packfs/examples/demo.c

/*
 * demo.c — a runnable, human-readable demonstration of PackFS, not another
 * automated test. Run it and read the output; each step prints what it did
 * and what it found, so the library's core behaviors (mem backend, a
 * pack-backed overlay with copy-up and whiteout, compaction, persistence
 * across a re-open, and a sandboxed dir mount) are visible directly,
 * without reading test assertions.
 *
 * Usage: packfs_demo [pack-file-path]
 * (defaults to /tmp/packfs_demo.pack; re-run it to see the previous run's
 * data survive via the pack file written by the first run's vfs_sync.)
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "packfs.h"

static void die(const char *what, int err) {
    fprintf(stderr, "FAILED: %s (error code %d)\n", what, err);
    exit(1);
}

static void print_stat(Vfs *v, const char *path) {
    VfsStat st;
    int rc = vfs_stat(v, path, &st);
    if (rc != VFS_OK) {
        printf("  stat %-20s -> error %d\n", path, rc);
        return;
    }
    printf("  stat %-20s -> %s, size=%llu\n", path,
           st.kind == VFS_KIND_DIR ? "dir " : "file",
           (unsigned long long)st.size);
}

static void list_dir(Vfs *v, const char *path) {
    VfsDir dir;
    int rc = vfs_readdir(v, path, &dir);
    if (rc != VFS_OK) {
        printf("  readdir %s -> error %d\n", path, rc);
        return;
    }
    printf("  readdir %s -> %zu entr%s:\n", path, (size_t)dir.count, dir.count == 1 ? "y" : "ies");
    for (pfs_usize i = 0; i < dir.count; i++) {
        printf("    %s%s\n", dir.entries[i].name, dir.entries[i].kind == VFS_KIND_DIR ? "/" : "");
    }
    vfs_dir_free(&dir);
}

static void write_file(Vfs *v, const char *path, const char *content) {
    int err = 0;
    VfsFile *f = vfs_open(v, path, VFS_O_WRONLY | VFS_O_CREAT | VFS_O_TRUNC, &err);
    if (!f) die(path, err);
    pfs_usize len = (pfs_usize)strlen(content);
    if (vfs_write(f, content, len) != (pfs_isize)len) die("short write", VFS_ERR_IO);
    vfs_close(f);
    printf("  wrote   %-20s (%zu bytes)\n", path, (size_t)len);
}

static void read_file(Vfs *v, const char *path) {
    int err = 0;
    VfsFile *f = vfs_open(v, path, VFS_O_RDONLY, &err);
    if (!f) { printf("  read    %-20s -> error %d\n", path, err); return; }
    char buf[256] = {0};
    pfs_isize n = vfs_read(f, buf, sizeof(buf) - 1);
    vfs_close(f);
    printf("  read    %-20s -> \"%.*s\"\n", path, (int)n, buf);
}

int main(int argc, char **argv) {
    const char *pack_path = argc > 1 ? argv[1] : "/tmp/packfs_demo.pack";

    printf("=== PackFS demo: pack-backed overlay at %s ===\n", pack_path);
    printf("(if this file already exists from a previous run, its contents\n"
           " should reappear below, proving compaction + reload persistence)\n\n");

    Vfs *v = vfs_new();
    Backend *mem = backend_mem_new();
    int oerr = 0;
    Backend *overlay = backend_overlay_new(pack_path, mem, &oerr);
    if (!overlay) die("backend_overlay_new", oerr);
    if (vfs_mount(v, "/", overlay) != VFS_OK) die("vfs_mount /", 0);

    printf("-- initial state (from the pack file, if one existed) --\n");
    list_dir(v, "/");

    printf("\n-- writing files --\n");
    write_file(v, "/hello.txt", "Hello from PackFS!");
    write_file(v, "/config.json", "{\"greeting\":\"hi\"}");
    if (vfs_mkdir(v, "/notes") != VFS_OK) printf("  (/notes already existed)\n");
    else printf("  mkdir   /notes\n");
    write_file(v, "/notes/todo.txt", "buy milk");

    printf("\n-- reading back --\n");
    read_file(v, "/hello.txt");
    read_file(v, "/notes/todo.txt");

    printf("\n-- directory listing --\n");
    list_dir(v, "/");
    list_dir(v, "/notes");

    printf("\n-- stat --\n");
    print_stat(v, "/hello.txt");
    print_stat(v, "/notes");
    print_stat(v, "/does-not-exist.txt");

    printf("\n-- copy-up + whiteout: deleting a file that lives in the pack --\n");
    /* On the second run, /hello.txt (if it survived compaction) lives in
     * the read-only pack layer; this unlink is served by a whiteout
     * (Section 4.2), not a mutation of the pack. */
    int rc = vfs_unlink(v, "/config.json");
    printf("  unlink  /config.json -> %s\n", rc == VFS_OK ? "ok (whiteout or plain removal)" : "error");
    list_dir(v, "/");

    printf("\n-- compacting the overlay into a fresh pack (vfs_sync) --\n");
    if (vfs_sync(v, "/") != VFS_OK) die("vfs_sync", 0);
    printf("  wrote %s\n", pack_path);

    vfs_unmount(v, "/");
    backend_free(overlay);
    backend_free(mem);
    vfs_free(v);

    printf("\n=== sandboxed dir mount demo ===\n");
    {
        char tmpl[] = "/tmp/packfs_demo_sandbox_XXXXXX";
        char *sandbox = mkdtemp(tmpl);
        if (!sandbox) die("mkdtemp", 0);
        printf("sandbox root: %s\n", sandbox);

        Vfs *v2 = vfs_new();
        int derr = 0;
        Backend *dir = backend_dir_new(sandbox, &derr);
        if (!dir) die("backend_dir_new", derr);
        vfs_mount(v2, "/data", dir);

        write_file(v2, "/data/inside.txt", "this file is contained to the sandbox root");
        read_file(v2, "/data/inside.txt");

        printf("  attempting /data/../../../etc/passwd (must be rejected) ...\n");
        int err2 = 0;
        VfsFile *escape = vfs_open(v2, "/data/../../../etc/passwd", VFS_O_RDONLY, &err2);
        printf("  -> %s (error code %d)\n", escape ? "UNEXPECTEDLY OPENED (bug!)" : "rejected, as required", err2);
        if (escape) vfs_close(escape);

        vfs_unmount(v2, "/data");
        backend_free(dir);
        vfs_free(v2);
    }

    printf("\n=== done ===\n");
    printf("Re-run this program (same pack path) to see /hello.txt and\n"
           "/notes/todo.txt survive, and /config.json stay deleted.\n");
    return 0;
}