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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:11:38 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-17 12:13:23 +0000 |
commit | 20431706a863f92cb37dc512fef6e48d192aaf2c (patch) | |
tree | 2867f13f5fd5437ba628c67d7f87309ccadcd286 /library/std/src/sys_common/once/mod.rs | |
parent | Releasing progress-linux version 1.65.0+dfsg1-2~progress7.99u1. (diff) | |
download | rustc-20431706a863f92cb37dc512fef6e48d192aaf2c.tar.xz rustc-20431706a863f92cb37dc512fef6e48d192aaf2c.zip |
Merging upstream version 1.66.0+dfsg1.
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | library/std/src/sys_common/once/mod.rs | 43 |
1 files changed, 43 insertions, 0 deletions
diff --git a/library/std/src/sys_common/once/mod.rs b/library/std/src/sys_common/once/mod.rs new file mode 100644 index 000000000..8742e68cc --- /dev/null +++ b/library/std/src/sys_common/once/mod.rs @@ -0,0 +1,43 @@ +// 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}; + } +} |