1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
#![deny(unsafe_code)]

//! Caching handle into the [ArcSwapAny].
//!
//! The [Cache] keeps a copy of the internal [Arc] for faster access.
//!
//! [Arc]: std::sync::Arc

use std::ops::Deref;
use std::sync::atomic::Ordering;

use super::ref_cnt::RefCnt;
use super::strategy::Strategy;
use super::ArcSwapAny;

/// Generalization of caches providing access to `T`.
///
/// This abstracts over all kinds of caches that can provide a cheap access to values of type `T`.
/// This is useful in cases where some code doesn't care if the `T` is the whole structure or just
/// a part of it.
///
/// See the example at [`Cache::map`].
pub trait Access<T> {
    /// Loads the value from cache.
    ///
    /// This revalidates the value in the cache, then provides the access to the cached value.
    fn load(&mut self) -> &T;
}

/// Caching handle for [`ArcSwapAny`][ArcSwapAny].
///
/// Instead of loading the [`Arc`][Arc] on every request from the shared storage, this keeps
/// another copy inside itself. Upon request it only cheaply revalidates it is up to
/// date. If it is, access is significantly faster. If it is stale, the [load_full] is done and the
/// cache value is replaced. Under a read-heavy loads, the measured speedup are 10-25 times,
/// depending on the architecture.
///
/// There are, however, downsides:
///
/// * The handle needs to be kept around by the caller (usually, one per thread). This is fine if
///   there's one global `ArcSwapAny`, but starts being tricky with eg. data structures build from
///   them.
/// * As it keeps a copy of the [Arc] inside the cache, the old value may be kept alive for longer
///   period of time ‒ it is replaced by the new value on [load][Cache::load]. You may not want to
///   use this if dropping the old value in timely manner is important (possibly because of
///   releasing large amount of RAM or because of closing file handles).
///
/// # Examples
///
/// ```rust
/// # fn do_something<V>(_v: V) { }
/// use std::sync::Arc;
/// use std::sync::atomic::{AtomicBool, Ordering};
///
/// use arc_swap::{ArcSwap, Cache};
///
/// let shared = Arc::new(ArcSwap::from_pointee(42));
/// # let mut threads = Vec::new();
/// let terminate = Arc::new(AtomicBool::new(false));
/// // Start 10 worker threads...
/// for _ in 0..10 {
///     let mut cache = Cache::new(Arc::clone(&shared));
///     let terminate = Arc::clone(&terminate);
///     # let thread =
///     std::thread::spawn(move || {
///         // Keep loading it like mad..
///         while !terminate.load(Ordering::Relaxed) {
///             let value = cache.load();
///             do_something(value);
///         }
///     });
///     # threads.push(thread);
/// }
/// shared.store(Arc::new(12));
/// # terminate.store(true, Ordering::Relaxed);
/// # for thread in threads { thread.join().unwrap() }
/// ```
///
/// Another one with using a thread local storage and explicit types:
///
/// ```rust
/// # use std::sync::Arc;
/// # use std::ops::Deref;
/// # use std::cell::RefCell;
/// #
/// # use arc_swap::ArcSwap;
/// # use arc_swap::cache::Cache;
/// # use once_cell::sync::Lazy;
/// #
/// # #[derive(Debug, Default)]
/// # struct Config;
/// #
/// static CURRENT_CONFIG: Lazy<ArcSwap<Config>> = Lazy::new(|| ArcSwap::from_pointee(Config::default()));
///
/// thread_local! {
///     static CACHE: RefCell<Cache<&'static ArcSwap<Config>, Arc<Config>>> = RefCell::new(Cache::from(CURRENT_CONFIG.deref()));
/// }
///
/// CACHE.with(|c| {
///     // * RefCell needed, because load on cache is `&mut`.
///     // * You want to operate inside the `with` ‒ cloning the Arc is comparably expensive as
///     //   ArcSwap::load itself and whatever you'd save by the cache would be lost on that.
///     println!("{:?}", c.borrow_mut().load());
/// });
/// ```
///
/// [Arc]: std::sync::Arc
/// [load_full]: ArcSwapAny::load_full
#[derive(Clone, Debug)]
pub struct Cache<A, T> {
    arc_swap: A,
    cached: T,
}

impl<A, T, S> Cache<A, T>
where
    A: Deref<Target = ArcSwapAny<T, S>>,
    T: RefCnt,
    S: Strategy<T>,
{
    /// Creates a new caching handle.
    ///
    /// The parameter is something dereferencing into an [`ArcSwapAny`] (eg. either to [`ArcSwap`]
    /// or [`ArcSwapOption`]). That can be [`ArcSwapAny`] itself, but that's not very useful. But
    /// it also can be a reference to it or `Arc`, which makes it possible to share the
    /// [`ArcSwapAny`] with multiple caches or access it in non-cached way too.
    ///
    /// [`ArcSwapOption`]: crate::ArcSwapOption
    /// [`ArcSwap`]: crate::ArcSwap
    pub fn new(arc_swap: A) -> Self {
        let cached = arc_swap.load_full();
        Self { arc_swap, cached }
    }

    /// Gives access to the (possibly shared) cached [`ArcSwapAny`].
    pub fn arc_swap(&self) -> &A::Target {
        &self.arc_swap
    }

    /// Loads the currently held value.
    ///
    /// This first checks if the cached value is up to date. This check is very cheap.
    ///
    /// If it is up to date, the cached value is simply returned without additional costs. If it is
    /// outdated, a load is done on the underlying shared storage. The newly loaded value is then
    /// stored in the cache and returned.
    #[inline]
    pub fn load(&mut self) -> &T {
        self.revalidate();
        self.load_no_revalidate()
    }

    #[inline]
    fn load_no_revalidate(&self) -> &T {
        &self.cached
    }

    #[inline]
    fn revalidate(&mut self) {
        let cached_ptr = RefCnt::as_ptr(&self.cached);
        // Node: Relaxed here is fine. We do not synchronize any data through this, we already have
        // it synchronized in self.cache. We just want to check if it changed, if it did, the
        // load_full will be responsible for any synchronization needed.
        let shared_ptr = self.arc_swap.ptr.load(Ordering::Relaxed);
        if cached_ptr != shared_ptr {
            self.cached = self.arc_swap.load_full();
        }
    }

    /// Turns this cache into a cache with a projection inside the cached value.
    ///
    /// You'd use this in case when some part of code needs access to fresh values of `U`, however
    /// a bigger structure containing `U` is provided by this cache. The possibility of giving the
    /// whole structure to the part of the code falls short in terms of reusability (the part of
    /// the code could be used within multiple contexts, each with a bigger different structure
    /// containing `U`) and code separation (the code shouldn't needs to know about the big
    /// structure).
    ///
    /// # Warning
    ///
    /// As the provided `f` is called inside every [`load`][Access::load], this one should be
    /// cheap. Most often it is expected to be just a closure taking reference of some inner field.
    ///
    /// For the same reasons, it should not have side effects and should never panic (these will
    /// not break Rust's safety rules, but might produce behaviour you don't expect).
    ///
    /// # Examples
    ///
    /// ```rust
    /// use arc_swap::ArcSwap;
    /// use arc_swap::cache::{Access, Cache};
    ///
    /// struct InnerCfg {
    ///     answer: usize,
    /// }
    ///
    /// struct FullCfg {
    ///     inner: InnerCfg,
    /// }
    ///
    /// fn use_inner<A: Access<InnerCfg>>(cache: &mut A) {
    ///     let value = cache.load();
    ///     println!("The answer is: {}", value.answer);
    /// }
    ///
    /// let full_cfg = ArcSwap::from_pointee(FullCfg {
    ///     inner: InnerCfg {
    ///         answer: 42,
    ///     }
    /// });
    /// let cache = Cache::new(&full_cfg);
    /// use_inner(&mut cache.map(|full| &full.inner));
    ///
    /// let inner_cfg = ArcSwap::from_pointee(InnerCfg { answer: 24 });
    /// let mut inner_cache = Cache::new(&inner_cfg);
    /// use_inner(&mut inner_cache);
    /// ```
    pub fn map<F, U>(self, f: F) -> MapCache<A, T, F>
    where
        F: FnMut(&T) -> &U,
    {
        MapCache {
            inner: self,
            projection: f,
        }
    }
}

impl<A, T, S> Access<T::Target> for Cache<A, T>
where
    A: Deref<Target = ArcSwapAny<T, S>>,
    T: Deref<Target = <T as RefCnt>::Base> + RefCnt,
    S: Strategy<T>,
{
    fn load(&mut self) -> &T::Target {
        self.load().deref()
    }
}

impl<A, T, S> From<A> for Cache<A, T>
where
    A: Deref<Target = ArcSwapAny<T, S>>,
    T: RefCnt,
    S: Strategy<T>,
{
    fn from(arc_swap: A) -> Self {
        Self::new(arc_swap)
    }
}

/// An implementation of a cache with a projection into the accessed value.
///
/// This is the implementation structure for [`Cache::map`]. It can't be created directly and it
/// should be used through the [`Access`] trait.
#[derive(Clone, Debug)]
pub struct MapCache<A, T, F> {
    inner: Cache<A, T>,
    projection: F,
}

impl<A, T, S, F, U> Access<U> for MapCache<A, T, F>
where
    A: Deref<Target = ArcSwapAny<T, S>>,
    T: RefCnt,
    S: Strategy<T>,
    F: FnMut(&T) -> &U,
{
    fn load(&mut self) -> &U {
        (self.projection)(self.inner.load())
    }
}

#[cfg(test)]
mod tests {
    use std::sync::Arc;

    use super::*;
    use crate::{ArcSwap, ArcSwapOption};

    #[test]
    fn cached_value() {
        let a = ArcSwap::from_pointee(42);
        let mut c1 = Cache::new(&a);
        let mut c2 = Cache::new(&a);

        assert_eq!(42, **c1.load());
        assert_eq!(42, **c2.load());

        a.store(Arc::new(43));
        assert_eq!(42, **c1.load_no_revalidate());
        assert_eq!(43, **c1.load());
    }

    #[test]
    fn cached_through_arc() {
        let a = Arc::new(ArcSwap::from_pointee(42));
        let mut c = Cache::new(Arc::clone(&a));
        assert_eq!(42, **c.load());
        a.store(Arc::new(0));
        drop(a); // A is just one handle, the ArcSwap is kept alive by the cache.
    }

    #[test]
    fn cache_option() {
        let a = ArcSwapOption::from_pointee(42);
        let mut c = Cache::new(&a);

        assert_eq!(42, **c.load().as_ref().unwrap());
        a.store(None);
        assert!(c.load().is_none());
    }

    struct Inner {
        answer: usize,
    }

    struct Outer {
        inner: Inner,
    }

    #[test]
    fn map_cache() {
        let a = ArcSwap::from_pointee(Outer {
            inner: Inner { answer: 42 },
        });

        let mut cache = Cache::new(&a);
        let mut inner = cache.clone().map(|outer| &outer.inner);
        let mut answer = cache.clone().map(|outer| &outer.inner.answer);

        assert_eq!(42, cache.load().inner.answer);
        assert_eq!(42, inner.load().answer);
        assert_eq!(42, *answer.load());

        a.store(Arc::new(Outer {
            inner: Inner { answer: 24 },
        }));

        assert_eq!(24, cache.load().inner.answer);
        assert_eq!(24, inner.load().answer);
        assert_eq!(24, *answer.load());
    }
}