OXYGENHUB
◆ v0.9.11 oxygenhub / crates / hub / src / render / cache.rs
4.1 KiB · Rust · last changed Cache rendered Markdown and highlighted files by content, in memory-bounded LRUs 1 month ago source blame history raw
f5c44337 Andy McCall ↶ 1 
//! Caches of rendered output keyed by what the output depends on (hub spec
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//! §16.3): the BLAKE3 of the content and of anything else that changes the
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//! result, such as a Markdown file's link bases or a file's name, which
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//! picks its syntax. Bounded by the bytes they hold; the least recently
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//! used entries go first.
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use std::collections::HashMap;
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use std::sync::{Arc, Mutex};
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pub struct Lru<V> {
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    inner: Mutex<Inner<V>>,
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}
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struct Inner<V> {
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    budget: usize,
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    used: usize,
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    tick: u64,
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    hits: u64,
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    misses: u64,
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    map: HashMap<[u8; 32], (u64, Arc<V>, usize)>,
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}
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/// Something whose size counts against a cache's budget.
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pub trait Weigh {
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    fn weight(&self) -> usize;
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}
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impl Weigh for String {
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    fn weight(&self) -> usize {
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        self.len() + 32
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    }
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}
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impl Weigh for Vec<String> {
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    fn weight(&self) -> usize {
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        self.iter().map(|s| s.len() + 24).sum::<usize>() + 32
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    }
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}
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/// The key for a set of parts: BLAKE3 of each part's length and bytes.
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pub fn key(parts: &[&[u8]]) -> [u8; 32] {
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    let mut h = blake3::Hasher::new();
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    for p in parts {
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        h.update(&(p.len() as u64).to_le_bytes());
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        h.update(p);
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    }
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    *h.finalize().as_bytes()
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}
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impl<V: Weigh> Lru<V> {
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    pub fn new(budget: usize) -> Lru<V> {
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        Lru { inner: Mutex::new(Inner { budget, used: 0, tick: 0, hits: 0, misses: 0, map: HashMap::new() }) }
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    }
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    /// The cached value for `key`, or `make()` stored and returned. `make`
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    /// runs without the lock held, so a slow render blocks nobody.
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    pub fn get_or(&self, key: [u8; 32], make: impl FnOnce() -> V) -> Arc<V> {
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        {
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            let mut g = self.inner.lock().unwrap_or_else(|p| p.into_inner());
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            g.tick += 1;
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            let tick = g.tick;
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            if let Some(entry) = g.map.get_mut(&key) {
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                entry.0 = tick;
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                let v = entry.1.clone();
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                g.hits += 1;
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                return v;
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            }
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            g.misses += 1;
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        }
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        let value = Arc::new(make());
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        let weight = value.weight();
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        let mut g = self.inner.lock().unwrap_or_else(|p| p.into_inner());
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        if weight > g.budget / 4 {
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            return value; // too big to be worth keeping
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        }
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        g.tick += 1;
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        let tick = g.tick;
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        if let Some(old) = g.map.insert(key, (tick, value.clone(), weight)) {
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            g.used -= old.2;
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        }
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        g.used += weight;
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        while g.used > g.budget {
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            let Some((&oldest, _)) = g.map.iter().min_by_key(|(_, e)| e.0) else { break };
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            if let Some(e) = g.map.remove(&oldest) {
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                g.used -= e.2;
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            }
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        }
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        value
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    }
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    /// (hits, misses, bytes held).
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    pub fn stats(&self) -> (u64, u64, usize) {
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        let g = self.inner.lock().unwrap_or_else(|p| p.into_inner());
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        (g.hits, g.misses, g.used)
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    }
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}
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#[cfg(test)]
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mod tests {
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    use super::*;
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    #[test]
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    fn hits_misses_and_eviction() {
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        let c: Lru<String> = Lru::new(400);
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        let mut runs = 0;
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        for _ in 0..3 {
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            c.get_or(key(&[b"a"]), || {
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                runs += 1;
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                "x".repeat(50)
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            });
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        }
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        assert_eq!(runs, 1, "rendered once");
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        assert_eq!(c.stats().0, 2);
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        // Filling past the budget drops the least recently used.
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        c.get_or(key(&[b"b"]), || "y".repeat(90));
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        c.get_or(key(&[b"a"]), || unreachable!());
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        c.get_or(key(&[b"c"]), || "z".repeat(90));
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        c.get_or(key(&[b"d"]), || "w".repeat(90));
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        assert!(c.stats().2 <= 400);
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        let mut again = false;
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        c.get_or(key(&[b"b"]), || {
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            again = true;
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            String::new()
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        });
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        assert!(again, "b was the least recently used and went first");
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        // Something larger than a quarter of the budget is not kept.
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        c.get_or(key(&[b"huge"]), || "h".repeat(200));
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        let mut rerun = false;
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        c.get_or(key(&[b"huge"]), || {
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            rerun = true;
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            String::new()
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        });
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        assert!(rerun);
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        // Parts are length-framed: ("ab","c") and ("a","bc") differ.
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        assert_ne!(key(&[b"ab", b"c"]), key(&[b"a", b"bc"]));
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    }
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}