# Renamore

More ways to rename files.

## Overview

The Rust standard library offers [`std::fs::rename`] for renaming files.
Sometimes, that's not enough. Consider the example of renaming a file but
aborting the operation if something already exists at the destination path.
That can be achieved using the Rust standard library but ensuring that the
operation is atomic requires platform-specific APIs. Without using
platform-specific APIs, a [TOCTTOU] bug can be introduced. This library aims
to provide a cross-platform interface to these APIs.

[`std::fs::rename`]: https://doc.rust-lang.org/std/fs/fn.rename.html
[TOCTTOU]: https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use

## Examples

Renaming a file without the possibility of accidentally overwriting anything
can be done using [`rename_exclusive`]. It should be noted that this feature
is not supported by all combinations of operating system and file system.
`rename_exclusive` will fail if it can't be done atomically.

[`rename_exclusive`]: https://docs.rs/renamore/latest/renamore/fn.rename_exclusive.html

```rust
use std::io::Result;

fn main() -> Result<()> {
    renamore::rename_exclusive("old.txt", "new.txt")
}
```

Alternatively, [`rename_exclusive_fallback`] can be used. This will try to
perform the operation atomically, and use a non-atomic fallback if that's
not supported. The return value will indicate what happened.

[`rename_exclusive_fallback`]: https://docs.rs/renamore/latest/renamore/fn.rename_exclusive_fallback.html

```rust
use std::io::Result;

fn main() -> Result<()> {
    if renamore::rename_exclusive_fallback("old.txt", "new.txt")? {
        // `new.txt` was definitely not overwritten.
        println!("The operation was atomic");
    } else {
        // `new.txt` was probably not overwritten.
        println!("The operation was not atomic");
    }

    Ok(())
}
```

## Platform-specific behaviour

On Linux, the `renameat2` syscall is used. A wrapper around this syscall is
provided by glibc since version 2.28 but not musl (yet?). The existence of
the wrapper is checked at build time and a wrapper is provided if one isn't
found. In case something goes wrong, there are two features that can be used
to bypass this mechanism.

 - `always-supported`. Assume that `renameat2` exists.
 - `always-fallback`. Assume that `renameat2` doesn't exist.

Hopefully using these features shouldn't be necessary. If they do become
necessary, then there might be a bug.

## 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.

## Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.