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/* * Copyright (c) Facebook, Inc. and its affiliates. * * This source code is licensed under both the MIT license found in the * LICENSE-MIT file in the root directory of this source tree and the Apache * License, Version 2.0 found in the LICENSE-APACHE file in the root directory * of this source tree. */ //! Provides `ThreadMap` structure for accessing `PerThread` thread local variables from a static //! context via `ThreadMap::for_each`. //! //! Notes: //! If we wanted to do a global accumulator for the per-thread stats we'd need to: //! //! 1. define a counter/stat type. It needs to be Sync to satisfy PerThread/ThreadMap's //! constraints. //! 2. set up a periodic thread/process to enumerate all the stats and accumulate them //! 3. give the stat type a Drop implementation which also updates the accumulator, so that stats //! are recorded when the thread dies (otherwise it loses stats after the last accumulator pass, //! and short-lived threads may never record stats at all) //! //! Examples: //! ``` //! use lazy_static::lazy_static; //! use perthread::{PerThread, ThreadMap}; //! //! // Set up the map of per-thread counters //! lazy_static! { //! static ref COUNTERS: ThreadMap<usize> = ThreadMap::default(); //! } //! //! // Declare a specific per-thread counter //! thread_local! { //! static COUNTER: PerThread<usize> = COUNTERS.register(0); //! } //! //! fn main() { //! COUNTER.with(|c| println!("COUNTER: {:?}", *c)); //! } //! ``` #![deny(warnings, missing_docs, clippy::all, broken_intra_doc_links)] use std::collections::HashMap; use std::fmt::{self, Debug}; use std::ops::Deref; use std::ptr::NonNull; use std::sync::Mutex; #[derive(Debug, Eq, PartialEq, Hash, Copy, Clone)] struct Handle(usize); /// This is a structure that lets you define a map with thread local variables, /// but also gives you access to all them behind a Mutex. pub struct ThreadMap<T> { inner: Mutex<ThreadMapLocked<T>>, } struct ThreadMapLocked<T> { idx: usize, map: HashMap<Handle, NonNull<T>>, } // The raw pointer makes ThreadMapLocked<T> not automatically Send which makes // Mutex<ThreadMapLocked<T>> neither Send nor Sync. // // The mutation of ThreadMapLocked are all synchronized by a mutex so sending or // sharing the mutex across threads won't cause data races. An empty map is fine // to send or share regardless of T. A nonempty map can only exist if T: Sync // because ThreadMap::register enforces that bound. Having T: Sync gives us // permission to share references to T across threads by either sending or // sharing the map; after the map is sent or shared the caller may use // ThreadMap::for_each to access &T from the other thread. unsafe impl<T> Send for ThreadMap<T> {} unsafe impl<T> Sync for ThreadMap<T> {} impl<T> Default for ThreadMap<T> { fn default() -> Self { Self { inner: Mutex::new(ThreadMapLocked { idx: 0, map: HashMap::new(), }), } } } impl<T> ThreadMap<T> { /// Register a new per-thread value with the thread map. pub fn register(&'static self, val: T) -> PerThread<T> where T: 'static + Sync, { // Does not require T: Send because ownership of T remains on its // original thread. The caller is free to move ownership of PerThread<T> // to a different thread themselves later. But that operation requires // PerThread<T>: Send which requires T: Send. let mut storage = Box::new(StableStorage { val, handle: Handle(0), // replaced below after locking map map: NonNull::from(self), }); let mut locked = self.inner.lock().expect("poisoned lock"); storage.handle = Handle(locked.idx); locked .map .insert(storage.handle, NonNull::from(&storage.val)); locked.idx += 1; // Beginning here, PerThread's Drop implementation is in charge of // removing the entry from the map. // // It's possible and legal for the caller to leak their PerThread<T> // without dropping it. In that case the inner StableStorage<T> will // continue to exist indefinitely which isn't a memory safety violation. PerThread { storage } } /// Enumerate the per-thread values /// /// This can't be an iterator because we need to control /// the lifetime of the returned reference, which is limited to the time /// we're holding the mutex (which is what's protecting the value from being /// destroyed while we're using it). pub fn for_each<F>(&self, mut cb: F) where // Note that we require the caller's closure to accept references with // an arbitrarily short lifetime. The trait bound `FnMut(&T)` is really // a higher-rank trait bound equivalent to `for<'r> FnMut(&'r T)`. The // values passed to the closure do not necessarily live as long as the // map does. In particular, the signature of for_each is *not* the same // as `fn for_each<'a, F>(&'a self, cb: F) where F: FnMut(&'a T)`. That // signature would be unsound! See comments in D13453346 for an example // of safe code triggering use-after-free if that were the signature. F: FnMut(&T), { let locked = self.inner.lock().expect("lock poisoned"); for val in locked.map.values() { cb(unsafe { val.as_ref() }); } } fn unregister(&self, h: Handle) { let mut locked = self.inner.lock().expect("poisoned lock"); locked.map.remove(&h); } } /// Values inserted into the map are returned to the caller inside this wrapper. /// The caller will hold on to this wrapper as long as they like, then when they /// drop it the corresponding entry is removed from the map. /// /// The map data structure holds a pointer NonNull<T> to the content of the /// storage box. We must not expose an API through which the owner of a /// PerThread<T> could invalidate that pointer, for example by moving the content /// or dropping the content outside of PerThread's Drop impl. pub struct PerThread<T> { storage: Box<StableStorage<T>>, } struct StableStorage<T> { val: T, handle: Handle, // Effectively &'static ThreadMap<T>. We enforce in ThreadMap::register that // T: 'static. We could use &'static ThreadMap<T> here as the field type but // then Rust would require an explicit `T: 'static` on every data structure // that transitively contains a generic PerThread<T>. Instead this approach // lets us require that bound only for producing any ThreadMap<T> i.e. we // know a PerThread<T> can only exist if T: 'static even without spelling // out that bound everywhere. map: NonNull<ThreadMap<T>>, } // Required in order for PerThread<T> to be held in a lazy_static in the single // threaded use case. The main motivating use case for ThreadMap and PerThread // does not involve sharing PerThread<T> instances across threads, but the // design is compatible with that and it is safe to do so. // // Sending PerThread<T> to another thread requires T: Send. The primary // situation of a type that is Sync but not Send is when an object needs to be // destroyed on the same thread that created it, for example because the Drop // impl accesses thread local storage. For such types it would not be safe to // send PerThread<T> either. // // Sync requires no further bounds because the T is already shared across // threads -- any thread can obtain a reference to the T through // ThreadMap::for_each. unsafe impl<T: Send> Send for StableStorage<T> {} unsafe impl<T> Sync for StableStorage<T> {} impl<T> Debug for PerThread<T> where T: Debug, { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { self.storage.val.fmt(fmt) } } impl<T> Drop for PerThread<T> { fn drop(&mut self) { let map = unsafe { self.storage.map.as_ref() }; map.unregister(self.storage.handle) } } impl<T> AsRef<T> for PerThread<T> { fn as_ref(&self) -> &T { &self.storage.val } } // Do not implement AsMut or DerefMut. Must not expose any accessor from // PerThread<T> to &mut T because some other thread may be iterating inside of // ThreadMap::for_each while this thread's exclusive reference would exist. An // exclusive reference here and a shared reference inside of for_each must not // be possible to exist at the same time. impl<T> Deref for PerThread<T> { type Target = T; fn deref(&self) -> &T { self.as_ref() } } #[cfg(test)] mod tests { use super::*; use lazy_static::lazy_static; use std::collections::HashSet; use std::hash::Hash; fn assert_map_content<T>(map: &ThreadMap<T>, expected: &HashSet<T>) where T: Clone + fmt::Debug + Hash + Eq, { let mut set = HashSet::new(); map.for_each(|el| assert!(set.insert(el.clone()))); assert_eq!(&set, expected); } #[test] fn test_single_thread() { lazy_static! { static ref TEST_MAP: ThreadMap<i64> = ThreadMap::default(); static ref TEST_VAL1: PerThread<i64> = TEST_MAP.register(42); static ref TEST_VAL2: PerThread<i64> = TEST_MAP.register(431); } let mut expected_values = HashSet::new(); assert_map_content(&*TEST_MAP, &expected_values); assert_eq!(**TEST_VAL1, 42); expected_values.insert(**TEST_VAL1); assert_map_content(&*TEST_MAP, &expected_values); assert_eq!(**TEST_VAL2, 431); expected_values.insert(**TEST_VAL2); assert_map_content(&*TEST_MAP, &expected_values); } #[test] fn test_integration_with_thread_local() { use std::sync::mpsc::sync_channel; struct Ack; lazy_static! { static ref TEST_MAP: ThreadMap<i64> = ThreadMap::default(); } thread_local! { static TEST_VAL1: PerThread<i64> = TEST_MAP.register(7); static TEST_VAL2: PerThread<i64> = TEST_MAP.register(42); } let (sender, receiver) = sync_channel(0); let (r_sender, r_receiver) = sync_channel(0); let test_thread = ::std::thread::spawn(move || { receiver.recv().unwrap(); TEST_VAL1.with(|val| assert_eq!(**val, 7)); r_sender.send(Ack).unwrap(); receiver.recv().unwrap(); TEST_VAL2.with(|val| assert_eq!(**val, 42)); r_sender.send(Ack).unwrap(); receiver.recv().unwrap(); }); let mut expected_values = HashSet::new(); assert_map_content(&*TEST_MAP, &expected_values); sender.send(Ack).unwrap(); r_receiver.recv().unwrap(); expected_values.insert(7); assert_map_content(&*TEST_MAP, &expected_values); sender.send(Ack).unwrap(); r_receiver.recv().unwrap(); expected_values.insert(42); assert_map_content(&*TEST_MAP, &expected_values); sender.send(Ack).unwrap(); test_thread.join().unwrap(); expected_values.clear(); assert_map_content(&*TEST_MAP, &expected_values); } }