efivars: Parse efivar as UTF-16 LE bytes

[Johan-Liebert1]

No description provided.

[gemini-code-assist]

Code Review

This pull request refactors the EFI variable reading logic into a new utility function read_uefi_var. This function correctly parses the variables as UTF-16 LE, which is a great improvement for correctness. The overall change improves code organization and maintainability. I've added a few suggestions to further improve code clarity and adhere to Rust idioms.

[gemini-code-assist]

The match arms for EfiError::SystemNotUEFI and EfiError::MissingVar have the same logic. They can be combined into a single arm using the | operator for conciseness and better readability.

Suggested change

	EfiError::SystemNotUEFI => return Ok(Bootloader::Grub),
	EfiError::MissingVar => return Ok(Bootloader::Grub),
	EfiError::SystemNotUEFI | EfiError::MissingVar => return Ok(Bootloader::Grub),

[gemini-code-assist]

The #[allow(dead_code)] attributes on the InvalidData and Io variants of EfiError are unnecessary. The InvalidData variant is used in read_uefi_var, and the Io variant is used via the From<std::io::Error> implementation and the ? operator. Removing these attributes will make the code cleaner and avoid suppressing potential future warnings about genuinely unused code.

Suggested change

	#[allow(dead_code)]
	InvalidData(&'static str),
	#[allow(dead_code)]
	Io(std::io::Error),
	InvalidData(&'static str),
	Io(std::io::Error),

[gemini-code-assist]

The read_uefi_var function can be refactored to be more idiomatic and readable by using the ? operator for error handling and avoiding multiple match statements with early returns. This improves maintainability.

pub fn read_uefi_var(var_name: &str) -> Result<String, EfiError> {
    use crate::install::EFIVARFS;
    use cap_std_ext::cap_std::ambient_authority;

    let efivarfs = Dir::open_ambient_dir(EFIVARFS, ambient_authority()).map_err(|e| {
        if e.kind() == std::io::ErrorKind::NotFound {
            EfiError::SystemNotUEFI
        } else {
            e.into()
        }
    })?;

    let loader_bytes = efivarfs.read(var_name).map_err(|e| {
        if e.kind() == std::io::ErrorKind::NotFound {
            EfiError::MissingVar
        } else {
            e.into()
        }
    })?;

    if loader_bytes.len() % 2 != 0 {
        return Err(EfiError::InvalidData(
            "EFI var length is not valid UTF-16 LE",
        ));
    }

    // EFI vars are UTF-16 LE
    let loader_u16_bytes: Vec<u16> = loader_bytes
        .chunks_exact(2)
        .map(|x| u16::from_le_bytes([x[0], x[1]]))
        .collect();

    String::from_utf16(&loader_u16_bytes)
        .map_err(|_| EfiError::InvalidData("EFI var is not UTF-16"))
}

[cgwalters]

I did coreos/cap-std-ext#78 motivated by this btw