monoio 0.2.4

// Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Scoped thread-local storage
//!
//! This module provides the ability to generate *scoped* thread-local
//! variables. In this sense, scoped indicates that thread local storage
//! actually stores a reference to a value, and this reference is only placed
//! in storage for a scoped amount of time.
//!
//! There are no restrictions on what types can be placed into a scoped
//! variable, but all scoped variables are initialized to the equivalent of
//! null. Scoped thread local storage is useful when a value is present for a known
//! period of time and it is not required to relinquish ownership of the
//! contents.

#![deny(missing_docs, warnings)]

use std::{cell::Cell, marker, thread::LocalKey};

/// The macro. See the module level documentation for the description and examples.
#[macro_export]
macro_rules! scoped_thread_local {
    ($(#[$attrs:meta])* $vis:vis static $name:ident: $ty:ty) => (
        $(#[$attrs])*
        $vis static $name: $crate::macros::scoped_tls::ScopedKey<$ty> = $crate::macros::scoped_tls::ScopedKey {
            inner: {
                thread_local!(static FOO: ::std::cell::Cell<*const ()> = {
                    ::std::cell::Cell::new(::std::ptr::null())
                });
                &FOO
            },
            _marker: ::std::marker::PhantomData,
        };
    )
}

/// Type representing a thread local storage key corresponding to a reference
/// to the type parameter `T`.
///
/// Keys are statically allocated and can contain a reference to an instance of
/// type `T` scoped to a particular lifetime. Keys provides two methods, `set`
/// and `with`, both of which currently use closures to control the scope of
/// their contents.
pub struct ScopedKey<T> {
    #[doc(hidden)]
    pub inner: &'static LocalKey<Cell<*const ()>>,
    #[doc(hidden)]
    pub _marker: marker::PhantomData<T>,
}

unsafe impl<T> Sync for ScopedKey<T> {}

impl<T> ScopedKey<T> {
    /// Inserts a value into this scoped thread local storage slot for a
    /// duration of a closure.
    ///
    /// While `cb` is running, the value `t` will be returned by `get` unless
    /// this function is called recursively inside of `cb`.
    ///
    /// Upon return, this function will restore the previous value, if any
    /// was available.
    pub fn set<F, R>(&'static self, t: &T, f: F) -> R
    where
        F: FnOnce() -> R,
    {
        struct Reset {
            key: &'static LocalKey<Cell<*const ()>>,
            val: *const (),
        }
        impl Drop for Reset {
            fn drop(&mut self) {
                self.key.with(|c| c.set(self.val));
            }
        }
        let prev = self.inner.with(|c| {
            let prev = c.get();
            c.set(t as *const T as *const ());
            prev
        });
        let _reset = Reset {
            key: self.inner,
            val: prev,
        };
        f()
    }

    /// Gets a value out of this scoped variable.
    ///
    /// This function takes a closure which receives the value of this
    /// variable.
    pub fn with<F, R>(&'static self, f: F) -> R
    where
        F: FnOnce(&T) -> R,
    {
        let val = self.inner.with(|c| c.get());
        assert!(
            !val.is_null(),
            "cannot access a scoped thread local variable without calling `set` first"
        );
        unsafe { f(&*(val as *const T)) }
    }

    /// Gets a value out of this scoped variable.
    pub fn try_with<F, R>(&'static self, f: F) -> R
    where
        F: FnOnce(Option<&T>) -> R,
    {
        let val = self.inner.with(|c| c.get());
        if val.is_null() {
            f(None)
        } else {
            unsafe { f(Some(&*(val as *const T))) }
        }
    }

    /// Test whether this TLS key has been `set` for the current thread.
    #[inline]
    pub fn is_set(&'static self) -> bool {
        self.inner.with(|c| !c.get().is_null())
    }
}