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Mark OnceState::poison as pub #509

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GrigorenkoPV opened this issue Dec 19, 2024 · 1 comment
Open

Mark OnceState::poison as pub #509

GrigorenkoPV opened this issue Dec 19, 2024 · 1 comment
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api-change-proposal A proposal to add or alter unstable APIs in the standard libraries T-libs-api

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@GrigorenkoPV
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GrigorenkoPV commented Dec 19, 2024

Proposal

To quote the tracking issue (I believe it describes the motivating use case as well as the alternatives)

The Once synchronization primitive provides a poison method for marking the internal OnceState as poisoned, which is combined with the call_once_force method in OnceLock::initialize to call initialization functions until one succeeds without panicking. This provides a way for users to use fallible initialization functions where they only observe the OnceLock being initialized once a function doesn't panic:

fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
    F: FnOnce() -> Result<T, E>,
{
    let mut res: Result<(), E> = Ok(());
    let slot = &self.value;
    // Ignore poisoning from other threads
    // If another thread panics, then we'll be able to run our closure
    self.once.call_once_force(|p| {
        match f() {
            Ok(value) => {
                unsafe { (&mut *slot.get()).write(value) };
            }
            Err(e) => {
                res = Err(e);
                // Treat the underlying `Once` as poisoned since we
                // failed to initialize our value.
                p.poison();
            }
        }
    });
    res
}

This tracking issue is for marking the poison method on OnceState as pub, rather than pub(crate). This has no impact on the OnceCell or OnceLock APIs, but allows downstream libraries to build out similar functionality. The motivation for this is for the twice-cell crate, where use of this API would simplify the implementation.

Public API

// std::sync::once

pub struct OnceState {
    pub(crate) inner: sys::OnceState,
}

impl OnceState {
     /// Poison the associated [`Once`] without explicitly panicking.
    #[inline]
    pub fn poison(&self) {
        self.inner.poison();
    }
}

Links and related work

@GrigorenkoPV GrigorenkoPV added api-change-proposal A proposal to add or alter unstable APIs in the standard libraries T-libs-api labels Dec 19, 2024
@programmerjake
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the link is broken, you want https://crates.io/crates/twice-cell

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Labels
api-change-proposal A proposal to add or alter unstable APIs in the standard libraries T-libs-api
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