Ticket #4630: readdir-wait.c

// This is free and unencumbered software released into the public domain.
#define _GNU_SOURCE    // make dlfcn.h define RTLD_NEXT
#include <dirent.h>
#include <dlfcn.h>
#include <errno.h>
#include <signal.h>
#include <stdio.h>     // perror()
#include <stdlib.h>    // abort()
#include <unistd.h>    // read()


static void
die_perror(const char *str)
{
        perror(str);
        exit(1);
}

static void
mc_test_wait(void)
{
        sigset_t orig_mask, sigwinch;
        static int done = 0;
        const int test_fd = 3;

        if (done) return;
        done = 1;

        sigemptyset(&sigwinch);
        sigaddset(&sigwinch, SIGWINCH);
        errno = pthread_sigmask(SIG_BLOCK, &sigwinch, &orig_mask);
        if (errno) {
                die_perror("pthread_sigmask");
        }

        // test the parent process we're ready for our SIGWINCH
        do {} while (write(test_fd, "", 1) < 0 && errno == EINTR);

        // wait for SIGWINCH
        sigsuspend(&orig_mask);
        do {} while (close(test_fd) < 0 && errno == EINTR);

        errno = pthread_sigmask(SIG_SETMASK, &orig_mask, NULL);
        if (errno) {
                die_perror("pthread_sigmask");
        }
}

struct dirent *
readdir(DIR *dirp)
{
        struct dirent *ret = NULL;
        struct dirent *(*real_fn)(DIR*);

        mc_test_wait();

        real_fn = dlsym(RTLD_NEXT, __func__);
        if (!real_fn) {
                errno = ELIBACC;
                goto out;
        }
        ret = real_fn(dirp);
out:
        return ret;
}