mpv/osdep/subprocess-posix.c

/*
 * This file is part of mpv.
 *
 * mpv is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * mpv is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with mpv.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <poll.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <errno.h>
#include <signal.h>

#include "osdep/subprocess.h"

#include "common/common.h"
#include "misc/thread_tools.h"
#include "osdep/io.h"
#include "stream/stream.h"

extern char **environ;

#define SAFE_CLOSE(fd) do { if ((fd) >= 0) close((fd)); (fd) = -1; } while (0)

// Async-signal-safe execvpe(). POSIX does not list it as async-signal-safe
// (POSIX is such a joke), so do it manually. While in theory the searching is
// apparently implementation dependent and not exposed (because POSIX is a
// joke?), the  expected rules are still relatively simple.
// Doesn't set errno correctly.
// Somewhat inspired by musl's src/process/execvp.c.
static int as_execvpe(const char *path, const char *file, char *const argv[],
                      char *const envp[])
{
    if (strchr(file, '/') || !file[0])
        return execve(file, argv, envp);

    size_t flen = strlen(file);
    while (path && path[0]) {
        size_t plen = strcspn(path, ":");
        // Ignore paths that are too long.
        char fn[PATH_MAX];
        if (plen + 1 + flen + 1 < sizeof(fn)) {
            memcpy(fn, path, plen);
            fn[plen] = '/';
            memcpy(fn + plen + 1, file, flen + 1);
            execve(fn, argv, envp);
            if (errno != EACCES && errno != ENOENT && errno != ENOTDIR)
                break;
        }
        path += plen + (path[plen] == ':' ? 1 : 0);
    }
    return -1;
}

// Returns 0 on any error, valid PID on success.
// This function must be async-signal-safe, as it may be called from a fork().
static pid_t spawn_process(const char *path, struct mp_subprocess_opts *opts,
                           int src_fds[])
{
    int p[2] = {-1, -1};
    pid_t fres = 0;
    sigset_t sigmask, oldmask;
    sigfillset(&sigmask);
    pthread_sigmask(SIG_BLOCK, &sigmask, &oldmask);

    // We setup a communication pipe to signal failure. Since the child calls
    // exec() and becomes the calling process, we don't know if or when the
    // child process successfully ran exec() just from the PID.
    // Use a CLOEXEC pipe to detect whether exec() was used. Obviously it will
    // be closed if exec() succeeds, and an error is written if not.
    // There are also some things further below in the code that need CLOEXEC.
    if (mp_make_cloexec_pipe(p) < 0)
        goto done;
    // Check whether CLOEXEC is really set. Important for correct operation.
    int p_flags = fcntl(p[0], F_GETFD);
    if (p_flags == -1 || !FD_CLOEXEC || !(p_flags & FD_CLOEXEC))
        goto done; // require CLOEXEC; unknown if fallback would be worth it

    fres = fork();
    if (fres < 0) {
        fres = 0;
        goto done;
    }
    if (fres == 0) {
        // child

        for (int n = 0; n < opts->num_fds; n++) {
            if (src_fds[n] == opts->fds[n].fd) {
                int flags = fcntl(opts->fds[n].fd, F_GETFD);
                if (flags == -1)
                    goto child_failed;
                flags &= ~(unsigned)FD_CLOEXEC;
                if (fcntl(opts->fds[n].fd, F_SETFD, flags) == -1)
                    goto child_failed;
            } else if (dup2(src_fds[n], opts->fds[n].fd) < 0) {
                goto child_failed;
            }
        }

        as_execvpe(path, opts->exe, opts->args, opts->env ? opts->env : environ);

    child_failed:
        write(p[1], &(char){1}, 1); // shouldn't be able to fail
        _exit(1);
    }

    SAFE_CLOSE(p[1]);

    int r;
    do {
        r = read(p[0], &(char){0}, 1);
    } while (r < 0 && errno == EINTR);

    // If exec()ing child failed, collect it immediately.
    if (r != 0) {
        while (waitpid(fres, &(int){0}, 0) < 0 && errno == EINTR) {}
        fres = 0;
    }

done:
    pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
    SAFE_CLOSE(p[0]);
    SAFE_CLOSE(p[1]);

    return fres;
}

void mp_subprocess2(struct mp_subprocess_opts *opts,
                    struct mp_subprocess_result *res)
{
    int status = -1;
    int comm_pipe[MP_SUBPROCESS_MAX_FDS][2];
    int src_fds[MP_SUBPROCESS_MAX_FDS];
    int devnull = -1;
    pid_t pid = 0;
    bool spawned = false;
    bool killed_by_us = false;
    int cancel_fd = -1;
    char *path = getenv("PATH");
    char path_storage[PATH_MAX];
    if (!path) {
        path = path_storage;
        size_t r = confstr(_CS_PATH, path, sizeof(path_storage));
        if (r == 0 || r >= sizeof(path_storage))
            path[0] = '\0'; // failure, who cares
    }

    *res = (struct mp_subprocess_result){0};

    for (int n = 0; n < opts->num_fds; n++)
        comm_pipe[n][0] = comm_pipe[n][1] = -1;

    if (opts->cancel) {
        cancel_fd = mp_cancel_get_fd(opts->cancel);
        if (cancel_fd < 0)
            goto done;
    }

    for (int n = 0; n < opts->num_fds; n++) {
        if (opts->fds[n].on_read && mp_make_cloexec_pipe(comm_pipe[n]) < 0)
            goto done;
    }

    devnull = open("/dev/null", O_RDONLY | O_CLOEXEC);
    if (devnull < 0)
        goto done;

    // redirect FDs
    for (int n = 0; n < opts->num_fds; n++) {
        int src_fd = devnull;
        if (comm_pipe[n][1] >= 0)
            src_fd = comm_pipe[n][1];
        if (opts->fds[n].src_fd >= 0)
            src_fd = opts->fds[n].src_fd;
        src_fds[n] = src_fd;
    }

    if (opts->detach) {
        // If we run it detached, we fork a child to start the process; then
        // it exits immediately, letting PID 1 inherit it. So we don't need
        // anything else to collect these child PIDs.
        sigset_t sigmask, oldmask;
        sigfillset(&sigmask);
        pthread_sigmask(SIG_BLOCK, &sigmask, &oldmask);
        pid_t fres = fork();
        if (fres < 0)
            goto done;
        if (fres == 0) {
            // child
            setsid();
            if (!spawn_process(path, opts, src_fds))
                _exit(1);
            _exit(0);
        }
        pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
        int child_status = 0;
        while (waitpid(fres, &child_status, 0) < 0 && errno == EINTR) {}
        if (!WIFEXITED(child_status) || WEXITSTATUS(child_status) != 0)
            goto done;
    } else {
        pid = spawn_process(path, opts, src_fds);
        if (!pid)
            goto done;
    }

    spawned = true;

    for (int n = 0; n < opts->num_fds; n++)
        SAFE_CLOSE(comm_pipe[n][1]);
    SAFE_CLOSE(devnull);

    while (1) {
        struct pollfd fds[MP_SUBPROCESS_MAX_FDS + 1];
        int map_fds[MP_SUBPROCESS_MAX_FDS + 1];
        int num_fds = 0;
        for (int n = 0; n < opts->num_fds; n++) {
            if (comm_pipe[n][0] >= 0) {
                map_fds[num_fds] = n;
                fds[num_fds++] = (struct pollfd){
                    .events = POLLIN,
                    .fd = comm_pipe[n][0],
                };
            }
        }
        if (!num_fds)
            break;
        if (cancel_fd >= 0) {
            map_fds[num_fds] = -1;
            fds[num_fds++] = (struct pollfd){.events = POLLIN, .fd = cancel_fd};
        }

        if (poll(fds, num_fds, -1) < 0 && errno != EINTR)
            break;

        for (int idx = 0; idx < num_fds; idx++) {
            if (fds[idx].revents) {
                int n = map_fds[idx];
                if (n < 0) {
                    // cancel_fd
                    if (pid)
                        kill(pid, SIGKILL);
                    killed_by_us = true;
                    break;
                } else {
                    char buf[4096];
                    ssize_t r = read(comm_pipe[n][0], buf, sizeof(buf));
                    if (r < 0 && errno == EINTR)
                        continue;
                    if (r > 0 && opts->fds[n].on_read)
                        opts->fds[n].on_read(opts->fds[n].on_read_ctx, buf, r);
                    if (r <= 0)
                        SAFE_CLOSE(comm_pipe[n][0]);
                }
            }
        }
    }

    // Note: it can happen that a child process closes the pipe, but does not
    //       terminate yet. In this case, we would have to run waitpid() in
    //       a separate thread and use pthread_cancel(), or use other weird
    //       and laborious tricks in order to react to mp_cancel.
    //       So this isn't handled yet.
    if (pid)
        while (waitpid(pid, &status, 0) < 0 && errno == EINTR) {}

done:
    for (int n = 0; n < opts->num_fds; n++) {
        SAFE_CLOSE(comm_pipe[n][0]);
        SAFE_CLOSE(comm_pipe[n][1]);
    }
    SAFE_CLOSE(devnull);

    if (!spawned || (pid && WIFEXITED(status) && WEXITSTATUS(status) == 127)) {
        res->error = MP_SUBPROCESS_EINIT;
    } else if (pid && WIFEXITED(status)) {
        res->exit_status = WEXITSTATUS(status);
    } else if (spawned && opts->detach) {
        // ok
    } else if (killed_by_us) {
        res->error = MP_SUBPROCESS_EKILLED_BY_US;
    } else {
        res->error = MP_SUBPROCESS_EGENERIC;
    }
}