Line data Source code
1 : use std::{
2 : borrow::Borrow,
3 : hash::Hash,
4 : time::{Duration, Instant},
5 : };
6 : use tracing::debug;
7 :
8 : // This seems to make more sense than `lru` or `cached`:
9 : //
10 : // * `near/nearcore` ditched `cached` in favor of `lru`
11 : // (https://github.com/near/nearcore/issues?q=is%3Aissue+lru+is%3Aclosed).
12 : //
13 : // * `lru` methods use an obscure `KeyRef` type in their contraints (which is deliberately excluded from docs).
14 : // This severely hinders its usage both in terms of creating wrappers and supported key types.
15 : //
16 : // On the other hand, `hashlink` has good download stats and appears to be maintained.
17 : use hashlink::{linked_hash_map::RawEntryMut, LruCache};
18 :
19 : use super::{common::Cached, timed_lru, Cache};
20 :
21 : /// An implementation of timed LRU cache with fixed capacity.
22 : /// Key properties:
23 : ///
24 : /// * Whenever a new entry is inserted, the least recently accessed one is evicted.
25 : /// The cache also keeps track of entry's insertion time (`created_at`) and TTL (`expires_at`).
26 : ///
27 : /// * If `update_ttl_on_retrieval` is `true`. When the entry is about to be retrieved, we check its expiration timestamp.
28 : /// If the entry has expired, we remove it from the cache; Otherwise we bump the
29 : /// expiration timestamp (e.g. +5mins) and change its place in LRU list to prolong
30 : /// its existence.
31 : ///
32 : /// * There's an API for immediate invalidation (removal) of a cache entry;
33 : /// It's useful in case we know for sure that the entry is no longer correct.
34 : /// See [`timed_lru::LookupInfo`] & [`timed_lru::Cached`] for more information.
35 : ///
36 : /// * Expired entries are kept in the cache, until they are evicted by the LRU policy,
37 : /// or by a successful lookup (i.e. the entry hasn't expired yet).
38 : /// There is no background job to reap the expired records.
39 : ///
40 : /// * It's possible for an entry that has not yet expired entry to be evicted
41 : /// before expired items. That's a bit wasteful, but probably fine in practice.
42 : pub(crate) struct TimedLru<K, V> {
43 : /// Cache's name for tracing.
44 : name: &'static str,
45 :
46 : /// The underlying cache implementation.
47 : cache: parking_lot::Mutex<LruCache<K, Entry<V>>>,
48 :
49 : /// Default time-to-live of a single entry.
50 : ttl: Duration,
51 :
52 : update_ttl_on_retrieval: bool,
53 : }
54 :
55 : impl<K: Hash + Eq, V> Cache for TimedLru<K, V> {
56 : type Key = K;
57 : type Value = V;
58 : type LookupInfo<Key> = LookupInfo<Key>;
59 :
60 4 : fn invalidate(&self, info: &Self::LookupInfo<K>) {
61 4 : self.invalidate_raw(info);
62 4 : }
63 : }
64 :
65 : struct Entry<T> {
66 : created_at: Instant,
67 : expires_at: Instant,
68 : ttl: Duration,
69 : update_ttl_on_retrieval: bool,
70 : value: T,
71 : }
72 :
73 : impl<K: Hash + Eq, V> TimedLru<K, V> {
74 : /// Construct a new LRU cache with timed entries.
75 7 : pub(crate) fn new(
76 7 : name: &'static str,
77 7 : capacity: usize,
78 7 : ttl: Duration,
79 7 : update_ttl_on_retrieval: bool,
80 7 : ) -> Self {
81 7 : Self {
82 7 : name,
83 7 : cache: LruCache::new(capacity).into(),
84 7 : ttl,
85 7 : update_ttl_on_retrieval,
86 7 : }
87 7 : }
88 :
89 : /// Drop an entry from the cache if it's outdated.
90 4 : #[tracing::instrument(level = "debug", fields(cache = self.name), skip_all)]
91 : fn invalidate_raw(&self, info: &LookupInfo<K>) {
92 : let now = Instant::now();
93 :
94 : // Do costly things before taking the lock.
95 : let mut cache = self.cache.lock();
96 : let raw_entry = match cache.raw_entry_mut().from_key(&info.key) {
97 : RawEntryMut::Vacant(_) => return,
98 : RawEntryMut::Occupied(x) => x,
99 : };
100 :
101 : // Remove the entry if it was created prior to lookup timestamp.
102 : let entry = raw_entry.get();
103 : let (created_at, expires_at) = (entry.created_at, entry.expires_at);
104 : let should_remove = created_at <= info.created_at || expires_at <= now;
105 :
106 : if should_remove {
107 : raw_entry.remove();
108 : }
109 :
110 : drop(cache); // drop lock before logging
111 : debug!(
112 : created_at = format_args!("{created_at:?}"),
113 : expires_at = format_args!("{expires_at:?}"),
114 : entry_removed = should_remove,
115 : "processed a cache entry invalidation event"
116 : );
117 : }
118 :
119 : /// Try retrieving an entry by its key, then execute `extract` if it exists.
120 0 : #[tracing::instrument(level = "debug", fields(cache = self.name), skip_all)]
121 : fn get_raw<Q, R>(&self, key: &Q, extract: impl FnOnce(&K, &Entry<V>) -> R) -> Option<R>
122 : where
123 : K: Borrow<Q>,
124 : Q: Hash + Eq + ?Sized,
125 : {
126 : let now = Instant::now();
127 :
128 : // Do costly things before taking the lock.
129 : let mut cache = self.cache.lock();
130 : let mut raw_entry = match cache.raw_entry_mut().from_key(key) {
131 : RawEntryMut::Vacant(_) => return None,
132 : RawEntryMut::Occupied(x) => x,
133 : };
134 :
135 : // Immeditely drop the entry if it has expired.
136 : let entry = raw_entry.get();
137 : if entry.expires_at <= now {
138 : raw_entry.remove();
139 : return None;
140 : }
141 :
142 : let value = extract(raw_entry.key(), entry);
143 : let (created_at, expires_at) = (entry.created_at, entry.expires_at);
144 :
145 : // Update the deadline and the entry's position in the LRU list.
146 : let deadline = now.checked_add(raw_entry.get().ttl).expect("time overflow");
147 : if raw_entry.get().update_ttl_on_retrieval {
148 : raw_entry.get_mut().expires_at = deadline;
149 : }
150 : raw_entry.to_back();
151 :
152 : drop(cache); // drop lock before logging
153 : debug!(
154 : created_at = format_args!("{created_at:?}"),
155 : old_expires_at = format_args!("{expires_at:?}"),
156 : new_expires_at = format_args!("{deadline:?}"),
157 : "accessed a cache entry"
158 : );
159 :
160 : Some(value)
161 : }
162 :
163 : /// Insert an entry to the cache. If an entry with the same key already
164 : /// existed, return the previous value and its creation timestamp.
165 10 : #[tracing::instrument(level = "debug", fields(cache = self.name), skip_all)]
166 : fn insert_raw(&self, key: K, value: V) -> (Instant, Option<V>) {
167 : self.insert_raw_ttl(key, value, self.ttl, self.update_ttl_on_retrieval)
168 : }
169 :
170 : /// Insert an entry to the cache. If an entry with the same key already
171 : /// existed, return the previous value and its creation timestamp.
172 10 : #[tracing::instrument(level = "debug", fields(cache = self.name), skip_all)]
173 : fn insert_raw_ttl(
174 : &self,
175 : key: K,
176 : value: V,
177 : ttl: Duration,
178 : update: bool,
179 : ) -> (Instant, Option<V>) {
180 : let created_at = Instant::now();
181 : let expires_at = created_at.checked_add(ttl).expect("time overflow");
182 :
183 : let entry = Entry {
184 : created_at,
185 : expires_at,
186 : ttl,
187 : update_ttl_on_retrieval: update,
188 : value,
189 : };
190 :
191 : // Do costly things before taking the lock.
192 : let old = self
193 : .cache
194 : .lock()
195 : .insert(key, entry)
196 0 : .map(|entry| entry.value);
197 :
198 : debug!(
199 : created_at = format_args!("{created_at:?}"),
200 : expires_at = format_args!("{expires_at:?}"),
201 : replaced = old.is_some(),
202 : "created a cache entry"
203 : );
204 :
205 : (created_at, old)
206 : }
207 : }
208 :
209 : impl<K: Hash + Eq + Clone, V: Clone> TimedLru<K, V> {
210 0 : pub(crate) fn insert_ttl(&self, key: K, value: V, ttl: Duration) {
211 0 : self.insert_raw_ttl(key, value, ttl, false);
212 0 : }
213 :
214 10 : pub(crate) fn insert_unit(&self, key: K, value: V) -> (Option<V>, Cached<&Self, ()>) {
215 10 : let (created_at, old) = self.insert_raw(key.clone(), value);
216 10 :
217 10 : let cached = Cached {
218 10 : token: Some((self, LookupInfo { created_at, key })),
219 10 : value: (),
220 10 : };
221 10 :
222 10 : (old, cached)
223 10 : }
224 : }
225 :
226 : impl<K: Hash + Eq, V: Clone> TimedLru<K, V> {
227 : /// Retrieve a cached entry in convenient wrapper.
228 0 : pub(crate) 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(crate) 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 : }
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