Cache with max capacity of zero should not cache anything

[tatsuya6502]

Description

All Moka versions (up to v0.10.0) are affected by this issue.

When a cache is configured with the max capacity of zero, users would expect that it should not cache anything. However, the current cache implementations are to cache everything for a short period of time (up to 0.5 seconds). This is because these implementations temporary admit new entries no matter if the cache is full or not, and evict the entries later in batch.

This behavior is not intuitive, so it needs to be changed to not caching anything.

// Cargo.toml
// [dependencies]
// moka = { version = "0.10.0", features = ["future"] }
// tokio = { version = "1.25.0", features = ["rt-multi-thread", "time"] }

fn main() {
    sync_cache();

    let rt = tokio::runtime::Runtime::new().unwrap();
    rt.block_on(future_cache());
}

fn sync_cache() {
    let cache = moka::sync::Cache::new(0);
    cache.insert(0, ());

    dbg!(cache.contains_key(&0));
    std::thread::sleep(std::time::Duration::from_millis(550));
    dbg!(cache.contains_key(&0));
}

async fn future_cache() {
    let cache = moka::future::Cache::new(0);
    cache.insert(1, ()).await;

    dbg!(cache.contains_key(&1));
    tokio::time::sleep(std::time::Duration::from_millis(550)).await;
    dbg!(cache.contains_key(&1));
}

$ cargo run
[src/main.rs:12] cache.contains_key(&0) = true
[src/main.rs:14] cache.contains_key(&0) = false
[src/main.rs:21] cache.contains_key(&1) = true
[src/main.rs:23] cache.contains_key(&1) = false

Workaround

Users can workaround this issue by setting time_to_live to ZERO. This will still cache new entries for a short period of time, but get* methods will never return the entries as they are already expired.

    let cache = moka::sync::Cache::builder()
        .max_capacity(0)
        .time_to_live(std::time::Duration::ZERO)
        .build();

CC: @Swatinem