src/lib.rs

//! 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)]
#![cfg_attr(not(feature = "std"), no_std)]
#![cfg_attr(feature = "nightly", feature(cfg_target_has_atomic))]
#![cfg_attr(feature = "nightly", feature(const_fn))]
// matches! requires Rust 1.42
#![allow(clippy::match_like_matches_macro)]

use cfg_if::cfg_if;

#[cfg_attr(feature = "nightly", cfg(target_has_atomic = "ptr"))]
cfg_if! {
    if #[cfg(feature = "alloc")] {
        extern crate alloc;

        mod atomic;
        mod collector;
        mod deferred;
        mod epoch;
        mod guard;
        mod internal;
        mod sync;

        pub use self::atomic::{Pointable, Atomic, CompareAndSetError, CompareAndSetOrdering, Owned, Pointer, Shared};
        pub use self::collector::{Collector, LocalHandle};
        pub use self::guard::{unprotected, Guard};
    }
}

cfg_if! {
    if #[cfg(feature = "std")] {
        mod default;
        pub use self::default::{default_collector, is_pinned, pin};
    }
}