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// A "once" is a relatively simple primitive, and it's also typically provided
// by the OS as well (see `pthread_once` or `InitOnceExecuteOnce`). The OS
// primitives, however, tend to have surprising restrictions, such as the Unix
// one doesn't allow an argument to be passed to the function.
//
// As a result, we end up implementing it ourselves in the standard library.
// This also gives us the opportunity to optimize the implementation a bit which
// should help the fast path on call sites.
//
// So to recap, the guarantees of a Once are that it will call the
// initialization closure at most once, and it will never return until the one
// that's running has finished running. This means that we need some form of
// blocking here while the custom callback is running at the very least.
// Additionally, we add on the restriction of **poisoning**. Whenever an
// initialization closure panics, the Once enters a "poisoned" state which means
// that all future calls will immediately panic as well.
//
// So to implement this, one might first reach for a `Mutex`, but those cannot
// be put into a `static`. It also gets a lot harder with poisoning to figure
// out when the mutex needs to be deallocated because it's not after the closure
// finishes, but after the first successful closure finishes.
//
// All in all, this is instead implemented with atomics and lock-free
// operations! Whee!
cfg_if::cfg_if! {
if #[cfg(any(
target_os = "linux",
target_os = "android",
all(target_arch = "wasm32", target_feature = "atomics"),
target_os = "freebsd",
target_os = "openbsd",
target_os = "dragonfly",
target_os = "fuchsia",
target_os = "hermit",
))] {
mod futex;
pub use futex::{Once, OnceState};
} else {
mod generic;
pub use generic::{Once, OnceState};
}
}
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