refactor(tvix/store/fs): Decouple TvixStoreFs from Nix concepts r/7222

To support tvix builds, we need to be able to use the `TvixStoreFs` to
materialize the sandbox's /nix/store filesystem with just the list of
inputs needed for the build. Currently we'd need to seed an in-memory
`PathInfoService`, which includes more functionality than what is
required for `TvixStoreFs`. Additionally, the `PathInfoService` is
specific to Nix. By decoupling `TvixStoreFs` and `PathInfoService`,
we allow for usage of `TvixStoreFs` with `tvix-castore` without needing
a `PathInfoService`.

This introduces a new `RootNodes` trait which provides a way for the
filesystem to look up CA nodes via their basename in the root directory
of the filesystem. We then implement `RootNodes` for any
`PathInfoService`. Additionally, the filesystem root inode tracker now
stores basenames rather than `StorePath`s since `StorePath`s are
specific to Nix.

As a followup we can rename `TvixStoreFs` to `TvixCaStoreFs` and move
it to the `castore` crate (or its own crate).

b/341

Change-Id: I928372955017c23b1bf2b37190cbc508a4ed10d5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10363
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

1 files changed, 61 insertions, 0 deletions

diff --git a/tvix/store/src/fs/root_nodes.rs b/tvix/store/src/fs/root_nodes.rs
new file mode 100644
index 000000000000..e672c6e647f3
--- /dev/null
+++ b/tvix/store/src/fs/root_nodes.rs
@@ -0,0 +1,61 @@
+use std::{ops::Deref, pin::Pin};
+
+use futures::{Stream, StreamExt};
+use nix_compat::store_path::StorePath;
+use tonic::async_trait;
+use tvix_castore::{proto::node::Node, Error};
+
+use crate::pathinfoservice::PathInfoService;
+
+/// Provides an interface for looking up root nodes  in tvix-castore by given
+/// a lookup key (usually the basename), and optionally allow a listing.
+///
+#[async_trait]
+pub trait RootNodes: Send + Sync {
+    /// Looks up a root CA node based on the basename of the node in the root
+    /// directory of the filesystem.
+    async fn get_by_basename(&self, name: &[u8]) -> Result<Option<Node>, Error>;
+
+    /// Lists all root CA nodes in the filesystem. An error can be returned
+    /// in case listing is not allowed
+    fn list(&self) -> Pin<Box<dyn Stream<Item = Result<Node, Error>> + Send>>;
+}
+
+/// Implements root node lookup for any [PathInfoService]. This represents a flat
+/// directory structure like /nix/store where each entry in the root filesystem
+/// directory corresponds to a CA node.
+#[async_trait]
+impl<T> RootNodes for T
+where
+    T: Deref<Target = dyn PathInfoService> + Send + Sync,
+{
+    async fn get_by_basename(&self, name: &[u8]) -> Result<Option<Node>, Error> {
+        let Ok(store_path) = StorePath::from_bytes(name) else {
+            return Ok(None);
+        };
+
+        Ok(self
+            .deref()
+            .get(*store_path.digest())
+            .await?
+            .map(|path_info| {
+                path_info
+                    .node
+                    .expect("missing root node")
+                    .node
+                    .expect("empty node")
+            }))
+    }
+
+    fn list(&self) -> Pin<Box<dyn Stream<Item = Result<Node, Error>> + Send>> {
+        Box::pin(self.deref().list().map(|result| {
+            result.map(|path_info| {
+                path_info
+                    .node
+                    .expect("missing root node")
+                    .node
+                    .expect("empty node")
+            })
+        }))
+    }
+}