Re: [PATCH v2 2/2] rust: arc: remove `ArcBorrow` in favour of `WithRef`

[Benno Lossin]

On 25.09.23 18:02, Boqun Feng wrote:
> On Mon, Sep 25, 2023 at 05:30:05PM +0200, Alice Ryhl wrote:
>> On 9/25/23 17:17, Boqun Feng wrote:
>>> On Mon, Sep 25, 2023 at 03:00:47PM +0000, Alice Ryhl wrote:
>>>>>>> I'm concerned about this change, because an `&WithRef<T>` only has
>>>>>>> immutable permissions for the allocation. No pointer derived from it
>>>>>>> may be used to modify the value in the Arc, however, the drop
>>>>>>> implementation of Arc will do exactly that.
>>>>>>
>>>>>> That is indeed a problem. We could put the value in an `UnsafeCell`, but
>>>>>> that would lose us niche optimizations and probably also other optimizations.
>>>>>>
>>>>>
>>>>> Not sure I understand the problem here, why do we allow modifying the
>>>>> value in the Arc if you only have a shared ownership?
>>>>
>>>> Well, usually it's when you have exclusive access even though the value
>>>> is in an `Arc`.
>>>>
>>>> The main example of this is the destructor of the `Arc`. When the last
>>>> refcount drops to zero, this gives you exclusive access. This lets you
>>>> run the destructor. The destructor requires mutable access.
>>>>
>>>> Another example would be converting the `Arc` back into an `UniqueArc`
>>>> by checking that the refcount is 1. Once you have a `UniqueArc`, you can
>>>> use it to mutate the inner value.
>>>>
>>>> Finally, there are methods like `Arc::get_mut_unchecked`, where you
>>>> unsafely assert that nobody else is using the value while you are
>>>> modifying it. We don't have that in our version of `Arc` right now, but
>>>> we might want to add it later.
>>>>
>>>
>>> Hmm.. but the only way to get an `Arc` from `&WithRef` is
>>>
>>> 	impl From<&WithRef<T>> for Arc<T> {
>>> 	    ...
>>> 	}
>>>
>>> , and we clone `Arc` in the that function (i.e. copying the raw
>>> pointer), so we are still good?
>>>
>>
>> No, the raw pointer in the Arc was created from the immutable reference, so
>> the raw pointer has the same restrictions as the immutable reference did.
>>
> 
> I see, this was the part I was missing. Thanks!
> 
> Looks like the only fix is replacing `&WithRef<T>` with
> `&UnsafeCell<WithRef<T>>`? But that's a bit wordy and I'm not sure
> whether it's better than `ArcBorrow<'_, T>`...

It should be sufficient to change only the type of the field
within `WithRef`, so no visible API change needed.

--
Cheers,
Benno