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

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Feb 16, 2026 at 11:26:11AM +0000, Alice Ryhl wrote:
> On Mon, Feb 16, 2026 at 12:10:16PM +0100, Andreas Hindborg wrote:
> > "Alice Ryhl" <aliceryhl@google.com> writes:
> > 
> > > On Sun, Feb 15, 2026 at 09:27:17PM +0100, 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.
> > >>
> > >> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
> > >
> > >> +    /// Get a reference to the contained object, or populate the [`SetOnce`]
> > >> +    /// with the value returned by `callable` and return a reference to that
> > >> +    /// object.
> > >> +    pub fn as_ref_or_populate_with(&self, callable: impl FnOnce() -> Result<T>) -> Result<&T> {
> > >> +        if !self.populate_with(callable)? {
> > >> +            while self.init.load(Acquire) != 2 {
> > >> +                core::hint::spin_loop();
> > >> +            }
> > >
> > > We should not be implementing our own spinlocks.
> > 
> > That is a great proverb. I'd be happy to receive a suggestion on an
> > alternate approach for this particular context.
> 
> You can add a spinlock to SetOnce. Like I mentioned previously [1],
> support for waiting will require the addition of extra fields.

By the way, back then I suggested renaming it from OnceLock to SetOnce
because you did not support waiting for the value to be populated, and
you said you didn't need that. If you add that feature, then we should
rename it back to OnceLock, or create a new type OnceLock for users who
need that additional feature.

Alice