Date:   Sun, 22 Jan 2023 03:31:12 -0300
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] rust: types: introduce `ScopeGuard`

On Fri, 20 Jan 2023 at 03:24, Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Thu, Jan 19, 2023 at 02:40:32PM -0300, Wedson Almeida Filho wrote:
> > 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>
> > ---
> >  rust/kernel/types.rs | 127 ++++++++++++++++++++++++++++++++++++++++++-
> >  1 file changed, 126 insertions(+), 1 deletion(-)
> >
> > diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> > index e84e51ec9716..f0ad4472292d 100644
> > --- a/rust/kernel/types.rs
> > +++ b/rust/kernel/types.rs
> > @@ -2,7 +2,132 @@
> >
> >  //! Kernel types.
> >
> > -use core::{cell::UnsafeCell, mem::MaybeUninit};
> > +use alloc::boxed::Box;
> > +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<(), Box<dyn FnOnce(())>> {
>
> How about `fn(())` here as a placeholder? I.e
>
>         impl ScopeGuard<(), fn(())>
>

That's simpler, I like it. I'll change this for v2. Thanks!

> Regards,
> Boqun
>
> > +    /// 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.
> >  ///
> > --
> > 2.34.1
> >

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