/***********************************************************************
 * This is a STC to show that flock occasionally does not work.
 *
 * It tries to use flock() for file locking. It creates a temporary
 * file, the uses fork to spawn a number of children. Each child opens
 * the file, then repeatedly uses flock to lock and unlock it.
 *
 * While each child has the lock, it increments a counter stored in
 * shared memory in a racy way, passing the current value to a function
 * which sleeps briefly, then returns the incremented counter.
 *
 * If all works correctly, the counter should end up be incremented
 * by each child iteration.
 *
 * However, this is failing for me occasionally. The counter ends up
 * being less than the expected value.
 *
 * This test was extracted from the APR test suite.
 *
 * Compile: gcc -Wall -o stc-flock-fork stc-flock-fork.c
 ***********************************************************************/

#include <sys/types.h>
#include <sys/file.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

#define MAX_ITER 200
#define CHILDREN 6
#define MAX_COUNT (MAX_ITER * CHILDREN)

/* Counter stored in shared memory. */
static volatile int *x;

/* A temporary file used for flock. */
char tmpfilename[] = "/tmp/flocktstXXXXXX";

/* a slower more racy way to implement (*x)++ */
static int increment(int n)
{
    usleep(1);
    return n+1;
}

/* Fork and use flock to lock and unlock the file repeatedly in the child. */
void make_child(int trylock, pid_t *pid)
{
    if ((*pid = fork()) < 0) {
        perror("fork failed");
        exit(1);
    }
    else if (*pid == 0) {
        int fd2 = open(tmpfilename, O_RDONLY);
        if (fd2 < 0) {
            perror("child open");
            exit(1);
        }

        int rc;
        int i;
        for (i=0; i<MAX_ITER; ++i) {
            /* Get the lock. */
            do {
                rc = flock(fd2, LOCK_EX);
            } while (rc < 0 && errno == EINTR);
            if (rc < 0) {
                perror("lock");
                exit(1);
            }

            /* Have the lock. Increment the counter. */
            *x = increment(*x);

            /* Release the lock. */
            do {
                rc = flock(fd2, LOCK_UN);
            } while (rc < 0 && errno == EINTR);
            if (rc < 0) {
                perror("unlock");
                exit(1);
            }
        }
        exit(0);
    }
}

/* Wait for the child to finish. */
void await_child(pid_t pid)
{
    pid_t pstatus;
    int exit_int;

    do {
        pstatus = waitpid(pid, &exit_int, WUNTRACED);
    } while (pstatus < 0 && errno == EINTR);
}

/* Allocate and attach shared memory */
void init_shm ()
{
    int shmid;
    if ((shmid = shmget(IPC_PRIVATE, sizeof(int), S_IRUSR | S_IWUSR | IPC_CREAT)) < 0) {
        perror("shmget failed");
        exit(1);
    }
    if ((x = shmat(shmid, NULL, 0)) == (void *) -1) {
        perror("shmat failed");
        exit(1);
    }
}

int main(int argc, const char * const * argv, const char * const *env)
{
    pid_t child[CHILDREN];
    int i;
    int n;
    int fd;

    /* Create the temporary file. */
    fd = mkstemp(tmpfilename);
    if (fd < 0) {
        perror("open failed");
        exit(1);
    }
    close(fd);

    /* Initialize shared memory */
    init_shm();

    /* Perform the test multiple times, since this fails only intermittedly. */
    for (i = 0; i < 100; ++i) {
        /* Initialize counter */
        *x = 0;

        /* Create the children. */
        for (n = 0; n < CHILDREN; n++)
            make_child(0, &child[n]);

        /* Wait for them to finish. */
        for (n = 0; n < CHILDREN; n++)
            await_child(child[n]);

        /* Check counter */
        if (*x != MAX_COUNT) {
            printf("Iteration %d: FAILED: *x (%d) != MAX_COUNT (%d)\n", i, *x, MAX_COUNT);
            exit(1);
        }
    }

    /* Clean up. */
    unlink(tmpfilename);

    return 0;
}