
}

static void
exit_signal_handler(int sig)
{
        int saved_errno;

        /*
         * If we get killed before we've started the grandchild then just exit
         * with that signal, otherwise kill all our descendants with that
         * signal and let the main program pick up the grandchild's death.
         */
        if (grandchild_pid == -1) {
                reproduce_signal_death(sig);
                exit(EXIT_FAILURE);
        }

        saved_errno = errno;
        kill_descendants(sig);
        errno = saved_errno;
}

static void
kill_wait_all_descendants(int sig)
{
        kill_descendants(sig);
        wait_all_descendants();
}

static void
kill_wait_all_descendants_err_exit(int eval __unused)
{
        kill_wait_all_descendants(SIGTERM);
}

static void __dead2
grandchild_run(const char **argv, const sigset_t *oset)
{
        sig_t orig;

        /* Restore signals */
        orig = signal(SIGALRM, SIG_DFL);
        if (orig == SIG_ERR)
                err(EX_OSERR, "could not restore SIGALRM");
        orig = signal(SIGCHLD, SIG_DFL);
        if (orig == SIG_ERR)
                err(EX_OSERR, "could not restore SIGCHLD");
        orig = signal(SIGTERM, SIG_DFL);
        if (orig == SIG_ERR)
                err(EX_OSERR, "could not restore SIGTERM");
        orig = signal(SIGINT, SIG_DFL);
        if (orig == SIG_ERR)
                err(EX_OSERR, "could not restore SIGINT");
        orig = signal(SIGQUIT, SIG_DFL);
        if (orig == SIG_ERR)
                err(EX_OSERR, "could not restore SIGQUIT");
        orig = signal(SIGPIPE, SIG_DFL);
        if (orig == SIG_ERR)
                err(EX_OSERR, "could not restore SIGPIPE");
        orig = signal(SIGTTOU, SIG_DFL);
        if (orig == SIG_ERR)
                err(EX_OSERR, "could not restore SIGTTOU");

        /* Now safe to unmask signals */
        sigprocmask(SIG_SETMASK, oset, NULL);

        /* Only run with stdin/stdout/stderr */
        closefrom(3);

        /* Ready to execute the requested program */
        execvp(argv[0], __DECONST(char * const *, argv));
        err(EX_OSERR, "cannot execvp %s", argv[0]);
}

static int
wait_grandchild_descendants(void)
{
        pipe_barrier_wait(&wait_grandchild_barrier);

        /*
         * Once the grandchild itself has exited, kill any lingering
         * descendants and wait until we've reaped them all.
         */
        kill_wait_all_descendants(SIGTERM);

        if (grandchild_pid != 0)
                errx(EX_SOFTWARE, "failed to reap grandchild");

        return (grandchild_status);
}

void
child_leader_run(const char *name, int fd, bool new_session, const char **argv,
    const sigset_t *oset, struct pipe_barrier *start_children_barrier)
{
        struct pipe_barrier start_grandchild_barrier;
        pid_t pid, sid, pgid;
        struct sigaction sa;
        int error, status;
        sigset_t set;

        setproctitle("%s [%s]", getprogname(), name);

        error = procctl(P_PID, getpid(), PROC_REAP_ACQUIRE, NULL);
        if (error != 0)
                err(EX_OSERR, "could not acquire reaper status");

        /*
         * Set up our own signal handlers for everything the parent overrides
         * other than SIGPIPE and SIGTTOU which we leave as ignored, since we
         * also use pipe-based synchronisation and want to be able to print
         * errors.
         */
        sa.sa_flags = SA_RESTART;
        sa.sa_handler = sigchld_handler;
        sigfillset(&sa.sa_mask);
        error = sigaction(SIGCHLD, &sa, NULL);
        if (error != 0)
                err(EX_OSERR, "could not enable SIGCHLD handler");
        sa.sa_handler = sigalrm_handler;
        error = sigaction(SIGALRM, &sa, NULL);
        if (error != 0)
                err(EX_OSERR, "could not enable SIGALRM handler");
        sa.sa_handler = exit_signal_handler;
        error = sigaction(SIGTERM, &sa, NULL);
        if (error != 0)
                err(EX_OSERR, "could not enable SIGTERM handler");
        error = sigaction(SIGINT, &sa, NULL);
        if (error != 0)
                err(EX_OSERR, "could not enable SIGINT handler");
        error = sigaction(SIGQUIT, &sa, NULL);
        if (error != 0)
                err(EX_OSERR, "could not enable SIGQUIT handler");

        /*
         * Now safe to unmask signals. Note that creating the barriers used by
         * the SIGCHLD handler with signals unmasked is safe since they won't
         * be used if the grandchild hasn't been forked (and reaped), which
         * comes later.
         */
        sigprocmask(SIG_SETMASK, oset, NULL);

        error = pipe_barrier_init(&start_grandchild_barrier);
        if (error != 0)
                err(EX_OSERR, "could not create start grandchild barrier");

        error = pipe_barrier_init(&wait_grandchild_barrier);
        if (error != 0)
                err(EX_OSERR, "could not create wait grandchild barrier");

        error = pipe_barrier_init(&wait_all_descendants_barrier);
        if (error != 0)
                err(EX_OSERR, "could not create wait all descendants barrier");

        /*
         * Create a new session if this is on a different terminal to
         * the current one, otherwise just create a new process group to keep
         * things as similar as possible between the two cases.
         */
        if (new_session) {
                sid = setsid();
                pgid = sid;
                if (sid == -1)
                        err(EX_OSERR, "could not create session");
        } else {
                sid = -1;
                pgid = getpid();
                error = setpgid(0, pgid);
                if (error == -1)
                        err(EX_OSERR, "could not create process group");
        }

        /* Wait until parent is ready for us to start */
        pipe_barrier_destroy_ready(start_children_barrier);
        pipe_barrier_wait(start_children_barrier);

        /*
         * Use the console for stdin/stdout/stderr.
         *
         * NB: dup2(2) is a no-op if the two fds are equal, and the call to
         * closefrom(2) later in the grandchild will close the fd if it isn't
         * one of stdin/stdout/stderr already. This means we do not need to
         * handle that special case differently.
         */
        error = dup2(fd, STDIN_FILENO);
        if (error == -1)
                err(EX_IOERR, "could not dup %s to stdin", name);
        error = dup2(fd, STDOUT_FILENO);
        if (error == -1)
                err(EX_IOERR, "could not dup %s to stdout", name);
        error = dup2(fd, STDERR_FILENO);
        if (error == -1)
                err(EX_IOERR, "could not dup %s to stderr", name);

        /*
         * If we created a new session, make the console our controlling
         * terminal. Either way, also make this the foreground process group.
         */
        if (new_session) {
                error = tcsetsid(STDIN_FILENO, sid);
                if (error != 0)
                        err(EX_IOERR, "could not set session for %s", name);
        } else {
                error = tcsetpgrp(STDIN_FILENO, pgid);
                if (error != 0)
                        err(EX_IOERR, "could not set process group for %s",
                            name);
        }

        /*
         * Temporarily block signals again; forking, setting grandchild_pid and
         * calling err_set_exit need to all be atomic for similar reasons as
         * the parent when forking us.
         */
        sigfillset(&set);
        sigprocmask(SIG_BLOCK, &set, NULL);
        pid = fork();
        if (pid == -1)
                err(EX_OSERR, "could not fork");

        if (pid == 0) {
                /*
                 * We need to destroy the ready ends so we don't block these
                 * child leader-only self-pipes, and might as well destroy the
                 * wait ends too given we're not going to use them.
                 */
                pipe_barrier_destroy(&wait_grandchild_barrier);
                pipe_barrier_destroy(&wait_all_descendants_barrier);

                /* Wait until the parent has put us in a new process group */
                pipe_barrier_destroy_ready(&start_grandchild_barrier);
                pipe_barrier_wait(&start_grandchild_barrier);
                grandchild_run(argv, oset);
        }

        grandchild_pid = pid;

        /*
         * Now the grandchild exists make sure to clean it up, and any of its
         * descendants, on exit.
         */
        err_set_exit(kill_wait_all_descendants_err_exit);

        sigprocmask(SIG_SETMASK, oset, NULL);

        /* Start the grandchild and wait for it and its descendants to exit */
        pipe_barrier_ready(&start_grandchild_barrier);

        status = wait_grandchild_descendants();

        if (WIFSIGNALED(status))
                reproduce_signal_death(WTERMSIG(status));

        if (WIFEXITED(status))
                exit(WEXITSTATUS(status));

        exit(EXIT_FAILURE);
}