chore(tvix): move store/fs to castore/fs

With the recent introduction of the RootNodes trait, there's nothing in
the fs module pulling in tvix-store dependencies, so it can live in
tvix-castore.

This allows other crates to make use of TvixStoreFS, without having to
pull in tvix-store.

For example, a tvix-build using a fuse mountpoint at /nix/store doesn't
need a PathInfoService to hold the root nodes that should be present,
but just a list.

tvix-store now has a pathinfoservice/fs module, which contains the
necessary glue logic to implement the RootNodes trait for a
PathInfoService.

To satisfy Rust orphan rules for trait implementations, we had to add a
small wrapper struct. It's mostly hidden away by the make_fs helper
function returning a TvixStoreFs.

It can't be entirely private, as its still leaking into the concrete
type of TvixStoreFS.

tvix-store still has `fuse` and `virtiofs` features, but they now simply
enable these features in the `tvix-castore` crate they depend on.

The tests for the fuse functionality stay in tvix-store for now, as
they populate the root nodes through a PathInfoService.

Once above mentioned "list of root nodes" implementation exists, we
might want to shuffle this around one more time.

Fixes b/341.

Change-Id: I989f664827a5a361b23b34368d242d10c157c756
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10378
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/store/src/pathinfoservice/fs/mod.rs

@@ -0,0 +1,84 @@
+use futures::Stream;
+use futures::StreamExt;
+use std::ops::Deref;
+use std::pin::Pin;
+use tonic::async_trait;
+use tvix_castore::fs::{RootNodes, TvixStoreFs};
+use tvix_castore::proto as castorepb;
+use tvix_castore::Error;
+use tvix_castore::{blobservice::BlobService, directoryservice::DirectoryService};
+
+use super::PathInfoService;
+
+#[cfg(test)]
+mod tests;
+
+/// Helper to construct a [TvixStoreFs] from a [BlobService], [DirectoryService]
+/// and [PathInfoService].
+/// This avoids users to have to interact with the wrapper struct directly, as
+/// it leaks into the type signature of TvixStoreFS.
+pub fn make_fs<BS, DS, PS>(
+    blob_service: BS,
+    directory_service: DS,
+    path_info_service: PS,
+    list_root: bool,
+) -> TvixStoreFs<BS, DS, RootNodesWrapper<PS>>
+where
+    BS: Deref<Target = dyn BlobService> + Send + Clone + 'static,
+    DS: Deref<Target = dyn DirectoryService> + Send + Clone + 'static,
+    PS: Deref<Target = dyn PathInfoService> + Send + Sync + Clone + 'static,
+{
+    TvixStoreFs::new(
+        blob_service,
+        directory_service,
+        RootNodesWrapper(path_info_service),
+        list_root,
+    )
+}
+
+/// Wrapper to satisfy Rust's orphan rules for trait implementations, as
+/// RootNodes is coming from the [tvix-castore] crate.
+#[doc(hidden)]
+#[derive(Clone, Debug)]
+pub struct RootNodesWrapper<T>(pub(crate) T);
+
+/// 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.
+#[cfg(any(feature = "fuse", feature = "virtiofs"))]
+#[async_trait]
+impl<T> RootNodes for RootNodesWrapper<T>
+where
+    T: Deref<Target = dyn PathInfoService> + Send + Sync,
+{
+    async fn get_by_basename(&self, name: &[u8]) -> Result<Option<castorepb::node::Node>, Error> {
+        let Ok(store_path) = nix_compat::store_path::StorePath::from_bytes(name) else {
+            return Ok(None);
+        };
+
+        Ok(self
+            .0
+            .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<castorepb::node::Node, Error>> + Send>> {
+        Box::pin(self.0.deref().list().map(|result| {
+            result.map(|path_info| {
+                path_info
+                    .node
+                    .expect("missing root node")
+                    .node
+                    .expect("empty node")
+            })
+        }))
+    }
+}

tvix/store/src/pathinfoservice/mod.rs

@@ -4,6 +4,9 @@ mod memory;
 mod nix_http;
 mod sled;
 
+#[cfg(any(feature = "fuse", feature = "virtiofs"))]
+mod fs;
+
 use futures::Stream;
 use std::pin::Pin;
 use tonic::async_trait;
@@ -18,6 +21,9 @@ pub use self::memory::MemoryPathInfoService;
 pub use self::nix_http::NixHTTPPathInfoService;
 pub use self::sled::SledPathInfoService;
 
+#[cfg(any(feature = "fuse", feature = "virtiofs"))]
+pub use self::fs::make_fs;
+
 /// The base trait all PathInfo services need to implement.
 #[async_trait]
 pub trait PathInfoService: Send + Sync {