3 files changed, 128 insertions, 0 deletions
diff --git a/tvix/store/Cargo.toml b/tvix/store/Cargo.toml
index f82cdef30077..bf04e9a1fbdf 100644
--- a/tvix/store/Cargo.toml
+++ b/tvix/store/Cargo.toml
@@ -41,6 +41,7 @@ tvix-castore = { path = "../castore" }
 url = "2.4.0"
 walkdir = "2.4.0"
 reqwest = { version = "0.11.22", features = ["rustls-tls-native-roots", "stream"], default-features = false }
+lru = "0.12.3"
 
 [dependencies.tonic-reflection]
 optional = true
diff --git a/tvix/store/src/pathinfoservice/lru.rs b/tvix/store/src/pathinfoservice/lru.rs
new file mode 100644
index 000000000000..f3790a9054fd
--- /dev/null
+++ b/tvix/store/src/pathinfoservice/lru.rs
@@ -0,0 +1,125 @@
+use std::num::NonZeroUsize;
+use std::sync::Arc;
+use tokio::sync::RwLock;
+
+use async_stream::try_stream;
+use futures::stream::BoxStream;
+use lru::LruCache;
+use tonic::async_trait;
+
+use crate::proto::PathInfo;
+use tvix_castore::Error;
+
+use super::PathInfoService;
+
+pub struct LruPathInfoService {
+    lru: Arc<RwLock<LruCache<[u8; 20], PathInfo>>>,
+}
+
+impl LruPathInfoService {
+    pub fn with_capacity(capacity: NonZeroUsize) -> Self {
+        Self {
+            lru: Arc::new(RwLock::new(LruCache::new(capacity))),
+        }
+    }
+}
+
+#[async_trait]
+impl PathInfoService for LruPathInfoService {
+    async fn get(&self, digest: [u8; 20]) -> Result<Option<PathInfo>, Error> {
+        Ok(self.lru.write().await.get(&digest).cloned())
+    }
+
+    async fn put(&self, path_info: PathInfo) -> Result<PathInfo, Error> {
+        // call validate
+        let store_path = path_info
+            .validate()
+            .map_err(|e| Error::InvalidRequest(format!("invalid PathInfo: {}", e)))?;
+
+        self.lru
+            .write()
+            .await
+            .put(*store_path.digest(), path_info.clone());
+
+        Ok(path_info)
+    }
+
+    fn list(&self) -> BoxStream<'static, Result<PathInfo, Error>> {
+        let lru = self.lru.clone();
+        Box::pin(try_stream! {
+            let lru = lru.read().await;
+            let it = lru.iter();
+
+            for (_k,v) in it {
+                yield v.clone()
+            }
+        })
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use std::num::NonZeroUsize;
+
+    use crate::{
+        pathinfoservice::{LruPathInfoService, PathInfoService},
+        proto::PathInfo,
+        tests::fixtures::PATH_INFO_WITH_NARINFO,
+    };
+    use lazy_static::lazy_static;
+    use tvix_castore::proto as castorepb;
+
+    lazy_static! {
+        static ref PATHINFO_1: PathInfo = PATH_INFO_WITH_NARINFO.clone();
+        static ref PATHINFO_1_DIGEST: [u8; 20] = [0; 20];
+        static ref PATHINFO_2: PathInfo = {
+            let mut p = PATHINFO_1.clone();
+            let root_node = p.node.as_mut().unwrap();
+            if let castorepb::Node { node: Some(node) } = root_node {
+                let n = node.to_owned();
+                *node = n.rename("11111111111111111111111111111111-dummy2".into());
+            } else {
+                unreachable!()
+            }
+            p
+        };
+        static ref PATHINFO_2_DIGEST: [u8; 20] = *(PATHINFO_2.validate().unwrap()).digest();
+    }
+
+    #[tokio::test]
+    async fn evict() {
+        let svc = LruPathInfoService::with_capacity(NonZeroUsize::new(1).unwrap());
+
+        // pathinfo_1 should not be there
+        assert!(svc
+            .get(*PATHINFO_1_DIGEST)
+            .await
+            .expect("no error")
+            .is_none());
+
+        // insert it
+        svc.put(PATHINFO_1.clone()).await.expect("no error");
+
+        // now it should be there.
+        assert_eq!(
+            Some(PATHINFO_1.clone()),
+            svc.get(*PATHINFO_1_DIGEST).await.expect("no error")
+        );
+
+        // insert pathinfo_2. This will evict pathinfo 1
+        svc.put(PATHINFO_2.clone()).await.expect("no error");
+
+        // now pathinfo 2 should be there.
+        assert_eq!(
+            Some(PATHINFO_2.clone()),
+            svc.get(*PATHINFO_2_DIGEST).await.expect("no error")
+        );
+
+        // … but pathinfo 1 not anymore.
+        assert!(svc
+            .get(*PATHINFO_1_DIGEST)
+            .await
+            .expect("no error")
+            .is_none());
+    }
+}
diff --git a/tvix/store/src/pathinfoservice/mod.rs b/tvix/store/src/pathinfoservice/mod.rs
index 64c54c7267df..5f5da2bff296 100644
--- a/tvix/store/src/pathinfoservice/mod.rs
+++ b/tvix/store/src/pathinfoservice/mod.rs
@@ -1,5 +1,6 @@
 mod from_addr;
 mod grpc;
+mod lru;
 mod memory;
 mod nix_http;
 mod sled;
@@ -18,6 +19,7 @@ use crate::proto::PathInfo;
 
 pub use self::from_addr::from_addr;
 pub use self::grpc::GRPCPathInfoService;
+pub use self::lru::LruPathInfoService;
 pub use self::memory::MemoryPathInfoService;
 pub use self::nix_http::NixHTTPPathInfoService;
 pub use self::sled::SledPathInfoService;