refactor(tvix/store): move Cache to cache mod

This being in combinators makes it harder to find.

Change-Id: If7984bdbd43f164c670548639bb4846d859f6695
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12750
Reviewed-by: yuka <yuka@yuka.dev>
Tested-by: BuildkiteCI

tvix/store/src/pathinfoservice/cache.rs

@@ -0,0 +1,156 @@
+use std::sync::Arc;
+
+use futures::stream::BoxStream;
+use nix_compat::nixbase32;
+use tonic::async_trait;
+use tracing::{debug, instrument};
+use tvix_castore::composition::{CompositionContext, ServiceBuilder};
+use tvix_castore::Error;
+
+use super::{PathInfo, PathInfoService};
+
+/// Asks near first, if not found, asks far.
+/// If found in there, returns it, and *inserts* it into
+/// near.
+/// There is no negative cache.
+/// Inserts and listings are not implemented for now.
+pub struct Cache<PS1, PS2> {
+    instance_name: String,
+    near: PS1,
+    far: PS2,
+}
+
+impl<PS1, PS2> Cache<PS1, PS2> {
+    pub fn new(instance_name: String, near: PS1, far: PS2) -> Self {
+        Self {
+            instance_name,
+            near,
+            far,
+        }
+    }
+}
+
+#[async_trait]
+impl<PS1, PS2> PathInfoService for Cache<PS1, PS2>
+where
+    PS1: PathInfoService,
+    PS2: PathInfoService,
+{
+    #[instrument(level = "trace", skip_all, fields(path_info.digest = nixbase32::encode(&digest), instance_name = %self.instance_name))]
+    async fn get(&self, digest: [u8; 20]) -> Result<Option<PathInfo>, Error> {
+        match self.near.get(digest).await? {
+            Some(path_info) => {
+                debug!("serving from cache");
+                Ok(Some(path_info))
+            }
+            None => {
+                debug!("not found in near, asking remote…");
+                match self.far.get(digest).await? {
+                    None => Ok(None),
+                    Some(path_info) => {
+                        debug!("found in remote, adding to cache");
+                        self.near.put(path_info.clone()).await?;
+                        Ok(Some(path_info))
+                    }
+                }
+            }
+        }
+    }
+
+    async fn put(&self, _path_info: PathInfo) -> Result<PathInfo, Error> {
+        Err(Error::StorageError("unimplemented".to_string()))
+    }
+
+    fn list(&self) -> BoxStream<'static, Result<PathInfo, Error>> {
+        Box::pin(tokio_stream::once(Err(Error::StorageError(
+            "unimplemented".to_string(),
+        ))))
+    }
+}
+
+#[derive(serde::Deserialize)]
+pub struct CacheConfig {
+    pub near: String,
+    pub far: String,
+}
+
+impl TryFrom<url::Url> for CacheConfig {
+    type Error = Box<dyn std::error::Error + Send + Sync>;
+    fn try_from(_url: url::Url) -> Result<Self, Self::Error> {
+        Err(Error::StorageError(
+            "Instantiating a CombinedPathInfoService from a url is not supported".into(),
+        )
+        .into())
+    }
+}
+
+#[async_trait]
+impl ServiceBuilder for CacheConfig {
+    type Output = dyn PathInfoService;
+    async fn build<'a>(
+        &'a self,
+        instance_name: &str,
+        context: &CompositionContext,
+    ) -> Result<Arc<dyn PathInfoService>, Box<dyn std::error::Error + Send + Sync + 'static>> {
+        let (near, far) = futures::join!(
+            context.resolve::<Self::Output>(self.near.clone()),
+            context.resolve::<Self::Output>(self.far.clone())
+        );
+        Ok(Arc::new(Cache {
+            instance_name: instance_name.to_string(),
+            near: near?,
+            far: far?,
+        }))
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use std::num::NonZeroUsize;
+
+    use crate::{
+        pathinfoservice::{LruPathInfoService, MemoryPathInfoService, PathInfoService},
+        tests::fixtures::PATH_INFO,
+    };
+
+    /// Helper function setting up an instance of a "far" and "near"
+    /// PathInfoService.
+    async fn create_pathinfoservice() -> super::Cache<LruPathInfoService, MemoryPathInfoService> {
+        // Create an instance of a "far" PathInfoService.
+        let far = MemoryPathInfoService::default();
+
+        // … and an instance of a "near" PathInfoService.
+        let near = LruPathInfoService::with_capacity("test".into(), NonZeroUsize::new(1).unwrap());
+
+        // create a Pathinfoservice combining the two and return it.
+        super::Cache::new("test".into(), near, far)
+    }
+
+    /// Getting from the far backend is gonna insert it into the near one.
+    #[tokio::test]
+    async fn test_populate_cache() {
+        let svc = create_pathinfoservice().await;
+
+        // query the PathInfo, things should not be there.
+        assert!(svc
+            .get(*PATH_INFO.store_path.digest())
+            .await
+            .unwrap()
+            .is_none());
+
+        // insert it into the far one.
+        svc.far.put(PATH_INFO.clone()).await.unwrap();
+
+        // now try getting it again, it should succeed.
+        assert_eq!(
+            Some(PATH_INFO.clone()),
+            svc.get(*PATH_INFO.store_path.digest()).await.unwrap()
+        );
+
+        // peek near, it should now be there.
+        assert_eq!(
+            Some(PATH_INFO.clone()),
+            svc.near.get(*PATH_INFO.store_path.digest()).await.unwrap()
+        );
+    }
+}