tuxdock/tuxreaperdgnu.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <signal.h>
#include <sys/prctl.h>
#include <sys/wait.h>
#include <dirent.h>
#include <errno.h>

/* Apache hijacks SIGWINCH (window resize) for graceful shutdown. Yes,
   really. So when the outside world sends SIGTERM, we have to translate
   it to SIGWINCH for anything whose /proc/<pid>/exe smells like apache2.
   This is insane, but Apache is a good web server so I give it a pass. */
static const char * const apache_exes[] = {
    "/usr/sbin/apache2",
    "/usr/sbin/httpd",
    "/usr/local/apache2/bin/httpd",
    0
};
#define APACHE_IN_SIG  SIGTERM
#define APACHE_OUT_SIG SIGWINCH

static volatile pid_t g_main_child = 0;
static volatile sig_atomic_t g_child_exited = 0;
static volatile int g_main_status = 0;
static volatile sig_atomic_t g_pending_signal = 0;

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

    // Reap all terminated orphans
    while ((pid = waitpid(-1, &status, WNOHANG)) > 0) {
        if (pid == g_main_child) {
            g_child_exited = 1;
            g_main_status = status;
        }
    }
}

static void proxy_signal_handler(int sig) {
    // Just record the signal; the main loop does the /proc scan so we
    // stay async-signal-safe here.
    g_pending_signal = sig;
}

static void proc_remap_signal(int in_sig, int out_sig, const char *const *target_exes) {
    DIR *proc = opendir("/proc");
    if (!proc) return;

    struct dirent *de;

    while ((de = readdir(proc)) != NULL) {
        if (de->d_name[0] == '.') continue;

        char *endptr;
        long pid = strtol(de->d_name, &endptr, 10);
        if (*endptr != '\0' || pid <= 1) continue;

        char path[64];
        snprintf(path, sizeof(path), "/proc/%ld/exe", pid);

        char linkbuf[256];
        ssize_t linklen = readlink(path, linkbuf, sizeof(linkbuf) - 1);
        if (linklen <= 0) continue;
        linkbuf[linklen] = '\0';

        int matched = 0;
        for (int i = 0; target_exes[i]; i++) {
            size_t target_len = strlen(target_exes[i]);
            if ((size_t)linklen == target_len && strcmp(linkbuf, target_exes[i]) == 0) {
                matched = 1;
                break;
            }
        }

        if (matched) {
            kill(pid, out_sig);
        } else {
            kill(pid, in_sig);
        }
    }

    closedir(proc);
}

int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <command> [args...]\n", argv[0]);
        return 1;
    }

    // Become a subreaper for any reparented processes down the tree
    if (prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0) < 0) {
        perror("prctl(PR_SET_CHILD_SUBREAPER)");
        return 1;
    }

    // Set up signal forwarding
    struct sigaction sa_forward;
    sa_forward.sa_handler = proxy_signal_handler;
    sigemptyset(&sa_forward.sa_mask);
    sa_forward.sa_flags = SA_RESTART;
    sigaction(SIGTERM, &sa_forward, NULL);
    sigaction(SIGQUIT, &sa_forward, NULL);
    sigaction(SIGINT, &sa_forward, NULL);
    sigaction(SIGHUP, &sa_forward, NULL);
    sigaction(SIGUSR1, &sa_forward, NULL);
    sigaction(SIGUSR2, &sa_forward, NULL);

    // Set up SIGCHLD reaper handler
    struct sigaction sa_chld;
    sa_chld.sa_handler = sigchld_handler;
    sigemptyset(&sa_chld.sa_mask);
    sa_chld.sa_flags = SA_RESTART | SA_NOCLDSTOP;
    sigaction(SIGCHLD, &sa_chld, NULL);

    // Block the signals we wait on so the check/suspend loop is race-free
    sigset_t block_mask, old_mask;
    sigemptyset(&block_mask);
    sigaddset(&block_mask, SIGCHLD);
    sigaddset(&block_mask, SIGTERM);
    sigaddset(&block_mask, SIGQUIT);
    sigaddset(&block_mask, SIGINT);
    sigaddset(&block_mask, SIGHUP);
    sigaddset(&block_mask, SIGUSR1);
    sigaddset(&block_mask, SIGUSR2);
    sigprocmask(SIG_BLOCK, &block_mask, &old_mask);

    // Spawn the primary workload
    g_main_child = fork();
    if (g_main_child < 0) {
        perror("fork");
        return 1;
    }

    if (g_main_child == 0) {
        sigprocmask(SIG_SETMASK, &old_mask, NULL);
        execvp(argv[1], &argv[1]);
        perror("execvp");
        _exit(127);
    }

    // Block until the primary process exits
    while (!g_child_exited) {
        sigsuspend(&old_mask);

        if (g_pending_signal != 0) {
            int sig = g_pending_signal;
            g_pending_signal = 0;
            if (sig == APACHE_IN_SIG) {
                proc_remap_signal(APACHE_IN_SIG, APACHE_OUT_SIG, apache_exes);
            } else {
                // Unmapped signals get the old broadcast treatment.
                kill(-1, sig);
            }
        }
    }

    // Final sweep of any remaining lingering zombies
    while (waitpid(-1, NULL, WNOHANG) > 0);

    if (WIFEXITED(g_main_status)) {
        return WEXITSTATUS(g_main_status);
    } else if (WIFSIGNALED(g_main_status)) {
        return 128 + WTERMSIG(g_main_status);
    }

    return 0;
}