Vendor things

third-party/vendor/crossbeam-epoch/src/lib.rs

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+//! Epoch-based memory reclamation.
+//!
+//! An interesting problem concurrent collections deal with comes from the remove operation.
+//! Suppose that a thread removes an element from a lock-free map, while another thread is reading
+//! that same element at the same time. The first thread must wait until the second thread stops
+//! reading the element. Only then it is safe to destruct it.
+//!
+//! Programming languages that come with garbage collectors solve this problem trivially. The
+//! garbage collector will destruct the removed element when no thread can hold a reference to it
+//! anymore.
+//!
+//! This crate implements a basic memory reclamation mechanism, which is based on epochs. When an
+//! element gets removed from a concurrent collection, it is inserted into a pile of garbage and
+//! marked with the current epoch. Every time a thread accesses a collection, it checks the current
+//! epoch, attempts to increment it, and destructs some garbage that became so old that no thread
+//! can be referencing it anymore.
+//!
+//! That is the general mechanism behind epoch-based memory reclamation, but the details are a bit
+//! more complicated. Anyhow, memory reclamation is designed to be fully automatic and something
+//! users of concurrent collections don't have to worry much about.
+//!
+//! # Pointers
+//!
+//! Concurrent collections are built using atomic pointers. This module provides [`Atomic`], which
+//! is just a shared atomic pointer to a heap-allocated object. Loading an [`Atomic`] yields a
+//! [`Shared`], which is an epoch-protected pointer through which the loaded object can be safely
+//! read.
+//!
+//! # Pinning
+//!
+//! Before an [`Atomic`] can be loaded, a participant must be [`pin`]ned. By pinning a participant
+//! we declare that any object that gets removed from now on must not be destructed just
+//! yet. Garbage collection of newly removed objects is suspended until the participant gets
+//! unpinned.
+//!
+//! # Garbage
+//!
+//! Objects that get removed from concurrent collections must be stashed away until all currently
+//! pinned participants get unpinned. Such objects can be stored into a thread-local or global
+//! storage, where they are kept until the right time for their destruction comes.
+//!
+//! There is a global shared instance of garbage queue. You can [`defer`](Guard::defer) the execution of an
+//! arbitrary function until the global epoch is advanced enough. Most notably, concurrent data
+//! structures may defer the deallocation of an object.
+//!
+//! # APIs
+//!
+//! For majority of use cases, just use the default garbage collector by invoking [`pin`]. If you
+//! want to create your own garbage collector, use the [`Collector`] API.
+
+#![doc(test(
+    no_crate_inject,
+    attr(
+        deny(warnings, rust_2018_idioms),
+        allow(dead_code, unused_assignments, unused_variables)
+    )
+))]
+#![warn(
+    missing_docs,
+    missing_debug_implementations,
+    rust_2018_idioms,
+    unreachable_pub
+)]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(crossbeam_loom)]
+extern crate loom_crate as loom;
+
+#[cfg(crossbeam_loom)]
+#[allow(unused_imports, dead_code)]
+mod primitive {
+    pub(crate) mod cell {
+        pub(crate) use loom::cell::UnsafeCell;
+    }
+    pub(crate) mod sync {
+        pub(crate) mod atomic {
+            pub(crate) use loom::sync::atomic::{fence, AtomicPtr, AtomicUsize, Ordering};
+
+            // FIXME: loom does not support compiler_fence at the moment.
+            // https://github.com/tokio-rs/loom/issues/117
+            // we use fence as a stand-in for compiler_fence for the time being.
+            // this may miss some races since fence is stronger than compiler_fence,
+            // but it's the best we can do for the time being.
+            pub(crate) use self::fence as compiler_fence;
+        }
+        pub(crate) use loom::sync::Arc;
+    }
+    pub(crate) use loom::thread_local;
+}
+#[cfg(target_has_atomic = "ptr")]
+#[cfg(not(crossbeam_loom))]
+#[allow(unused_imports, dead_code)]
+mod primitive {
+    pub(crate) mod cell {
+        #[derive(Debug)]
+        #[repr(transparent)]
+        pub(crate) struct UnsafeCell<T>(::core::cell::UnsafeCell<T>);
+
+        // loom's UnsafeCell has a slightly different API than the standard library UnsafeCell.
+        // Since we want the rest of the code to be agnostic to whether it's running under loom or
+        // not, we write this small wrapper that provides the loom-supported API for the standard
+        // library UnsafeCell. This is also what the loom documentation recommends:
+        // https://github.com/tokio-rs/loom#handling-loom-api-differences
+        impl<T> UnsafeCell<T> {
+            #[inline]
+            pub(crate) const fn new(data: T) -> UnsafeCell<T> {
+                UnsafeCell(::core::cell::UnsafeCell::new(data))
+            }
+
+            #[inline]
+            pub(crate) fn with<R>(&self, f: impl FnOnce(*const T) -> R) -> R {
+                f(self.0.get())
+            }
+
+            #[inline]
+            pub(crate) fn with_mut<R>(&self, f: impl FnOnce(*mut T) -> R) -> R {
+                f(self.0.get())
+            }
+        }
+    }
+    pub(crate) mod sync {
+        pub(crate) mod atomic {
+            pub(crate) use core::sync::atomic::{
+                compiler_fence, fence, AtomicPtr, AtomicUsize, Ordering,
+            };
+        }
+        #[cfg(feature = "alloc")]
+        pub(crate) use alloc::sync::Arc;
+    }
+
+    #[cfg(feature = "std")]
+    pub(crate) use std::thread_local;
+}
+
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+extern crate alloc;
+
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+mod atomic;
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+mod collector;
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+mod deferred;
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+mod epoch;
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+mod guard;
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+mod internal;
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+mod sync;
+
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+#[allow(deprecated)]
+pub use crate::atomic::{CompareAndSetError, CompareAndSetOrdering};
+#[cfg(all(feature = "alloc", target_has_atomic = "ptr"))]
+pub use crate::{
+    atomic::{Atomic, CompareExchangeError, Owned, Pointable, Pointer, Shared},
+    collector::{Collector, LocalHandle},
+    guard::{unprotected, Guard},
+};
+
+#[cfg(feature = "std")]
+mod default;
+#[cfg(feature = "std")]
+pub use crate::default::{default_collector, is_pinned, pin};