2 files changed, 144 insertions, 0 deletions
diff --git a/tvix/castore/src/directoryservice/combinators.rs b/tvix/castore/src/directoryservice/combinators.rs
new file mode 100644
index 000000000000..d3f351d6b689
--- /dev/null
+++ b/tvix/castore/src/directoryservice/combinators.rs
@@ -0,0 +1,142 @@
+use futures::stream::BoxStream;
+use futures::StreamExt;
+use futures::TryFutureExt;
+use futures::TryStreamExt;
+use tonic::async_trait;
+use tracing::{instrument, trace};
+
+use super::{DirectoryGraph, DirectoryService, RootToLeavesValidator, SimplePutter};
+use crate::directoryservice::DirectoryPutter;
+use crate::proto;
+use crate::B3Digest;
+use crate::Error;
+
+/// Asks near first, if not found, asks far.
+/// If found in there, returns it, and *inserts* it into
+/// near.
+/// Specifically, it always obtains the entire directory closure from far and inserts it into near,
+/// which is useful when far does not support accessing intermediate directories (but near does).
+/// There is no negative cache.
+/// Inserts and listings are not implemented for now.
+#[derive(Clone)]
+pub struct Cache<DS1, DS2> {
+    near: DS1,
+    far: DS2,
+}
+
+impl<DS1, DS2> Cache<DS1, DS2> {
+    pub fn new(near: DS1, far: DS2) -> Self {
+        Self { near, far }
+    }
+}
+
+#[async_trait]
+impl<DS1, DS2> DirectoryService for Cache<DS1, DS2>
+where
+    DS1: DirectoryService + Clone + 'static,
+    DS2: DirectoryService + Clone + 'static,
+{
+    #[instrument(skip(self, digest), fields(directory.digest = %digest))]
+    async fn get(&self, digest: &B3Digest) -> Result<Option<proto::Directory>, Error> {
+        match self.near.get(digest).await? {
+            Some(directory) => {
+                trace!("serving from cache");
+                Ok(Some(directory))
+            }
+            None => {
+                trace!("not found in near, asking remote…");
+
+                let mut copy = DirectoryGraph::with_order(
+                    RootToLeavesValidator::new_with_root_digest(digest.clone()),
+                );
+
+                let mut stream = self.far.get_recursive(digest);
+                let root = stream.try_next().await?;
+
+                if let Some(root) = root.clone() {
+                    copy.add(root)
+                        .map_err(|e| Error::StorageError(e.to_string()))?;
+                }
+
+                while let Some(dir) = stream.try_next().await? {
+                    copy.add(dir)
+                        .map_err(|e| Error::StorageError(e.to_string()))?;
+                }
+
+                let copy = copy
+                    .validate()
+                    .map_err(|e| Error::StorageError(e.to_string()))?;
+
+                let mut put = self.near.put_multiple_start();
+                for dir in copy.drain_leaves_to_root() {
+                    put.put(dir).await?;
+                }
+                put.close().await?;
+
+                Ok(root)
+            }
+        }
+    }
+
+    #[instrument(skip_all)]
+    async fn put(&self, _directory: proto::Directory) -> Result<B3Digest, Error> {
+        Err(Error::StorageError("unimplemented".to_string()))
+    }
+
+    #[instrument(skip_all, fields(directory.digest = %root_directory_digest))]
+    fn get_recursive(
+        &self,
+        root_directory_digest: &B3Digest,
+    ) -> BoxStream<'static, Result<proto::Directory, Error>> {
+        let near = self.near.clone();
+        let far = self.far.clone();
+        let digest = root_directory_digest.clone();
+        Box::pin(
+            (async move {
+                let mut stream = near.get_recursive(&digest);
+                match stream.try_next().await? {
+                    Some(first) => {
+                        trace!("serving from cache");
+                        Ok(futures::stream::once(async { Ok(first) })
+                            .chain(stream)
+                            .left_stream())
+                    }
+                    None => {
+                        trace!("not found in near, asking remote…");
+
+                        let mut copy_for_near = DirectoryGraph::with_order(
+                            RootToLeavesValidator::new_with_root_digest(digest.clone()),
+                        );
+                        let mut copy_for_client = vec![];
+
+                        let mut stream = far.get_recursive(&digest);
+                        while let Some(dir) = stream.try_next().await? {
+                            copy_for_near
+                                .add(dir.clone())
+                                .map_err(|e| Error::StorageError(e.to_string()))?;
+                            copy_for_client.push(dir);
+                        }
+
+                        let copy_for_near = copy_for_near
+                            .validate()
+                            .map_err(|e| Error::StorageError(e.to_string()))?;
+                        let mut put = near.put_multiple_start();
+                        for dir in copy_for_near.drain_leaves_to_root() {
+                            put.put(dir).await?;
+                        }
+                        put.close().await?;
+
+                        Ok(futures::stream::iter(copy_for_client.into_iter().map(Ok))
+                            .right_stream())
+                    }
+                }
+            })
+            .try_flatten_stream(),
+        )
+    }
+
+    #[instrument(skip_all)]
+    fn put_multiple_start(&self) -> Box<(dyn DirectoryPutter + 'static)> {
+        Box::new(SimplePutter::new((*self).clone()))
+    }
+}
diff --git a/tvix/castore/src/directoryservice/mod.rs b/tvix/castore/src/directoryservice/mod.rs
index 0d717a8a3f74..eff4a685fa6d 100644
--- a/tvix/castore/src/directoryservice/mod.rs
+++ b/tvix/castore/src/directoryservice/mod.rs
@@ -2,6 +2,7 @@ use crate::{proto, B3Digest, Error};
 use futures::stream::BoxStream;
 use tonic::async_trait;
 
+mod combinators;
 mod directory_graph;
 mod from_addr;
 mod grpc;
@@ -15,6 +16,7 @@ pub mod tests;
 mod traverse;
 mod utils;
 
+pub use self::combinators::Cache;
 pub use self::directory_graph::DirectoryGraph;
 pub use self::from_addr::from_addr;
 pub use self::grpc::GRPCDirectoryService;