[pbs-devel] [PATCH v2 proxmox-backup 2/9] tools: add AsyncLruCache as a wrapper around sync LruCache

[Stefan Reiter]

Supports concurrent 'access' calls to the same key via a
BroadcastFuture. These are stored in a seperate HashMap, the LruCache
underneath is only modified once a valid value has been retrieved.

Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
---
 src/tools.rs                 |   1 +
 src/tools/async_lru_cache.rs | 135 +++++++++++++++++++++++++++++++++++
 2 files changed, 136 insertions(+)
 create mode 100644 src/tools/async_lru_cache.rs

diff --git a/src/tools.rs b/src/tools.rs
index 65049b1e..59599339 100644
--- a/src/tools.rs
+++ b/src/tools.rs
@@ -42,6 +42,7 @@ pub mod json;
 pub mod logrotate;
 pub mod loopdev;
 pub mod lru_cache;
+pub mod async_lru_cache;
 pub mod nom;
 pub mod runtime;
 pub mod serde_filter;
diff --git a/src/tools/async_lru_cache.rs b/src/tools/async_lru_cache.rs
new file mode 100644
index 00000000..cc385ec9
--- /dev/null
+++ b/src/tools/async_lru_cache.rs
@@ -0,0 +1,135 @@
+//! An 'async'-safe layer on the existing sync LruCache implementation. Supports multiple
+//! concurrent requests to the same key.
+
+use anyhow::Error;
+
+use std::collections::HashMap;
+use std::future::Future;
+use std::sync::{Arc, Mutex};
+
+use super::lru_cache::LruCache;
+use super::BroadcastFuture;
+
+/// Interface for asynchronously getting values on cache misses.
+pub trait AsyncCacher<K, V: Clone>: Sync + Send {
+    /// Fetch a value for key on cache miss.
+    ///
+    /// Works similar to non-async lru_cache::Cacher, but if the key has already been requested
+    /// and the result is not cached yet, the 'fetch' function will not be called and instead the
+    /// result of the original request cloned and returned upon completion.
+    ///
+    /// The underlying LRU structure is not modified until the returned future resolves to an
+    /// Ok(Some(_)) value.
+    fn fetch(&self, key: K) -> Box<dyn Future<Output = Result<Option<V>, Error>> + Send>;
+}
+
+/// See tools::lru_cache::LruCache, this implements an async-safe variant of that with the help of
+/// AsyncCacher.
+#[derive(Clone)]
+pub struct AsyncLruCache<K, V> {
+    maps: Arc<Mutex<(LruCache<K, V>, HashMap<K, BroadcastFuture<Option<V>>>)>>,
+}
+
+impl<K: std::cmp::Eq + std::hash::Hash + Copy, V: Clone + Send + 'static> AsyncLruCache<K, V> {
+    /// Create a new AsyncLruCache with the given maximum capacity.
+    pub fn new(capacity: usize) -> Self {
+        Self {
+            maps: Arc::new(Mutex::new((LruCache::new(capacity), HashMap::new()))),
+        }
+    }
+
+    /// Access an item either via the cache or by calling cacher.fetch. A return value of Ok(None)
+    /// means the item requested has no representation, Err(_) means a call to fetch() failed,
+    /// regardless of whether it was initiated by this call or a previous one.
+    pub async fn access(&self, key: K, cacher: &dyn AsyncCacher<K, V>) -> Result<Option<V>, Error> {
+        let (owner, result_fut) = {
+            // check if already requested
+            let mut maps = self.maps.lock().unwrap();
+            if let Some(fut) = maps.1.get(&key) {
+                // wait for the already scheduled future to resolve
+                (false, fut.listen())
+            } else {
+                // check if value is cached in LRU
+                if let Some(val) = maps.0.get_mut(key) {
+                    return Ok(Some(val.clone()));
+                }
+
+                // if neither, start broadcast future and put into map while we still have lock
+                let fut = cacher.fetch(key);
+                let broadcast = BroadcastFuture::new(fut);
+                let result_fut = broadcast.listen();
+                maps.1.insert(key, broadcast);
+                (true, result_fut)
+            }
+            // drop Mutex before awaiting any future
+        };
+
+        let result = result_fut.await;
+        match result {
+            Ok(Some(ref value)) if owner => {
+                // this call was the one initiating the request, put into LRU and remove from map
+                let mut maps = self.maps.lock().unwrap();
+                maps.0.insert(key, value.clone());
+                maps.1.remove(&key);
+            }
+            _ => {}
+        }
+        result
+    }
+}
+
+mod test {
+    use super::*;
+
+    struct TestAsyncCacher {
+        prefix: &'static str,
+    }
+
+    impl AsyncCacher<i32, String> for TestAsyncCacher {
+        fn fetch(
+            &self,
+            key: i32,
+        ) -> Box<dyn Future<Output = Result<Option<String>, Error>> + Send> {
+            let x = self.prefix;
+            Box::new(async move { Ok(Some(format!("{}{}", x, key))) })
+        }
+    }
+
+    #[test]
+    fn test_async_lru_cache() {
+        let rt = tokio::runtime::Runtime::new().unwrap();
+        rt.block_on(async move {
+            let cacher = TestAsyncCacher { prefix: "x" };
+            let cache: AsyncLruCache<i32, String> = AsyncLruCache::new(2);
+
+            assert_eq!(
+                cache.access(10, &cacher).await.unwrap(),
+                Some("x10".to_string())
+            );
+            assert_eq!(
+                cache.access(20, &cacher).await.unwrap(),
+                Some("x20".to_string())
+            );
+            assert_eq!(
+                cache.access(30, &cacher).await.unwrap(),
+                Some("x30".to_string())
+            );
+
+            for _ in 0..10 {
+                let c = cache.clone();
+                tokio::spawn(async move {
+                    let cacher = TestAsyncCacher { prefix: "y" };
+                    assert_eq!(
+                        c.access(40, &cacher).await.unwrap(),
+                        Some("y40".to_string())
+                    );
+                });
+            }
+
+            assert_eq!(
+                cache.access(20, &cacher).await.unwrap(),
+                Some("x20".to_string())
+            );
+        });
+    }
+}
-- 
2.30.2