arcbox-ext4 0.1.0

# arcbox-ext4

Pure-Rust ext4 filesystem formatter and reader. Creates and reads ext4 images entirely in userspace -- no kernel mount, no FUSE, no C dependencies.

Designed for converting OCI container image layers into ext4 block device images on macOS and Linux.

## Features

- **Formatter** -- create ext4 images from scratch (`mkfs` + file population)
- **Reader** -- parse and read existing ext4 images (files, directories, symlinks)
- **OCI unpack** -- stream tar layers into ext4 with whiteout semantics (`.wh.*`, `.wh..wh..opq`)
- Extended attributes (inline + block)
- Extent trees (depth 0 and 1)
- Hard links with correct reference counting
- Symlinks (inline for targets < 60 bytes, data-block otherwise)
- No journal (not needed for container rootfs)

## Quick start

```rust
use std::path::Path;
use arcbox_ext4::Formatter;

// Create a new ext4 image.
let mut fmt = Formatter::new(Path::new("rootfs.ext4"), 4096, 256 * 1024)?;
fmt.create("/hello.txt", 0x8000 | 0o644, None, None,
    Some(&mut "hello world".as_bytes()), None, None, None)?;
fmt.close()?;

// Read it back.
let mut reader = arcbox_ext4::Reader::new(Path::new("rootfs.ext4"))?;
let data = reader.read_file("/hello.txt", 0, None)?;
assert_eq!(&data, b"hello world");
```

## OCI layer unpacking

```rust
use std::io::Cursor;
use arcbox_ext4::Formatter;

let mut fmt = Formatter::new(path, 4096, 512 * 1024 * 1024)?;
fmt.unpack_tar(Cursor::new(&layer1_tar))?;
fmt.unpack_tar(Cursor::new(&layer2_tar))?; // overlay with whiteouts
fmt.close()?;
```

## Limitations

- Block size is fixed at 4096 bytes
- Maximum file size: 128 GiB
- No journal support
- No metadata checksumming (`metadata_csum`)
- Extent tree depth limited to 1

## License

Licensed under either of

- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or <http://www.apache.org/licenses/LICENSE-2.0>)
- MIT License ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)

at your option.