TLA Line data Source code
1 : use std::{
2 : borrow::Borrow,
3 : hash::Hash,
4 : ops::{Deref, DerefMut},
5 : time::{Duration, Instant},
6 : };
7 : use tracing::debug;
8 :
9 : // This seems to make more sense than `lru` or `cached`:
10 : //
11 : // * `near/nearcore` ditched `cached` in favor of `lru`
12 : // (https://github.com/near/nearcore/issues?q=is%3Aissue+lru+is%3Aclosed).
13 : //
14 : // * `lru` methods use an obscure `KeyRef` type in their contraints (which is deliberately excluded from docs).
15 : // This severely hinders its usage both in terms of creating wrappers and supported key types.
16 : //
17 : // On the other hand, `hashlink` has good download stats and appears to be maintained.
18 : use hashlink::{linked_hash_map::RawEntryMut, LruCache};
19 :
20 : /// A generic trait which exposes types of cache's key and value,
21 : /// as well as the notion of cache entry invalidation.
22 : /// This is useful for [`timed_lru::Cached`].
23 : pub trait Cache {
24 : /// Entry's key.
25 : type Key;
26 :
27 : /// Entry's value.
28 : type Value;
29 :
30 : /// Used for entry invalidation.
31 : type LookupInfo<Key>;
32 :
33 : /// Invalidate an entry using a lookup info.
34 : /// We don't have an empty default impl because it's error-prone.
35 : fn invalidate(&self, _: &Self::LookupInfo<Self::Key>);
36 : }
37 :
38 : impl<C: Cache> Cache for &C {
39 : type Key = C::Key;
40 : type Value = C::Value;
41 : type LookupInfo<Key> = C::LookupInfo<Key>;
42 :
43 UBC 0 : fn invalidate(&self, info: &Self::LookupInfo<Self::Key>) {
44 0 : C::invalidate(self, info)
45 0 : }
46 : }
47 :
48 : pub use timed_lru::TimedLru;
49 : pub mod timed_lru {
50 : use super::*;
51 :
52 : /// An implementation of timed LRU cache with fixed capacity.
53 : /// Key properties:
54 : ///
55 : /// * Whenever a new entry is inserted, the least recently accessed one is evicted.
56 : /// The cache also keeps track of entry's insertion time (`created_at`) and TTL (`expires_at`).
57 : ///
58 : /// * When the entry is about to be retrieved, we check its expiration timestamp.
59 : /// If the entry has expired, we remove it from the cache; Otherwise we bump the
60 : /// expiration timestamp (e.g. +5mins) and change its place in LRU list to prolong
61 : /// its existence.
62 : ///
63 : /// * There's an API for immediate invalidation (removal) of a cache entry;
64 : /// It's useful in case we know for sure that the entry is no longer correct.
65 : /// See [`timed_lru::LookupInfo`] & [`timed_lru::Cached`] for more information.
66 : ///
67 : /// * Expired entries are kept in the cache, until they are evicted by the LRU policy,
68 : /// or by a successful lookup (i.e. the entry hasn't expired yet).
69 : /// There is no background job to reap the expired records.
70 : ///
71 : /// * It's possible for an entry that has not yet expired entry to be evicted
72 : /// before expired items. That's a bit wasteful, but probably fine in practice.
73 : pub struct TimedLru<K, V> {
74 : /// Cache's name for tracing.
75 : name: &'static str,
76 :
77 : /// The underlying cache implementation.
78 : cache: parking_lot::Mutex<LruCache<K, Entry<V>>>,
79 :
80 : /// Default time-to-live of a single entry.
81 : ttl: Duration,
82 : }
83 :
84 : impl<K: Hash + Eq, V> Cache for TimedLru<K, V> {
85 : type Key = K;
86 : type Value = V;
87 : type LookupInfo<Key> = LookupInfo<Key>;
88 :
89 0 : fn invalidate(&self, info: &Self::LookupInfo<K>) {
90 0 : self.invalidate_raw(info)
91 0 : }
92 : }
93 :
94 : struct Entry<T> {
95 : created_at: Instant,
96 : expires_at: Instant,
97 : value: T,
98 : }
99 :
100 : impl<K: Hash + Eq, V> TimedLru<K, V> {
101 : /// Construct a new LRU cache with timed entries.
102 0 : pub fn new(name: &'static str, capacity: usize, ttl: Duration) -> Self {
103 0 : Self {
104 0 : name,
105 0 : cache: LruCache::new(capacity).into(),
106 0 : ttl,
107 0 : }
108 0 : }
109 :
110 : /// Drop an entry from the cache if it's outdated.
111 0 : #[tracing::instrument(level = "debug", fields(cache = self.name), skip_all)]
112 : fn invalidate_raw(&self, info: &LookupInfo<K>) {
113 : let now = Instant::now();
114 :
115 : // Do costly things before taking the lock.
116 : let mut cache = self.cache.lock();
117 : let raw_entry = match cache.raw_entry_mut().from_key(&info.key) {
118 : RawEntryMut::Vacant(_) => return,
119 : RawEntryMut::Occupied(x) => x,
120 : };
121 :
122 : // Remove the entry if it was created prior to lookup timestamp.
123 : let entry = raw_entry.get();
124 : let (created_at, expires_at) = (entry.created_at, entry.expires_at);
125 : let should_remove = created_at <= info.created_at || expires_at <= now;
126 :
127 : if should_remove {
128 : raw_entry.remove();
129 : }
130 :
131 : drop(cache); // drop lock before logging
132 0 : debug!(
133 0 : created_at = format_args!("{created_at:?}"),
134 0 : expires_at = format_args!("{expires_at:?}"),
135 0 : entry_removed = should_remove,
136 0 : "processed a cache entry invalidation event"
137 0 : );
138 : }
139 :
140 : /// Try retrieving an entry by its key, then execute `extract` if it exists.
141 0 : #[tracing::instrument(level = "debug", fields(cache = self.name), skip_all)]
142 : fn get_raw<Q, R>(&self, key: &Q, extract: impl FnOnce(&K, &Entry<V>) -> R) -> Option<R>
143 : where
144 : K: Borrow<Q>,
145 : Q: Hash + Eq + ?Sized,
146 : {
147 : let now = Instant::now();
148 : let deadline = now.checked_add(self.ttl).expect("time overflow");
149 :
150 : // Do costly things before taking the lock.
151 : let mut cache = self.cache.lock();
152 : let mut raw_entry = match cache.raw_entry_mut().from_key(key) {
153 : RawEntryMut::Vacant(_) => return None,
154 : RawEntryMut::Occupied(x) => x,
155 : };
156 :
157 : // Immeditely drop the entry if it has expired.
158 : let entry = raw_entry.get();
159 : if entry.expires_at <= now {
160 : raw_entry.remove();
161 : return None;
162 : }
163 :
164 : let value = extract(raw_entry.key(), entry);
165 : let (created_at, expires_at) = (entry.created_at, entry.expires_at);
166 :
167 : // Update the deadline and the entry's position in the LRU list.
168 : raw_entry.get_mut().expires_at = deadline;
169 : raw_entry.to_back();
170 :
171 : drop(cache); // drop lock before logging
172 0 : debug!(
173 0 : created_at = format_args!("{created_at:?}"),
174 0 : old_expires_at = format_args!("{expires_at:?}"),
175 0 : new_expires_at = format_args!("{deadline:?}"),
176 0 : "accessed a cache entry"
177 0 : );
178 :
179 : Some(value)
180 : }
181 :
182 : /// Insert an entry to the cache. If an entry with the same key already
183 : /// existed, return the previous value and its creation timestamp.
184 0 : #[tracing::instrument(level = "debug", fields(cache = self.name), skip_all)]
185 : fn insert_raw(&self, key: K, value: V) -> (Instant, Option<V>) {
186 : let created_at = Instant::now();
187 : let expires_at = created_at.checked_add(self.ttl).expect("time overflow");
188 :
189 : let entry = Entry {
190 : created_at,
191 : expires_at,
192 : value,
193 : };
194 :
195 : // Do costly things before taking the lock.
196 : let old = self
197 : .cache
198 : .lock()
199 : .insert(key, entry)
200 0 : .map(|entry| entry.value);
201 :
202 0 : debug!(
203 0 : created_at = format_args!("{created_at:?}"),
204 0 : expires_at = format_args!("{expires_at:?}"),
205 0 : replaced = old.is_some(),
206 0 : "created a cache entry"
207 0 : );
208 :
209 : (created_at, old)
210 : }
211 : }
212 :
213 : impl<K: Hash + Eq + Clone, V: Clone> TimedLru<K, V> {
214 0 : pub fn insert(&self, key: K, value: V) -> (Option<V>, Cached<&Self>) {
215 0 : let (created_at, old) = self.insert_raw(key.clone(), value.clone());
216 0 :
217 0 : let cached = Cached {
218 0 : token: Some((self, LookupInfo { created_at, key })),
219 0 : value,
220 0 : };
221 0 :
222 0 : (old, cached)
223 0 : }
224 : }
225 :
226 : impl<K: Hash + Eq, V: Clone> TimedLru<K, V> {
227 : /// Retrieve a cached entry in convenient wrapper.
228 0 : pub fn get<Q>(&self, key: &Q) -> Option<timed_lru::Cached<&Self>>
229 0 : where
230 0 : K: Borrow<Q> + Clone,
231 0 : Q: Hash + Eq + ?Sized,
232 0 : {
233 0 : self.get_raw(key, |key, entry| {
234 0 : let info = LookupInfo {
235 0 : created_at: entry.created_at,
236 0 : key: key.clone(),
237 0 : };
238 0 :
239 0 : Cached {
240 0 : token: Some((self, info)),
241 0 : value: entry.value.clone(),
242 0 : }
243 0 : })
244 0 : }
245 : }
246 :
247 : /// Lookup information for key invalidation.
248 : pub struct LookupInfo<K> {
249 : /// Time of creation of a cache [`Entry`].
250 : /// We use this during invalidation lookups to prevent eviction of a newer
251 : /// entry sharing the same key (it might've been inserted by a different
252 : /// task after we got the entry we're trying to invalidate now).
253 : created_at: Instant,
254 :
255 : /// Search by this key.
256 : key: K,
257 : }
258 :
259 : /// Wrapper for convenient entry invalidation.
260 : pub struct Cached<C: Cache> {
261 : /// Cache + lookup info.
262 : token: Option<(C, C::LookupInfo<C::Key>)>,
263 :
264 : /// The value itself.
265 : value: C::Value,
266 : }
267 :
268 : impl<C: Cache> Cached<C> {
269 : /// Place any entry into this wrapper; invalidation will be a no-op.
270 CBC 70 : pub fn new_uncached(value: C::Value) -> Self {
271 70 : Self { token: None, value }
272 70 : }
273 :
274 : /// Drop this entry from a cache if it's still there.
275 : pub fn invalidate(self) -> C::Value {
276 8 : if let Some((cache, info)) = &self.token {
277 UBC 0 : cache.invalidate(info);
278 CBC 8 : }
279 8 : self.value
280 8 : }
281 :
282 : /// Tell if this entry is actually cached.
283 8 : pub fn cached(&self) -> bool {
284 8 : self.token.is_some()
285 8 : }
286 : }
287 :
288 : impl<C: Cache> Deref for Cached<C> {
289 : type Target = C::Value;
290 :
291 166 : fn deref(&self) -> &Self::Target {
292 166 : &self.value
293 166 : }
294 : }
295 :
296 : impl<C: Cache> DerefMut for Cached<C> {
297 132 : fn deref_mut(&mut self) -> &mut Self::Target {
298 132 : &mut self.value
299 132 : }
300 : }
301 : }
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