Re: [PATCH v2] rust: xarray: Add an abstraction for XArray

[Asahi Lina]

Why do I always notice mistakes after sending the patch... this is
missing the comment I intended to add because I edited the file in the
wrong git checkout. Sorry!

The intended comment is below, I'll roll it into v3 when I make the 
Error API change.

On 24/03/2023 01.23, Asahi Lina wrote:
> The XArray is an abstract data type which behaves like a very large
> array of pointers. Add a Rust abstraction for this data type.
>
> The initial implementation uses explicit locking on get operations and
> returns a guard which blocks mutation, ensuring that the referenced
> object remains alive. To avoid excessive serialization, users are
> expected to use an inner type that can be efficiently cloned (such as
> Arc<T>), and eagerly clone and drop the guard to unblock other users
> after a lookup.
>
> Future variants may support using RCU instead to avoid mutex locking.
>
> This abstraction also introduces a reservation mechanism, which can be
> used by alloc-capable XArrays to reserve a free slot without immediately
> filling it, and then do so at a later time. If the reservation is
> dropped without being filled, the slot is freed again for other users,
> which eliminates the need for explicit cleanup code.
>
> Signed-off-by: Asahi Lina <lina@xxxxxxxxxxxxx>
[...]
> +impl<T: ForeignOwnable> Drop for XArray<T> {
> +    fn drop(&mut self) {
> +        // SAFETY: `self.xa` is valid by the type invariant, and as we have the only reference to
> +        // the `XArray` we can safely iterate its contents and drop everything.
> +        unsafe {
> +            let mut index: core::ffi::c_ulong = 0;
> +            let mut entry = bindings::xa_find(
> +                self.xa.get(),
> +                &mut index,
> +                core::ffi::c_ulong::MAX,
> +                bindings::BINDINGS_XA_PRESENT,
> +            );
> +            while !entry.is_null() {
> +                T::from_foreign(entry);
> +                entry = bindings::xa_find_after(
> +                    self.xa.get(),
> +                    &mut index,
> +                    core::ffi::c_ulong::MAX,
> +                    bindings::BINDINGS_XA_PRESENT,
> +                );
> +            }
> +

Add for v3:
> +            // Locked locks are not safe to drop. Normally we would want to try_lock()/unlock() here
> +            // for safety or something similar, but in this case xa_destroy() is guaranteed to
> +            // acquire the lock anyway. This will deadlock if a lock guard was improperly dropped,
> +            // but that is not UB, so it's sufficient for soundness purposes.

> +            bindings::xa_destroy(self.xa.get());
> +        }
> +    }
> +}
> +
> +// SAFETY: XArray is thread-safe and all mutation operations are internally locked.
> +unsafe impl<T: Send + ForeignOwnable> Send for XArray<T> {}
> +unsafe impl<T: Sync + ForeignOwnable> Sync for XArray<T> {}
>
> ---
> base-commit: 3ef2e9730ba4bb914dd8654ae34aef3f486c8c58
> change-id: 20230224-rust-xarray-f503f9e5455e
>
> Thank you,
> ~~ Lina

~~ Lina