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use crate::inner::{Inner, Values}; use std::borrow::Borrow; use std::cell; use std::collections::hash_map::RandomState; use std::hash::{BuildHasher, Hash}; use std::iter::{self, FromIterator}; use std::marker::PhantomData; use std::mem; use std::sync::atomic; use std::sync::atomic::AtomicPtr; use std::sync::{self, Arc}; /// A handle that may be used to read from the eventually consistent map. /// /// Note that any changes made to the map will not be made visible until the writer calls /// `refresh()`. In other words, all operations performed on a `ReadHandle` will *only* see writes /// to the map that preceeded the last call to `refresh()`. pub struct ReadHandle<K, V, M = (), S = RandomState> where K: Eq + Hash, S: BuildHasher, { pub(crate) inner: sync::Arc<AtomicPtr<Inner<K, V, M, S>>>, pub(crate) epochs: crate::Epochs, epoch: sync::Arc<sync::atomic::AtomicUsize>, my_epoch: sync::atomic::AtomicUsize, // Since a `ReadHandle` keeps track of its own epoch, it is not safe for multiple threads to // call `with_handle` at the same time. We *could* keep it `Sync` and make `with_handle` // require `&mut self`, but that seems overly excessive. It would also mean that all other // methods on `ReadHandle` would now take `&mut self`, *and* that `ReadHandle` can no longer be // `Clone`. Since optin_builtin_traits is still an unstable feature, we use this hack to make // `ReadHandle` be marked as `!Sync` (since it contains an `Cell` which is `!Sync`). _not_sync_no_feature: PhantomData<cell::Cell<()>>, } /// A type that is both `Sync` and `Send` and lets you produce new [`ReadHandle`] instances. /// /// This serves as a handy way to distribute read handles across many threads without requiring /// additional external locking to synchronize access to the non-`Sync` `ReadHandle` type. Note /// that this _internally_ takes a lock whenever you call [`ReadHandleFactory::handle`], so /// you should not expect producing new handles rapidly to scale well. pub struct ReadHandleFactory<K, V, M = (), S = RandomState> where K: Eq + Hash, S: BuildHasher, { inner: sync::Arc<AtomicPtr<Inner<K, V, M, S>>>, epochs: crate::Epochs, } impl<K, V, M, S> Clone for ReadHandleFactory<K, V, M, S> where K: Eq + Hash, S: BuildHasher, { fn clone(&self) -> Self { Self { inner: sync::Arc::clone(&self.inner), epochs: sync::Arc::clone(&self.epochs), } } } impl<K, V, M, S> ReadHandleFactory<K, V, M, S> where K: Eq + Hash, S: BuildHasher, { /// Produce a new [`ReadHandle`] to the same map as this factory was originally produced from. pub fn handle(&self) -> ReadHandle<K, V, M, S> { ReadHandle::new( sync::Arc::clone(&self.inner), sync::Arc::clone(&self.epochs), ) } } impl<K, V, M, S> Clone for ReadHandle<K, V, M, S> where K: Eq + Hash, S: BuildHasher, { fn clone(&self) -> Self { ReadHandle::new( sync::Arc::clone(&self.inner), sync::Arc::clone(&self.epochs), ) } } pub(crate) fn new<K, V, M, S>( inner: Inner<K, V, M, S>, epochs: crate::Epochs, ) -> ReadHandle<K, V, M, S> where K: Eq + Hash, S: BuildHasher, { let store = Box::into_raw(Box::new(inner)); ReadHandle::new(sync::Arc::new(AtomicPtr::new(store)), epochs) } impl<K, V, M, S> ReadHandle<K, V, M, S> where K: Eq + Hash, S: BuildHasher, { fn new(inner: sync::Arc<AtomicPtr<Inner<K, V, M, S>>>, epochs: crate::Epochs) -> Self { // tell writer about our epoch tracker let epoch = sync::Arc::new(atomic::AtomicUsize::new(0)); epochs.lock().unwrap().push(Arc::clone(&epoch)); Self { epochs, epoch, my_epoch: atomic::AtomicUsize::new(0), inner, _not_sync_no_feature: PhantomData, } } /// Create a new `Sync` type that can produce additional `ReadHandle`s for use in other /// threads. pub fn factory(&self) -> ReadHandleFactory<K, V, M, S> { ReadHandleFactory { inner: sync::Arc::clone(&self.inner), epochs: sync::Arc::clone(&self.epochs), } } } impl<K, V, M, S> ReadHandle<K, V, M, S> where K: Eq + Hash, S: BuildHasher, M: Clone, { fn with_handle<F, T>(&self, f: F) -> Option<T> where F: FnOnce(&Inner<K, V, M, S>) -> T, { // once we update our epoch, the writer can no longer do a swap until we set the MSB to // indicate that we've finished our read. however, we still need to deal with the case of a // race between when the writer reads our epoch and when they decide to make the swap. // // assume that there is a concurrent writer. it just swapped the atomic pointer from A to // B. the writer wants to modify A, and needs to know if that is safe. we can be in any of // the following cases when we atomically swap out our epoch: // // 1. the writer has read our previous epoch twice // 2. the writer has already read our previous epoch once // 3. the writer has not yet read our previous epoch // // let's discuss each of these in turn. // // 1. since writers assume they are free to proceed if they read an epoch with MSB set // twice in a row, this is equivalent to case (2) below. // 2. the writer will see our epoch change, and so will assume that we have read B. it // will therefore feel free to modify A. note that *another* pointer swap can happen, // back to A, but then the writer would be block on our epoch, and so cannot modify // A *or* B. consequently, using a pointer we read *after* the epoch swap is definitely // safe here. // 3. the writer will read our epoch, notice that MSB is not set, and will keep reading, // continuing to observe that it is still not set until we finish our read. thus, // neither A nor B are being modified, and we can safely use either. // // in all cases, using a pointer we read *after* updating our epoch is safe. // so, update our epoch tracker. let epoch = self.my_epoch.fetch_add(1, atomic::Ordering::Relaxed); self.epoch.store(epoch + 1, atomic::Ordering::Release); // ensure that the pointer read happens strictly after updating the epoch atomic::fence(atomic::Ordering::SeqCst); // then, atomically read pointer, and use the map being pointed to let r_handle = self.inner.load(atomic::Ordering::Acquire); let res = unsafe { r_handle.as_ref().map(f) }; // we've finished reading -- let the writer know self.epoch.store( (epoch + 1) | 1usize << (mem::size_of::<usize>() * 8 - 1), atomic::Ordering::Release, ); res } /// Returns the number of non-empty keys present in the map. pub fn len(&self) -> usize { self.with_handle(|inner| inner.data.len()).unwrap_or(0) } /// Returns true if the map contains no elements. pub fn is_empty(&self) -> bool { self.with_handle(|inner| inner.data.is_empty()) .unwrap_or(true) } /// Get the current meta value. pub fn meta(&self) -> Option<M> { self.with_handle(|inner| inner.meta.clone()) } /// Internal version of `get_and` fn get_raw<Q: ?Sized, F, T>(&self, key: &Q, then: F) -> Option<T> where F: FnOnce(&Values<V>) -> T, K: Borrow<Q>, Q: Hash + Eq, { self.with_handle(move |inner| { if !inner.is_ready() { None } else { inner.data.get(key).map(then) } }) .unwrap_or(None) } /// Applies a function to the values corresponding to the key, and returns the result. /// /// The key may be any borrowed form of the map's key type, but `Hash` and `Eq` on the borrowed /// form *must* match those for the key type. /// /// Note that not all writes will be included with this read -- only those that have been /// refreshed by the writer. If no refresh has happened, this function returns `None`. /// /// If no values exist for the given key, no refresh has happened, or the map has been /// destroyed, `then` will not be called, and `None` will be returned. #[inline] pub fn get_and<Q: ?Sized, F, T>(&self, key: &Q, then: F) -> Option<T> where F: FnOnce(&[V]) -> T, K: Borrow<Q>, Q: Hash + Eq, { // call `borrow` here to monomorphize `get_raw` fewer times self.get_raw(key.borrow(), |values| then(&**values)) } /// Applies a function to the values corresponding to the key, and returns the result alongside /// the meta information. /// /// The key may be any borrowed form of the map's key type, but `Hash` and `Eq` on the borrowed /// form *must* match those for the key type. /// /// Note that not all writes will be included with this read -- only those that have been /// refreshed by the writer. If no refresh has happened, or if the map has been closed by the /// writer, this function returns `None`. /// /// If no values exist for the given key, `then` will not be called, and `Some(None, _)` is /// returned. pub fn meta_get_and<Q: ?Sized, F, T>(&self, key: &Q, then: F) -> Option<(Option<T>, M)> where F: FnOnce(&[V]) -> T, K: Borrow<Q>, Q: Hash + Eq, { self.with_handle(move |inner| { if !inner.is_ready() { None } else { let res = inner.data.get(key).map(move |v| then(&**v)); let res = (res, inner.meta.clone()); Some(res) } }) .unwrap_or(None) } /// If the writer has destroyed this map, this method will return true. /// /// See `WriteHandle::destroy`. pub fn is_destroyed(&self) -> bool { self.with_handle(|_| ()).is_none() } /// Returns true if the map contains any values for the specified key. /// /// The key may be any borrowed form of the map's key type, but `Hash` and `Eq` on the borrowed /// form *must* match those for the key type. pub fn contains_key<Q: ?Sized>(&self, key: &Q) -> bool where K: Borrow<Q>, Q: Hash + Eq, { self.with_handle(move |inner| inner.data.contains_key(key)) .unwrap_or(false) } /// Read all values in the map, and transform them into a new collection. /// /// Be careful with this function! While the iteration is ongoing, any writer that tries to /// refresh will block waiting on this reader to finish. pub fn for_each<F>(&self, mut f: F) where F: FnMut(&K, &[V]), { self.with_handle(move |inner| { for (k, vs) in &inner.data { f(k, &vs[..]) } }); } /// Read all values in the map, and transform them into a new collection. pub fn map_into<Map, Collector, Target>(&self, mut f: Map) -> Collector where Map: FnMut(&K, &[V]) -> Target, Collector: FromIterator<Target>, { self.with_handle(move |inner| { Collector::from_iter(inner.data.iter().map(|(k, vs)| f(k, &vs[..]))) }) .unwrap_or_else(|| Collector::from_iter(iter::empty())) } } #[cfg(test)] mod test { use crate::new; // the idea of this test is to allocate 64 elements, and only use 17. The vector will // probably try to fit either exactly the length, to the next highest power of 2 from // the length, or something else entirely, E.g. 17, 32, etc., // but it should always end up being smaller than the original 64 elements reserved. #[test] fn reserve_and_fit() { const MIN: usize = (1 << 4) + 1; const MAX: usize = (1 << 6); let (r, mut w) = new(); w.reserve(0, MAX).refresh(); r.get_raw(&0, |vs| assert_eq!(vs.capacity(), MAX)).unwrap(); for i in 0..MIN { w.insert(0, i); } w.fit_all().refresh(); r.get_raw(&0, |vs| assert!(vs.capacity() < MAX)).unwrap(); } }