Centos-kernel-stream-10/rust/kernel/types.rs

// SPDX-License-Identifier: GPL-2.0

//! Kernel types.

use core::{
    cell::UnsafeCell,
    mem::MaybeUninit,
    ops::{Deref, DerefMut},
};

/// Runs a cleanup function/closure when dropped.
///
/// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
///
/// # Examples
///
/// In the example below, we have multiple exit paths and we want to log regardless of which one is
/// taken:
/// ```
/// # use kernel::ScopeGuard;
/// fn example1(arg: bool) {
///     let _log = ScopeGuard::new(|| pr_info!("example1 completed\n"));
///
///     if arg {
///         return;
///     }
///
///     pr_info!("Do something...\n");
/// }
///
/// # example1(false);
/// # example1(true);
/// ```
///
/// In the example below, we want to log the same message on all early exits but a different one on
/// the main exit path:
/// ```
/// # use kernel::ScopeGuard;
/// fn example2(arg: bool) {
///     let log = ScopeGuard::new(|| pr_info!("example2 returned early\n"));
///
///     if arg {
///         return;
///     }
///
///     // (Other early returns...)
///
///     log.dismiss();
///     pr_info!("example2 no early return\n");
/// }
///
/// # example2(false);
/// # example2(true);
/// ```
///
/// In the example below, we need a mutable object (the vector) to be accessible within the log
/// function, so we wrap it in the [`ScopeGuard`]:
/// ```
/// # use kernel::ScopeGuard;
/// fn example3(arg: bool) -> Result {
///     let mut vec =
///         ScopeGuard::new_with_data(Vec::new(), |v| pr_info!("vec had {} elements\n", v.len()));
///
///     vec.try_push(10u8)?;
///     if arg {
///         return Ok(());
///     }
///     vec.try_push(20u8)?;
///     Ok(())
/// }
///
/// # assert_eq!(example3(false), Ok(()));
/// # assert_eq!(example3(true), Ok(()));
/// ```
///
/// # Invariants
///
/// The value stored in the struct is nearly always `Some(_)`, except between
/// [`ScopeGuard::dismiss`] and [`ScopeGuard::drop`]: in this case, it will be `None` as the value
/// will have been returned to the caller. Since  [`ScopeGuard::dismiss`] consumes the guard,
/// callers won't be able to use it anymore.
pub struct ScopeGuard<T, F: FnOnce(T)>(Option<(T, F)>);

impl<T, F: FnOnce(T)> ScopeGuard<T, F> {
    /// Creates a new guarded object wrapping the given data and with the given cleanup function.
    pub fn new_with_data(data: T, cleanup_func: F) -> Self {
        // INVARIANT: The struct is being initialised with `Some(_)`.
        Self(Some((data, cleanup_func)))
    }

    /// Prevents the cleanup function from running and returns the guarded data.
    pub fn dismiss(mut self) -> T {
        // INVARIANT: This is the exception case in the invariant; it is not visible to callers
        // because this function consumes `self`.
        self.0.take().unwrap().0
    }
}

impl ScopeGuard<(), fn(())> {
    /// Creates a new guarded object with the given cleanup function.
    pub fn new(cleanup: impl FnOnce()) -> ScopeGuard<(), impl FnOnce(())> {
        ScopeGuard::new_with_data((), move |_| cleanup())
    }
}

impl<T, F: FnOnce(T)> Deref for ScopeGuard<T, F> {
    type Target = T;

    fn deref(&self) -> &T {
        // The type invariants guarantee that `unwrap` will succeed.
        &self.0.as_ref().unwrap().0
    }
}

impl<T, F: FnOnce(T)> DerefMut for ScopeGuard<T, F> {
    fn deref_mut(&mut self) -> &mut T {
        // The type invariants guarantee that `unwrap` will succeed.
        &mut self.0.as_mut().unwrap().0
    }
}

impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> {
    fn drop(&mut self) {
        // Run the cleanup function if one is still present.
        if let Some((data, cleanup)) = self.0.take() {
            cleanup(data)
        }
    }
}

/// Stores an opaque value.
///
/// This is meant to be used with FFI objects that are never interpreted by Rust code.
#[repr(transparent)]
pub struct Opaque<T>(MaybeUninit<UnsafeCell<T>>);

impl<T> Opaque<T> {
    /// Creates a new opaque value.
    pub const fn new(value: T) -> Self {
        Self(MaybeUninit::new(UnsafeCell::new(value)))
    }

    /// Creates an uninitialised value.
    pub const fn uninit() -> Self {
        Self(MaybeUninit::uninit())
    }

    /// Returns a raw pointer to the opaque data.
    pub fn get(&self) -> *mut T {
        UnsafeCell::raw_get(self.0.as_ptr())
    }
}

/// A sum type that always holds either a value of type `L` or `R`.
pub enum Either<L, R> {
    /// Constructs an instance of [`Either`] containing a value of type `L`.
    Left(L),

    /// Constructs an instance of [`Either`] containing a value of type `R`.
    Right(R),
}