Tracking Issue for once_wait

[tgross35]

Feature gate: #![feature(once_wait)]

This is a tracking issue for adding a blocking .wait() method on Once and OnceLock, to allow other threads to synchronize on their completion. This will provide the same interface as once_cell's OnceCell::wait.

Public API

impl Once {
    /// Block the current thread until the `Once` has been initialized.
    fn wait(&self);
    /// Block the current thread until the `Once` has been initialized, ignoring poisoning.
    fn wait_force(&self);
}

impl OnceLock {
    /// Block the current thread until the value is set, then return it.
    fn wait(&self) -> &T;
}

Steps / History

• ACP: Add OnceLock::wait libs-team#405
• Implementation: std: implement the once_wait feature #127567
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• Async API (proposed in the ACP): naming, how it works, interaction with other synchronization primitives since e.g. Mutex does not have async methods. This would probably get its own feature gate.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[PhilipDaniels]

This would be very useful, frankly I was surprised when I ran into the problem of get being non-blocking! Pretty-please can this be moved forward to stabilisation?