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