From 8dd16259287f58f9273002717ec4d27e97127719 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Wed, 12 Jun 2024 07:43:14 +0200 Subject: Merging upstream version 127.0. Signed-off-by: Daniel Baumann --- third_party/rust/getrandom/src/lazy.rs | 56 ++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) create mode 100644 third_party/rust/getrandom/src/lazy.rs (limited to 'third_party/rust/getrandom/src/lazy.rs') diff --git a/third_party/rust/getrandom/src/lazy.rs b/third_party/rust/getrandom/src/lazy.rs new file mode 100644 index 0000000000..100ce1eaf5 --- /dev/null +++ b/third_party/rust/getrandom/src/lazy.rs @@ -0,0 +1,56 @@ +use core::sync::atomic::{AtomicUsize, Ordering::Relaxed}; + +// This structure represents a lazily initialized static usize value. Useful +// when it is preferable to just rerun initialization instead of locking. +// unsync_init will invoke an init() function until it succeeds, then return the +// cached value for future calls. +// +// unsync_init supports init() "failing". If the init() method returns UNINIT, +// that value will be returned as normal, but will not be cached. +// +// Users should only depend on the _value_ returned by init() functions. +// Specifically, for the following init() function: +// fn init() -> usize { +// a(); +// let v = b(); +// c(); +// v +// } +// the effects of c() or writes to shared memory will not necessarily be +// observed and additional synchronization methods may be needed. +pub(crate) struct LazyUsize(AtomicUsize); + +impl LazyUsize { + pub const fn new() -> Self { + Self(AtomicUsize::new(Self::UNINIT)) + } + + // The initialization is not completed. + pub const UNINIT: usize = usize::max_value(); + + // Runs the init() function at most once, returning the value of some run of + // init(). Multiple callers can run their init() functions in parallel. + // init() should always return the same value, if it succeeds. + pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize { + // Relaxed ordering is fine, as we only have a single atomic variable. + let mut val = self.0.load(Relaxed); + if val == Self::UNINIT { + val = init(); + self.0.store(val, Relaxed); + } + val + } +} + +// Identical to LazyUsize except with bool instead of usize. +pub(crate) struct LazyBool(LazyUsize); + +impl LazyBool { + pub const fn new() -> Self { + Self(LazyUsize::new()) + } + + pub fn unsync_init(&self, init: impl FnOnce() -> bool) -> bool { + self.0.unsync_init(|| init() as usize) != 0 + } +} -- cgit v1.2.3