Re: [PATCH 2/9] rust: list: add tracking for ListArc

[Alice Ryhl]

On Wed, Apr 3, 2024 at 5:52 PM Benno Lossin <benno.lossin@xxxxxxxxx> wrote:
>
> I think the commit one-line description sounds a bit strange, how about
> "rust: list: add ListArc tracking strategies"?
>
> On 02.04.24 14:16, Alice Ryhl wrote:
> > @@ -33,19 +34,64 @@ pub trait ListArcSafe<const ID: u64 = 0> {
> >      unsafe fn on_drop_list_arc(&self);
> >  }
> >
> > +/// Declares that this type is able to safely attempt to create `ListArc`s at any time.
> > +///
> > +/// # Safety
> > +///
> > +/// Implementers must ensure that `try_new_list_arc` does not return `true` if a `ListArc` already
> > +/// exists.
> > +pub unsafe trait TryNewListArc<const ID: u64 = 0>: ListArcSafe<ID> {
> > +    /// Attempts to convert an `Arc<Self>` into an `ListArc<Self>`. Returns `true` if the
> > +    /// conversion was successful.
> > +    fn try_new_list_arc(&self) -> bool;
> > +}
> > +
> >  /// Declares that this type supports [`ListArc`].
> >  ///
> > -/// When using this macro, it will only be possible to create a [`ListArc`] from a [`UniqueArc`].
> > +/// When using this macro, you may choose between the `untracked` strategy where it is not tracked
> > +/// whether a [`ListArc`] exists, and the `tracked_by` strategy where the tracking is deferred to a
> > +/// field of the struct. The `tracked_by` strategy can be combined with a field of type
> > +/// [`AtomicListArcTracker`] to track whether a [`ListArc`] exists.
> >  #[macro_export]
> >  macro_rules! impl_list_arc_safe {
> >      (impl$({$($generics:tt)*})? ListArcSafe<$num:tt> for $t:ty { untracked; } $($rest:tt)*) => {
> > -        impl$(<$($generics)*>)? $crate::list::ListArcSafe<$num> for $t {
> > +        impl$(<$($generics)*>)? ListArcSafe<$num> for $t {
>
> This change seems unintentional.

Will fix.

> >              unsafe fn on_create_list_arc_from_unique(&mut self) {}
> >              unsafe fn on_drop_list_arc(&self) {}
> >          }
> >          $crate::list::impl_list_arc_safe! { $($rest)* }
> >      };
> >
> > +    (impl$({$($generics:tt)*})? ListArcSafe<$num:tt> for $t:ty {
> > +        tracked_by $field:ident : $fty:ty;
> > +    } $($rest:tt)*) => {
> > +        impl$(<$($generics)*>)? ListArcSafe<$num> for $t {
>
> Here you also want to access `ListArcSafe` via
> `$crate::list::ListArcSafe`.

Will fix.

> > +            unsafe fn on_create_list_arc_from_unique(&mut self) {
> > +                let me = self as *mut Self;
> > +                let field: *mut $fty = unsafe { ::core::ptr::addr_of_mut!((*me).$field) };
>
> I think we should also have `SAFETY` comments in macros.
>
> Also why can't this be done using safe code?:
>
>      let field: &mut $fty = &mut self.$field;

Not sure why. Probably a historical accident. Will change it to be safe.

> > +                unsafe { <$fty as $crate::list::ListArcSafe<$num>>::on_create_list_arc_from_unique(
> > +                        &mut *field
> > +                ) };
>
> Formatting? rustfmt gives me this:
>
>                  unsafe {
>                      <$fty as $crate::list::ListArcSafe<$num>>::on_create_list_arc_from_unique(
>                          &mut *field
>                      )
>                  };
>
> (maybe the `;` should be inside the `unsafe` block in this case?)

I can make the change, but rustfmt does not affect macros.

Alice