Date:   Mon, 30 Jan 2023 21:07:07 +0100
From:   Andreas Hindborg <nmi@...aspace.dk>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Vincenzo Palazzo <vincenzopalazzodev@...il.com>,
        Andreas Hindborg <a.hindborg@...sung.com>
Subject: Re: [PATCH v2 1/5] rust: types: introduce `ScopeGuard`


Wedson Almeida Filho <wedsonaf@...il.com> writes:

> This allows us to run some code when the guard is dropped (e.g.,
> implicitly when it goes out of scope). We can also prevent the
> guard from running by calling its `dismiss()` method.
>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@...il.com>
> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@...il.com>
> ---

Reviewed-by: Andreas Hindborg <a.hindborg@...sung.com>

> v1 -> v2: Simpler type for `ScopeGuard::new()` impl block
>
>  rust/kernel/types.rs | 126 ++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 125 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index e84e51ec9716..dd834bfcb57b 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -2,7 +2,131 @@
>  
>  //! Kernel types.
>  
> -use core::{cell::UnsafeCell, mem::MaybeUninit};
> +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.
>  ///

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