fs_txn.c

#define _POSIX_C_SOURCE 200809L
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>

typedef struct {
    const char *path;
    const char *tmp;
    const char *data;
} txn_entry_t;

static int write_temp(const char *tmp, const char *data)
{
    int fd = open(tmp, O_WRONLY | O_CREAT | O_TRUNC | O_EXCL, 0644);
    if (fd < 0) return -1;
    size_t len = strlen(data);
    ssize_t n = write(fd, data, len);
    if (n != (ssize_t)len) {
        int e = errno;
        close(fd);
        unlink(tmp);
        errno = e;
        return -1;
    }
    if (fsync(fd) < 0) {
        int e = errno;
        close(fd);
        unlink(tmp);
        errno = e;
        return -1;
    }
    if (close(fd) < 0) {
        int e = errno;
        unlink(tmp);
        errno = e;
        return -1;
    }
    return 0;
}

static int txn_commit(txn_entry_t *entries, size_t n)
{
    for (size_t i = 0; i < n; i++) {
        if (write_temp(entries[i].tmp, entries[i].data) < 0) return -1;
    }
    for (size_t i = 0; i < n; i++) {
        if (rename(entries[i].tmp, entries[i].path) < 0) return -1;
    }
    for (size_t i = 0; i < n; i++) {
        char dir[4096];
        strncpy(dir, entries[i].path, sizeof dir);
        dir[sizeof dir - 1] = 0;
        char *slash = strrchr(dir, '/');
        if (slash) {
            *slash = 0;
            int dfd = open(dir, O_RDONLY | O_DIRECTORY);
            if (dfd >= 0) {
                fsync(dfd);
                close(dfd);
            }
        }
    }
    return 0;
}

int main(int argc, char **argv)
{
    if (argc < 3) {
        fprintf(stderr, "usage: %s file1 file2\n", argv[0]);
        return 1;
    }
    char tmp1[4096], tmp2[4096];
    snprintf(tmp1, sizeof tmp1, "%s.tmp.%ld", argv[1], (long)getpid());
    snprintf(tmp2, sizeof tmp2, "%s.tmp.%ld", argv[2], (long)getpid());
    txn_entry_t e[2];
    e[0].path = argv[1];
    e[0].tmp = tmp1;
    e[0].data = "file1 contents\n";
    e[1].path = argv[2];
    e[1].tmp = tmp2;
    e[1].data = "file2 contents\n";
    if (txn_commit(e, 2) < 0) {
        perror("txn_commit");
        return 1;
    }
    printf("committed\n");
    return 0;
}