SchOS/cmd/locktest/main.c

// locktest: flock() and fcntl() record locks, each property a program
// relies on checked between a parent and a child -- the only way to see a
// lock is to be someone it keeps out.

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/file.h>
#include <sys/wait.h>
#include <unistd.h>

static int bad;

#define CHECK(cond, ...) do {                                   \
        if (!(cond)) { printf("locktest: FAIL " __VA_ARGS__); printf("\n"); bad++; } \
    } while (0)

#define PATH "/tmp/locktest"

// Run `fn` in a child and return its exit status.
static int
child(int (*fn)(void))
{
    pid_t p = fork();
    if (p == 0)
        _exit(fn());
    int st = 0;
    waitpid(p, &st, 0);
    return WIFEXITED(st) ? WEXITSTATUS(st) : 99;
}

static int
try_flock_ex(void)
{
    int fd = open(PATH, O_RDWR);
    return flock(fd, LOCK_EX | LOCK_NB) == 0 ? 0 : (errno == EWOULDBLOCK ? 1 : 2);
}

static int
try_flock_sh(void)
{
    int fd = open(PATH, O_RDWR);
    return flock(fd, LOCK_SH | LOCK_NB) == 0 ? 0 : 1;
}

static int
set(int fd, int cmd, short type, off_t start, off_t len)
{
    struct flock fl = { .l_type = type, .l_whence = SEEK_SET,
                        .l_start = start, .l_len = len };
    return fcntl(fd, cmd, &fl);
}

static int
try_range_10_19(void)
{
    int fd = open(PATH, O_RDWR);
    return set(fd, F_SETLK, F_WRLCK, 10, 10) == 0 ? 0 : 1;
}

static int
try_range_50_59(void)
{
    int fd = open(PATH, O_RDWR);
    return set(fd, F_SETLK, F_WRLCK, 50, 10) == 0 ? 0 : 1;
}

static int
wait_range_10_19(void)
{
    int fd = open(PATH, O_RDWR);
    return set(fd, F_SETLKW, F_WRLCK, 10, 10) == 0 ? 0 : 1;
}

static int
lock_and_exit(void)
{
    int fd = open(PATH, O_RDWR);
    return set(fd, F_SETLK, F_WRLCK, 0, 0) == 0 ? 0 : 1;
}

int
main(void)
{
    int fd = open(PATH, O_RDWR | O_CREAT | O_TRUNC, 0644);
    CHECK(fd >= 0, "open: %s", strerror(errno));
    write(fd, "0123456789012345678901234567890123456789", 40);

    // flock: exclusive keeps everyone out, shared lets shared in.
    CHECK(flock(fd, LOCK_EX) == 0, "flock LOCK_EX");
    CHECK(child(try_flock_ex) == 1, "a second LOCK_EX is refused with EWOULDBLOCK");
    CHECK(flock(fd, LOCK_SH) == 0, "convert to LOCK_SH");
    CHECK(child(try_flock_sh) == 0, "LOCK_SH alongside LOCK_SH");
    CHECK(child(try_flock_ex) == 1, "LOCK_EX against LOCK_SH");
    CHECK(flock(fd, LOCK_UN) == 0 && child(try_flock_ex) == 0, "LOCK_UN releases");

    // Closing the last descriptor on the description drops a flock.
    int fd2 = open(PATH, O_RDWR);
    CHECK(flock(fd2, LOCK_EX) == 0, "flock on a second description");
    close(fd2);
    CHECK(child(try_flock_ex) == 0, "close releases flock");

    // flock and fcntl locks do not see each other.
    CHECK(flock(fd, LOCK_EX) == 0 && child(try_range_10_19) == 0,
          "fcntl ignores flock");
    flock(fd, LOCK_UN);

    // Ranges: a lock on 0..39 with 15..16 unlocked is two locks.
    CHECK(set(fd, F_SETLK, F_WRLCK, 0, 40) == 0, "F_SETLK 0..39");
    CHECK(child(try_range_50_59) == 0, "outside the range is free");
    CHECK(child(try_range_10_19) == 1, "inside the range is not");
    CHECK(set(fd, F_SETLK, F_UNLCK, 10, 10) == 0, "unlock 10..19");
    CHECK(child(try_range_10_19) == 0, "the hole is free");

    struct flock q = { .l_type = F_WRLCK, .l_whence = SEEK_SET, .l_start = 0, .l_len = 5 };
    pid_t p = fork();
    if (p == 0) {
        int cfd = open(PATH, O_RDWR);
        _exit(fcntl(cfd, F_GETLK, &q) == 0 && q.l_type == F_WRLCK &&
              q.l_pid == getppid() && q.l_start == 0 && q.l_len == 10 ? 0 : 1);
    }
    int st;
    waitpid(p, &st, 0);
    CHECK(WIFEXITED(st) && WEXITSTATUS(st) == 0, "F_GETLK reports the holder");

    // A process's record locks go when it closes *any* descriptor on the file.
    int fd3 = open(PATH, O_RDONLY);
    close(fd3);
    CHECK(child(try_range_10_19) == 0 && child(try_range_50_59) == 0,
          "closing another descriptor releases the process's locks");

    // A blocked waiter is woken when the lock goes.
    CHECK(set(fd, F_SETLK, F_WRLCK, 0, 40) == 0, "relock");
    p = fork();
    if (p == 0)
        _exit(wait_range_10_19());
    usleep(100000);
    CHECK(waitpid(p, &st, WNOHANG) == 0, "F_SETLKW waits");
    set(fd, F_SETLK, F_UNLCK, 0, 0);
    waitpid(p, &st, 0);
    CHECK(WIFEXITED(st) && WEXITSTATUS(st) == 0, "F_SETLKW is woken by the unlock");

    // And they go when the process exits.
    CHECK(child(lock_and_exit) == 0 && set(fd, F_SETLK, F_WRLCK, 0, 0) == 0,
          "exit releases record locks");

    close(fd);
    unlink(PATH);
    printf(bad ? "locktest: %d failed\n" : "locktest: ok\n", bad);
    return bad != 0;
}