lib.rs

#![cfg_attr(not(test), no_std)]
#![cfg_attr(docsrs, feature(doc_cfg))]
// FIXME
#![allow(clippy::missing_safety_doc)]

/*!

[littlefs](https://github.com/littlefs-project/littlefs) is a filesystem for microcontrollers
written in C, that claims to be *fail-safe*:
- power-loss resilience, by virtue of copy-on-write guarantees
- bounded RAM/ROM, with stack-allocated buffers

Since [version 2](https://github.com/littlefs-project/littlefs/releases/tag/v2.0.0), it has
some nifty features such as:
- dynamic wear-leveling, including detection of bad Flash blocks
- custom user attributes
- inline files, avoiding block waste

For more background, see its [design notes](https://github.com/littlefs-project/littlefs/blob/master/DESIGN.md)
and the [specification](https://github.com/littlefs-project/littlefs/blob/master/SPEC.md) of its format.

### What is this?

This library, [`littlefs2`](https://lib.rs/littlefs2), offers an idiomatic Rust API for littlefs.

It follows the design of [`std::fs`](https://doc.rust-lang.org/std/fs/index.html) as much as reasonable,
and builds on the bindings [`littlefs2-sys`](https://lib.rs/littlefs2-sys).

Some complications arise due to the lack of const generics in Rust, we work around these
with the [`generic-array`](https://lib.rs/generic-array) library, and long for the day when
constants associated to traits will be treated as constants by the compiler.

Another complication is the fact that files (and directories) need to be closed before they go out of scope,
since the main littlefs state structure contains a linked list which would exhibit UB (undefined behaviour)
otherwise, see [issue #3](https://github.com/trussed-dev/littlefs2/issues/3) and
[issue #5](https://github.com/trussed-dev/littlefs2/issues/5). We choose *not* to call `close` in `drop` (as
`std::fs` does), since these operations could panic if for instance `littlefs` detects Flash corruption
(from which the application might otherwise recover).

For this reason, the various `File`-related `open` methods are marked as `unsafe`.
Instead, a closure-based API is offered (`open_and_then` and friends),
the same is done for `Filesystem::read_dir`. Under the hood, this API first calls the unsafe constructor,
then calls the user-supplied closure, and finally closes the object.

**FOLLOWING SECTION OUT-OF-DATE**

⯈ [**The best place to start reading the API docs is here**](fs/index.html). ⯇

### Usage

To use this library, implement `littlefs2::driver::Storage`.
The macro `ram_storage!` generates examples of this.

Roughly speaking, the [`Storage`](driver/trait.Storage.html) trait defines a block device in
terms of actual and `typenum` constants, and an implementation supplies methods to read, erase and write.

The filesystem and each open file need memory for state and caching, this has to be allocated
beforehand and passed to constructors.

### `no_std`

This library is `no_std` compatible, but there are two gotchas.

- The dev-dependency `memchr` of `littlefs2-sys` has its `std` features activated. To prevent this, upgrade to at least Rust 1.51
  and add `resolver = "2"` in the consuming code's `[package]` section. This will be the default in Rust 2021 edition.

- At link time, `lfs.c` has a dependency on `strcpy`. When not linking to a `libc` with this symbol, activate the `c-stubs` feature
  to provide an implementation.

### Design notes

All operations on the filesystem require passing a `&mut Storage`, which guarantees by Rust's
borrow checker that only one thread can manipulate the filesystem.
This design choice (as opposed to consuming the Storage, which would be less verbose) was made to
enable use of the underlying flash peripheral outside of the filesystem (the `Storage` can be
dropped and reconstructed).  For instance, one could setup an additional filesystem,
or handle some flash data manually.

As an experiment, we implemented [`ReadDirWith`](fs/struct.ReadDirWith.html). It converts a
[`ReadDir`](fs/struct.ReadDir.html) (which needs mutable references, and so is "not quite an iterator"
over the files of a directory), into a true iterator, by temporarily binding the mutable references.

Currying with lifetime gymnastics!

In the future, we may extend this approach to other operations, thus adding a secondary API layer.

<https://play.rust-lang.org/?edition=2018&gist=c86abf99fc87551cfe3136e398a45d19>

Separately, keeping track of the allocations is a chore, we hope that
[`Pin`](https://doc.rust-lang.org/core/pin/index.html) magic will help fix this.

### Example

```
# use littlefs2::fs::{Filesystem, File, OpenOptions};
# use littlefs2::io::prelude::*;
# use littlefs2::path;
#
# use littlefs2::{consts, ram_storage, driver, io::Result};
#
#
// example storage backend
ram_storage!(tiny);
let mut ram = Ram::default();
let mut storage = RamStorage::new(&mut ram);

// must format before first mount
Filesystem::format(&mut storage).unwrap();
// must allocate state statically before use
let mut alloc = Filesystem::allocate();
let mut fs = Filesystem::mount(&mut alloc, &mut storage).unwrap();

// may use common `OpenOptions`
let mut buf = [0u8; 11];
fs.open_file_with_options_and_then(
    |options| options.read(true).write(true).create(true),
    path!("example.txt"),
    |file| {
        file.write(b"Why is black smoke coming out?!")?;
        file.seek(SeekFrom::End(-24)).unwrap();
        assert_eq!(file.read(&mut buf)?, 11);
        Ok(())
    }
).unwrap();
assert_eq!(&buf, b"black smoke");
```
*/

#[cfg(feature = "alloc")]
extern crate alloc;

/// Low-level bindings
pub use littlefs2_sys as ll;

// Re-exports for compatibility with older versions
pub use littlefs2_core::path;

/// Traits and types for core I/O functionality.
pub mod io {
    pub use littlefs2_core::{Error, OpenSeekFrom, Read, Result, Seek, SeekFrom, Write};

    pub mod prelude {
        //! Export of the Read, Write and Seek traits for ease of use.

        pub use super::{Read, Seek, SeekFrom, Write};
    }
}

/// Paths
pub mod path {
    pub use littlefs2_core::{Ancestors, Iter, Path, PathBuf, PathError as Error};
    pub type Result<T> = core::result::Result<T, Error>;
}

#[macro_use]
extern crate delog;
generate_macros!();

/// cf. Macros documentation
#[macro_use]
pub mod macros;

#[cfg(feature = "c-stubs")]
mod c_stubs;

pub mod consts;
pub mod driver;
pub mod fs;
pub mod object_safe;

/// The version of the C backend.
pub const BACKEND_VERSION: Version = Version(ll::LFS_VERSION);

/// The on-disk version used by [`Filesystem`][`fs::Filesystem`].
///
/// Note that this is not the same as the littlefs [`LFS_DISK_VERSION`][`ll::LFS_DISK_VERSION`]
/// constant as this crate uses the multiversion feature to select on-disk version 2.0 instead of
/// using the latest on-disk version (currently 2.1).
pub const DISK_VERSION: Version = Version(0x0002_0000);

/// A littlefs version number.
#[derive(Clone, Copy, Debug)]
pub struct Version(u32);

impl Version {
    /// The major version component (top-nibble).
    pub const fn major(&self) -> u16 {
        let [high, low, _, _] = self.0.to_be_bytes();
        u16::from_be_bytes([high, low])
    }

    /// The minor version component (bottom-nibble).
    pub const fn minor(&self) -> u16 {
        let [_, _, high, low] = self.0.to_be_bytes();
        u16::from_be_bytes([high, low])
    }
}

impl From<u32> for Version {
    fn from(version: u32) -> Self {
        Self(version)
    }
}

impl From<Version> for u32 {
    fn from(version: Version) -> u32 {
        version.0
    }
}

#[cfg(test)]
mod tests;