Re: [PATCH] rust: sync: add lazy initialization methods to SetOnce

["Gary Guo" <gary@garyguo.net>]

On Tue May 12, 2026 at 9:07 AM BST, Andreas Hindborg wrote:
> Andreas Hindborg <a.hindborg@kernel.org> writes:
>
>> "Gary Guo" <gary@garyguo.net> writes:
>>
>>> On Sun Feb 15, 2026 at 8:27 PM GMT, Andreas Hindborg wrote:
>>>> Add methods to get a reference to the contained value or populate the
>>>> SetOnce if empty. The new `as_ref_or_populate` method accepts a value
>>>> directly, while `as_ref_or_populate_with` accepts a fallible closure,
>>>> allowing for lazy initialization that may fail. Both methods spin-wait
>>>> if another thread is concurrently initializing the container.
>>>>
>>>> Also add `populate_with` which takes a fallible closure and serves as
>>>> the implementation basis for the other populate methods.
>>>
>>> Hi Andreas, in an earlier call I mentioned that I'm working on getting SetOnce
>>> to work with pin-init, the capability of which I think is a superset of you have
>>> here.
>>>
>>> The API I have is
>>>
>>> impl<T> SetOnce<T> {
>>>     pub fn init<E>(&self, init: impl Init<T, E>) -> Result<&T, InitError<E>>;
>>>     pub fn pin_init<E>(self, Pin<&Self>, init: impl PinInit<T, E>) -> Result<&T, InitError<E>>;
>>> }
>>>
>>> To achieve what you need with a function, you can simply write:
>>>
>>>     set_once.init(pin_init::init_scope(your_fn))
>>>
>>> The patch that implement the API is here:
>>> https://github.com/nbdd0121/linux/commit/4aabdbcf20b11626c253f203745b1d55c37ab2ee
>>> in tree
>>> https://github.com/nbdd0121/linux/tree/lazy_revocable_nova_wip/
>>>
>>> which I haven't submitted to the list as the user side of this API isn't ready.
>>
>> I probably evicted that from cache.
>>
>> It looks like that could replace the `populate` in my patch? We would
>> still need the synchronization in `as_ref_or_populate`, right? (or
>> `as_ref_or_init` if you will)
>
> Looks like this is not ready yet. I think we can move forward with the
> current suggestion and then wire up pin-init when it lands.
>

Alvin's been carrying the patch in his series:
https://lore.kernel.org/rust-for-linux/20260326-b4-tyr-debugfs-v1-1-074badd18716@linux.dev/

Best,
Gary