thread_local_key.rs

//! OS-based thread local storage
//!
//! This module provides an implementation of OS-based thread local storage,
//! using the native OS-provided facilities (think `TlsAlloc` or
//! `pthread_setspecific`). The interface of this differs from the other types
//! of thread-local-storage provided in this crate in that OS-based TLS can only
//! get/set pointer-sized data, possibly with an associated destructor.
//!
//! This module also provides two flavors of TLS. One is intended for static
//! initialization, and does not contain a `Drop` implementation to deallocate
//! the OS-TLS key. The other is a type which does implement `Drop` and hence
//! has a safe interface.
//!
//! # Usage
//!
//! This module should likely not be used directly unless other primitives are
//! being built on. Types such as `thread_local::spawn::Key` are likely much
//! more useful in practice than this OS-based version which likely requires
//! unsafe code to interoperate with.
//!
//! # Examples
//!
//! Using a dynamically allocated TLS key. Note that this key can be shared
//! among many threads via an `Arc`.
//!
//! ```ignore (cannot-doctest-private-modules)
//! let key = Key::new(None);
//! assert!(key.get().is_null());
//! key.set(1 as *mut u8);
//! assert!(!key.get().is_null());
//!
//! drop(key); // deallocate this TLS slot.
//! ```
//!
//! Sometimes a statically allocated key is either required or easier to work
//! with, however.
//!
//! ```ignore (cannot-doctest-private-modules)
//! static KEY: StaticKey = INIT;
//!
//! unsafe {
//!     assert!(KEY.get().is_null());
//!     KEY.set(1 as *mut u8);
//! }
//! ```

#![allow(non_camel_case_types)]
#![unstable(feature = "thread_local_internals", issue = "none")]
#![allow(dead_code)]

#[cfg(test)]
mod tests;

use crate::sync::atomic::{self, AtomicUsize, Ordering};
use crate::sys::thread_local_key as imp;

/// A type for TLS keys that are statically allocated.
///
/// This type is entirely `unsafe` to use as it does not protect against
/// use-after-deallocation or use-during-deallocation.
///
/// The actual OS-TLS key is lazily allocated when this is used for the first
/// time. The key is also deallocated when the Rust runtime exits or `destroy`
/// is called, whichever comes first.
///
/// # Examples
///
/// ```ignore (cannot-doctest-private-modules)
/// use tls::os::{StaticKey, INIT};
///
/// // Use a regular global static to store the key.
/// static KEY: StaticKey = INIT;
///
/// // The state provided via `get` and `set` is thread-local.
/// unsafe {
///     assert!(KEY.get().is_null());
///     KEY.set(1 as *mut u8);
/// }
/// ```
pub struct StaticKey {
    /// Inner static TLS key (internals).
    key: AtomicUsize,
    /// Destructor for the TLS value.
    ///
    /// See `Key::new` for information about when the destructor runs and how
    /// it runs.
    dtor: Option<unsafe extern "C" fn(*mut u8)>,
}

/// Constant initialization value for static TLS keys.
///
/// This value specifies no destructor by default.
pub const INIT: StaticKey = StaticKey::new(None);

// Define a sentinel value that is likely not to be returned
// as a TLS key.
#[cfg(not(target_os = "nto"))]
const KEY_SENTVAL: usize = 0;
// On QNX Neutrino, 0 is always returned when currently not in use.
// Using 0 would mean to always create two keys and remote the first
// one (with value of 0) immediately afterwards.
#[cfg(target_os = "nto")]
const KEY_SENTVAL: usize = libc::PTHREAD_KEYS_MAX + 1;

impl StaticKey {
    #[rustc_const_unstable(feature = "thread_local_internals", issue = "none")]
    pub const fn new(dtor: Option<unsafe extern "C" fn(*mut u8)>) -> StaticKey {
        StaticKey { key: atomic::AtomicUsize::new(KEY_SENTVAL), dtor }
    }

    /// Gets the value associated with this TLS key
    ///
    /// This will lazily allocate a TLS key from the OS if one has not already
    /// been allocated.
    #[inline]
    pub unsafe fn get(&self) -> *mut u8 {
        imp::get(self.key())
    }

    /// Sets this TLS key to a new value.
    ///
    /// This will lazily allocate a TLS key from the OS if one has not already
    /// been allocated.
    #[inline]
    pub unsafe fn set(&self, val: *mut u8) {
        imp::set(self.key(), val)
    }

    #[inline]
    unsafe fn key(&self) -> imp::Key {
        match self.key.load(Ordering::Acquire) {
            KEY_SENTVAL => self.lazy_init() as imp::Key,
            n => n as imp::Key,
        }
    }

    unsafe fn lazy_init(&self) -> usize {
        // POSIX allows the key created here to be KEY_SENTVAL, but the compare_exchange
        // below relies on using KEY_SENTVAL as a sentinel value to check who won the
        // race to set the shared TLS key. As far as I know, there is no
        // guaranteed value that cannot be returned as a posix_key_create key,
        // so there is no value we can initialize the inner key with to
        // prove that it has not yet been set. As such, we'll continue using a
        // value of KEY_SENTVAL, but with some gyrations to make sure we have a non-KEY_SENTVAL
        // value returned from the creation routine.
        // FIXME: this is clearly a hack, and should be cleaned up.
        let key1 = imp::create(self.dtor);
        let key = if key1 as usize != KEY_SENTVAL {
            key1
        } else {
            let key2 = imp::create(self.dtor);
            imp::destroy(key1);
            key2
        };
        rtassert!(key as usize != KEY_SENTVAL);
        match self.key.compare_exchange(
            KEY_SENTVAL,
            key as usize,
            Ordering::Release,
            Ordering::Acquire,
        ) {
            // The CAS succeeded, so we've created the actual key
            Ok(_) => key as usize,
            // If someone beat us to the punch, use their key instead
            Err(n) => {
                imp::destroy(key);
                n
            }
        }
    }
}