Chih-Hung Hsieh | e42c505 | 2020-04-16 10:44:21 -0700 | [diff] [blame] | 1 | // This module defines a common API for caching internal runtime state. |
| 2 | // The `thread_local` crate provides an extremely optimized version of this. |
| 3 | // However, if the perf-cache feature is disabled, then we drop the |
| 4 | // thread_local dependency and instead use a pretty naive caching mechanism |
| 5 | // with a mutex. |
| 6 | // |
| 7 | // Strictly speaking, the CachedGuard isn't necessary for the much more |
| 8 | // flexible thread_local API, but implementing thread_local's API doesn't |
| 9 | // seem possible in purely safe code. |
| 10 | |
| 11 | pub use self::imp::{Cached, CachedGuard}; |
| 12 | |
| 13 | #[cfg(feature = "perf-cache")] |
| 14 | mod imp { |
| 15 | use thread_local::CachedThreadLocal; |
| 16 | |
| 17 | #[derive(Debug)] |
| 18 | pub struct Cached<T: Send>(CachedThreadLocal<T>); |
| 19 | |
| 20 | #[derive(Debug)] |
| 21 | pub struct CachedGuard<'a, T: 'a>(&'a T); |
| 22 | |
| 23 | impl<T: Send> Cached<T> { |
| 24 | pub fn new() -> Cached<T> { |
| 25 | Cached(CachedThreadLocal::new()) |
| 26 | } |
| 27 | |
| 28 | pub fn get_or(&self, create: impl FnOnce() -> T) -> CachedGuard<T> { |
| 29 | CachedGuard(self.0.get_or(|| create())) |
| 30 | } |
| 31 | } |
| 32 | |
| 33 | impl<'a, T: Send> CachedGuard<'a, T> { |
| 34 | pub fn value(&self) -> &T { |
| 35 | self.0 |
| 36 | } |
| 37 | } |
| 38 | } |
| 39 | |
| 40 | #[cfg(not(feature = "perf-cache"))] |
| 41 | mod imp { |
| 42 | use std::marker::PhantomData; |
| 43 | use std::panic::UnwindSafe; |
| 44 | use std::sync::Mutex; |
| 45 | |
| 46 | #[derive(Debug)] |
| 47 | pub struct Cached<T: Send> { |
| 48 | stack: Mutex<Vec<T>>, |
| 49 | /// When perf-cache is enabled, the thread_local crate is used, and |
| 50 | /// its CachedThreadLocal impls Send, Sync and UnwindSafe, but NOT |
| 51 | /// RefUnwindSafe. However, a Mutex impls RefUnwindSafe. So in order |
| 52 | /// to keep the APIs consistent regardless of whether perf-cache is |
| 53 | /// enabled, we force this type to NOT impl RefUnwindSafe too. |
| 54 | /// |
| 55 | /// Ideally, we should always impl RefUnwindSafe, but it seems a little |
| 56 | /// tricky to do that right now. |
| 57 | /// |
| 58 | /// See also: https://github.com/rust-lang/regex/issues/576 |
| 59 | _phantom: PhantomData<Box<dyn Send + Sync + UnwindSafe>>, |
| 60 | } |
| 61 | |
| 62 | #[derive(Debug)] |
| 63 | pub struct CachedGuard<'a, T: 'a + Send> { |
| 64 | cache: &'a Cached<T>, |
| 65 | value: Option<T>, |
| 66 | } |
| 67 | |
| 68 | impl<T: Send> Cached<T> { |
| 69 | pub fn new() -> Cached<T> { |
| 70 | Cached { stack: Mutex::new(vec![]), _phantom: PhantomData } |
| 71 | } |
| 72 | |
| 73 | pub fn get_or(&self, create: impl FnOnce() -> T) -> CachedGuard<T> { |
| 74 | let mut stack = self.stack.lock().unwrap(); |
| 75 | match stack.pop() { |
| 76 | None => CachedGuard { cache: self, value: Some(create()) }, |
| 77 | Some(value) => CachedGuard { cache: self, value: Some(value) }, |
| 78 | } |
| 79 | } |
| 80 | |
| 81 | fn put(&self, value: T) { |
| 82 | let mut stack = self.stack.lock().unwrap(); |
| 83 | stack.push(value); |
| 84 | } |
| 85 | } |
| 86 | |
| 87 | impl<'a, T: Send> CachedGuard<'a, T> { |
| 88 | pub fn value(&self) -> &T { |
| 89 | self.value.as_ref().unwrap() |
| 90 | } |
| 91 | } |
| 92 | |
| 93 | impl<'a, T: Send> Drop for CachedGuard<'a, T> { |
| 94 | fn drop(&mut self) { |
| 95 | if let Some(value) = self.value.take() { |
| 96 | self.cache.put(value); |
| 97 | } |
| 98 | } |
| 99 | } |
| 100 | } |