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//! Per-object thread local storage. //! //! The `ThreadLocal` type which stores a distinct (nullable) value of some type for each thread //! that accesses it. //! //! A thread's values are destroyed when it exits, but the values associated with a `ThreadLocal` //! instance are not destroyed when it is dropped. These are in some ways the opposite semantics of //! those provided by the `thread_local` crate, where values are cleaned up when a `ThreadLocal` //! object is dropped, but not when individual threads exit. //! //! Because of this, this crate is an appropriate choice for use cases where you have long lived //! `ThreadLocal` instances which are widely shared among threads that are created and destroyed //! through the runtime of a program, while the `thread_local` crate is an appropriate choice for //! short lived values. //! //! # Examples //! //! ```rust //! use std::sync::Arc; //! use std::thread; //! use thread_local_object::ThreadLocal; //! //! let tls = Arc::new(ThreadLocal::new()); //! //! tls.set(1); //! //! let tls2 = tls.clone(); //! thread::spawn(move || { //! // the other thread doesn't see the 1 //! assert_eq!(tls2.get_cloned(), None); //! tls2.set(2); //! }).join().unwrap(); //! //! // we still see our original value //! assert_eq!(tls.get_cloned(), Some(1)); //! ``` #![warn(missing_docs)] #![doc(html_root_url="https://docs.rs/thread-local-object/0.1.0")] extern crate unsafe_any; use std::cell::RefCell; use std::collections::hash_map::{self, HashMap}; use std::marker::PhantomData; use std::mem; use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering}; use unsafe_any::UnsafeAny; thread_local! { static VALUES: RefCell<HashMap<usize, Box<UnsafeAny>>> = RefCell::new(HashMap::new()); } static NEXT_ID: AtomicUsize = ATOMIC_USIZE_INIT; // if IDs ever wrap around we'll run into soundness issues with downcasts, so panic if we're out of // IDs. On 64 bit platforms this can literally never happen (it'd take 584 years even if you were // generating a billion IDs per second), but is more realistic a concern on 32 bit platforms. // // FIXME use AtomicU64 when it's stable fn next_id() -> usize { let mut id = NEXT_ID.load(Ordering::SeqCst); loop { assert!(id != usize::max_value(), "thread local ID overflow"); let old = id; id = NEXT_ID.compare_and_swap(old, old + 1, Ordering::SeqCst); if id == old { return id; } } } /// A thread local variable wrapper. pub struct ThreadLocal<T: 'static> { id: usize, _p: PhantomData<T>, } impl<T: 'static> ThreadLocal<T> { /// Creates a new `ThreadLocal` with no values for any threads. /// /// # Panics /// /// Panics if more than `usize::max_value()` `ThreadLocal` objects have already been created. /// This can only ever realistically happen on 32 bit platforms. pub fn new() -> ThreadLocal<T> { ThreadLocal { id: next_id(), _p: PhantomData, } } /// Sets this thread's value, returning the previous value if present. /// /// # Panics /// /// Panics if called from within the execution of a closure provided to another method on this /// value. pub fn set(&self, value: T) -> Option<T> { self.entry(|e| match e { Entry::Occupied(mut e) => Some(e.insert(value)), Entry::Vacant(e) => { e.insert(value); None } }) } /// Removes this thread's value, returning it if it existed. /// /// # Panics /// /// Panics if called from within the execution of a closure provided to another method on this /// value. pub fn remove(&self) -> Option<T> { VALUES.with(|v| { v.borrow_mut() .remove(&self.id) .map(|v| unsafe { *v.downcast_unchecked::<T>() }) }) } /// Passes a handle to the current thread's value to a closure for in-place manipulation. /// /// The closure is required for the same soundness reasons it is required for the standard /// library's `thread_local!` values. /// /// # Panics /// /// Panics if called from within the execution of a closure provided to another method on this /// value. pub fn entry<F, R>(&self, f: F) -> R where F: FnOnce(Entry<T>) -> R { VALUES.with(|v| { let mut v = v.borrow_mut(); let entry = match v.entry(self.id) { hash_map::Entry::Occupied(e) => Entry::Occupied(OccupiedEntry(e, PhantomData)), hash_map::Entry::Vacant(e) => Entry::Vacant(VacantEntry(e, PhantomData)), }; f(entry) }) } /// Passes a mutable reference to the current thread's value to a closure. /// /// The closure is required for the same soundness reasons it is required for the standard /// library's `thread_local!` values. /// /// # Panics /// /// Panics if called from within the execution of a closure passed to `entry` or `get_mut` on /// this value. pub fn get<F, R>(&self, f: F) -> R where F: FnOnce(Option<&T>) -> R { VALUES.with(|v| { let v = v.borrow(); let value = v.get(&self.id) .map(|v| unsafe { v.downcast_ref_unchecked() }); f(value) }) } /// Passes a mutable reference to the current thread's value to a closure. /// /// The closure is required for the same soundness reasons it is required for the standard /// library's `thread_local!` values. /// /// # Panics /// /// Panics if called from within the execution of a closure provided to another method on this /// value. pub fn get_mut<F, R>(&self, f: F) -> R where F: FnOnce(Option<&mut T>) -> R { VALUES.with(|v| { let mut v = v.borrow_mut(); let value = v.get_mut(&self.id) .map(|v| unsafe { v.downcast_mut_unchecked() }); f(value) }) } } impl<T> ThreadLocal<T> where T: 'static + Clone { /// Returns a copy of the current thread's value. /// /// # Panics /// /// Panics if called from within the execution of a closure passed to `entry` or `get_mut` on /// this value. pub fn get_cloned(&self) -> Option<T> { VALUES.with(|v| { v.borrow() .get(&self.id) .map(|v| unsafe { v.downcast_ref_unchecked::<T>().clone() }) }) } } /// A view into a thread's slot in a `ThreadLocal` that may be empty. pub enum Entry<'a, T: 'static> { /// An occupied entry. Occupied(OccupiedEntry<'a, T>), /// A vacant entry. Vacant(VacantEntry<'a, T>), } impl<'a, T: 'static> Entry<'a, T> { /// Ensures a value is in the entry by inserting the default if it is empty, and returns a /// mutable reference to the value in the entry. pub fn or_insert(self, default: T) -> &'a mut T { match self { Entry::Occupied(e) => e.into_mut(), Entry::Vacant(e) => e.insert(default), } } /// Ensures a value is in the entry by inserting the result of the default function if it is /// empty, and returns a mutable reference to the value in the entry. pub fn or_insert_with<F>(self, default: F) -> &'a mut T where F: FnOnce() -> T { match self { Entry::Occupied(e) => e.into_mut(), Entry::Vacant(e) => e.insert(default()), } } } /// A view into a thread's slot in a `ThreadLocal` which is occupied. pub struct OccupiedEntry<'a, T: 'static>(hash_map::OccupiedEntry<'a, usize, Box<UnsafeAny>>, PhantomData<&'a mut T>); impl<'a, T: 'static> OccupiedEntry<'a, T> { /// Returns a reference to the value in the entry. pub fn get(&self) -> &T { unsafe { self.0.get().downcast_ref_unchecked() } } /// Returns a mutable reference to the value in the entry. pub fn get_mut(&mut self) -> &mut T { unsafe { self.0.get_mut().downcast_mut_unchecked() } } /// Converts an `OccupiedEntry` into a mutable reference to the value in the entry with a /// lifetime bound of the slot itself. pub fn into_mut(self) -> &'a mut T { unsafe { self.0.into_mut().downcast_mut_unchecked() } } /// Sets the value of the entry, and returns the entry's old value. pub fn insert(&mut self, value: T) -> T { mem::replace(self.get_mut(), value) } /// Takes the value out of the entry, and returns it. pub fn remove(self) -> T { unsafe { *self.0.remove().downcast_unchecked() } } } /// A view into a thread's slot in a `ThreadLocal` which is unoccupied. pub struct VacantEntry<'a, T: 'static>(hash_map::VacantEntry<'a, usize, Box<UnsafeAny>>, PhantomData<&'a mut T>); impl<'a, T: 'static> VacantEntry<'a, T> { /// Sets the value of the entry, and returns a mutable reference to it. pub fn insert(self, value: T) -> &'a mut T { unsafe { self.0.insert(Box::new(value)).downcast_mut_unchecked() } } }