chore(tvix): move store/fs to castore/fs r/7256

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>

3 files changed, 1261 insertions, 0 deletions

diff --git a/tvix/store/src/pathinfoservice/fs/mod.rs b/tvix/store/src/pathinfoservice/fs/mod.rs
new file mode 100644
index 000000000000..524aa10391f8
--- /dev/null
+++ b/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")
+            })
+        }))
+    }
+}
diff --git a/tvix/store/src/pathinfoservice/fs/tests.rs b/tvix/store/src/pathinfoservice/fs/tests.rs
new file mode 100644
index 000000000000..d12ee2f2a03e
--- /dev/null
+++ b/tvix/store/src/pathinfoservice/fs/tests.rs
@@ -0,0 +1,1171 @@
+use crate::pathinfoservice::PathInfoService;
+use crate::proto::PathInfo;
+use crate::tests::fixtures;
+use crate::tests::utils::{gen_blob_service, gen_directory_service, gen_pathinfo_service};
+use futures::StreamExt;
+use std::io::Cursor;
+use std::os::unix::prelude::MetadataExt;
+use std::path::Path;
+use std::sync::Arc;
+use tempfile::TempDir;
+use tokio::{fs, io};
+use tokio_stream::wrappers::ReadDirStream;
+use tvix_castore::blobservice::BlobService;
+use tvix_castore::directoryservice::DirectoryService;
+use tvix_castore::fs::fuse::FuseDaemon;
+use tvix_castore::proto as castorepb;
+
+use super::make_fs;
+
+const BLOB_A_NAME: &str = "00000000000000000000000000000000-test";
+const BLOB_B_NAME: &str = "55555555555555555555555555555555-test";
+const HELLOWORLD_BLOB_NAME: &str = "66666666666666666666666666666666-test";
+const SYMLINK_NAME: &str = "11111111111111111111111111111111-test";
+const SYMLINK_NAME2: &str = "44444444444444444444444444444444-test";
+const DIRECTORY_WITH_KEEP_NAME: &str = "22222222222222222222222222222222-test";
+const DIRECTORY_COMPLICATED_NAME: &str = "33333333333333333333333333333333-test";
+
+fn gen_svcs() -> (
+    Arc<dyn BlobService>,
+    Arc<dyn DirectoryService>,
+    Arc<dyn PathInfoService>,
+) {
+    let blob_service = gen_blob_service();
+    let directory_service = gen_directory_service();
+    let path_info_service = gen_pathinfo_service(blob_service.clone(), directory_service.clone());
+
+    (blob_service, directory_service, path_info_service)
+}
+
+fn do_mount<P: AsRef<Path>>(
+    blob_service: Arc<dyn BlobService>,
+    directory_service: Arc<dyn DirectoryService>,
+    path_info_service: Arc<dyn PathInfoService>,
+    mountpoint: P,
+    list_root: bool,
+) -> io::Result<FuseDaemon> {
+    let fs = make_fs(
+        blob_service,
+        directory_service,
+        path_info_service,
+        list_root,
+    );
+    FuseDaemon::new(fs, mountpoint.as_ref(), 4)
+}
+
+async fn populate_blob_a(
+    blob_service: &Arc<dyn BlobService>,
+    _directory_service: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // Upload BLOB_A
+    let mut bw = blob_service.open_write().await;
+    tokio::io::copy(&mut Cursor::new(fixtures::BLOB_A.to_vec()), &mut bw)
+        .await
+        .expect("must succeed uploading");
+    bw.close().await.expect("must succeed closing");
+
+    // Create a PathInfo for it
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::File(castorepb::FileNode {
+                name: BLOB_A_NAME.into(),
+                digest: fixtures::BLOB_A_DIGEST.clone().into(),
+                size: fixtures::BLOB_A.len() as u64,
+                executable: false,
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+async fn populate_blob_b(
+    blob_service: &Arc<dyn BlobService>,
+    _directory_service: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // Upload BLOB_B
+    let mut bw = blob_service.open_write().await;
+    tokio::io::copy(&mut Cursor::new(fixtures::BLOB_B.to_vec()), &mut bw)
+        .await
+        .expect("must succeed uploading");
+    bw.close().await.expect("must succeed closing");
+
+    // Create a PathInfo for it
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::File(castorepb::FileNode {
+                name: BLOB_B_NAME.into(),
+                digest: fixtures::BLOB_B_DIGEST.clone().into(),
+                size: fixtures::BLOB_B.len() as u64,
+                executable: false,
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+/// adds a blob containing helloworld and marks it as executable
+async fn populate_helloworld_blob(
+    blob_service: &Arc<dyn BlobService>,
+    _directory_service: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // Upload BLOB_B
+    let mut bw = blob_service.open_write().await;
+    tokio::io::copy(
+        &mut Cursor::new(fixtures::HELLOWORLD_BLOB_CONTENTS.to_vec()),
+        &mut bw,
+    )
+    .await
+    .expect("must succeed uploading");
+    bw.close().await.expect("must succeed closing");
+
+    // Create a PathInfo for it
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::File(castorepb::FileNode {
+                name: HELLOWORLD_BLOB_NAME.into(),
+                digest: fixtures::HELLOWORLD_BLOB_DIGEST.clone().into(),
+                size: fixtures::HELLOWORLD_BLOB_CONTENTS.len() as u64,
+                executable: true,
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+async fn populate_symlink(
+    _blob_service: &Arc<dyn BlobService>,
+    _directory_service: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // Create a PathInfo for it
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::Symlink(castorepb::SymlinkNode {
+                name: SYMLINK_NAME.into(),
+                target: BLOB_A_NAME.into(),
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+/// This writes a symlink pointing to /nix/store/somewhereelse,
+/// which is the same symlink target as "aa" inside DIRECTORY_COMPLICATED.
+async fn populate_symlink2(
+    _blob_service: &Arc<dyn BlobService>,
+    _directory_service: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // Create a PathInfo for it
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::Symlink(castorepb::SymlinkNode {
+                name: SYMLINK_NAME2.into(),
+                target: "/nix/store/somewhereelse".into(),
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+async fn populate_directory_with_keep(
+    blob_service: &Arc<dyn BlobService>,
+    directory_service: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // upload empty blob
+    let mut bw = blob_service.open_write().await;
+    assert_eq!(
+        fixtures::EMPTY_BLOB_DIGEST.as_slice(),
+        bw.close().await.expect("must succeed closing").as_slice(),
+    );
+
+    // upload directory
+    directory_service
+        .put(fixtures::DIRECTORY_WITH_KEEP.clone())
+        .await
+        .expect("must succeed uploading");
+
+    // upload pathinfo
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::Directory(castorepb::DirectoryNode {
+                name: DIRECTORY_WITH_KEEP_NAME.into(),
+                digest: fixtures::DIRECTORY_WITH_KEEP.digest().into(),
+                size: fixtures::DIRECTORY_WITH_KEEP.size(),
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+/// Insert [PathInfo] for DIRECTORY_WITH_KEEP, but don't provide the Directory
+/// itself.
+async fn populate_pathinfo_without_directory(
+    _: &Arc<dyn BlobService>,
+    _: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // upload pathinfo
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::Directory(castorepb::DirectoryNode {
+                name: DIRECTORY_WITH_KEEP_NAME.into(),
+                digest: fixtures::DIRECTORY_WITH_KEEP.digest().into(),
+                size: fixtures::DIRECTORY_WITH_KEEP.size(),
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+/// Insert , but don't provide the blob .keep is pointing to
+async fn populate_blob_a_without_blob(
+    _: &Arc<dyn BlobService>,
+    _: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // Create a PathInfo for blob A
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::File(castorepb::FileNode {
+                name: BLOB_A_NAME.into(),
+                digest: fixtures::BLOB_A_DIGEST.clone().into(),
+                size: fixtures::BLOB_A.len() as u64,
+                executable: false,
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+async fn populate_directory_complicated(
+    blob_service: &Arc<dyn BlobService>,
+    directory_service: &Arc<dyn DirectoryService>,
+    path_info_service: &Arc<dyn PathInfoService>,
+) {
+    // upload empty blob
+    let mut bw = blob_service.open_write().await;
+    assert_eq!(
+        fixtures::EMPTY_BLOB_DIGEST.as_slice(),
+        bw.close().await.expect("must succeed closing").as_slice(),
+    );
+
+    // upload inner directory
+    directory_service
+        .put(fixtures::DIRECTORY_WITH_KEEP.clone())
+        .await
+        .expect("must succeed uploading");
+
+    // uplodad parent directory
+    directory_service
+        .put(fixtures::DIRECTORY_COMPLICATED.clone())
+        .await
+        .expect("must succeed uploading");
+
+    // upload pathinfo
+    let path_info = PathInfo {
+        node: Some(castorepb::Node {
+            node: Some(castorepb::node::Node::Directory(castorepb::DirectoryNode {
+                name: DIRECTORY_COMPLICATED_NAME.into(),
+                digest: fixtures::DIRECTORY_COMPLICATED.digest().into(),
+                size: fixtures::DIRECTORY_COMPLICATED.size(),
+            })),
+        }),
+        ..Default::default()
+    };
+    path_info_service
+        .put(path_info)
+        .await
+        .expect("must succeed");
+}
+
+/// Ensure mounting itself doesn't fail
+#[tokio::test]
+async fn mount() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Ensure listing the root isn't allowed
+#[tokio::test]
+async fn root() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    {
+        // read_dir succeeds, but getting the first element will fail.
+        let mut it = ReadDirStream::new(fs::read_dir(tmpdir).await.expect("must succeed"));
+
+        let err = it
+            .next()
+            .await
+            .expect("must be some")
+            .expect_err("must be err");
+        assert_eq!(std::io::ErrorKind::PermissionDenied, err.kind());
+    }
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Ensure listing the root is allowed if configured explicitly
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn root_with_listing() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_blob_a(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        true, /* allow listing */
+    )
+    .expect("must succeed");
+
+    {
+        // read_dir succeeds, but getting the first element will fail.
+        let mut it = ReadDirStream::new(fs::read_dir(tmpdir).await.expect("must succeed"));
+
+        let e = it
+            .next()
+            .await
+            .expect("must be some")
+            .expect("must succeed");
+
+        let metadata = e.metadata().await.expect("must succeed");
+        assert!(metadata.is_file());
+        assert!(metadata.permissions().readonly());
+        assert_eq!(fixtures::BLOB_A.len() as u64, metadata.len());
+    }
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Ensure we can stat a file at the root
+#[tokio::test]
+async fn stat_file_at_root() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_blob_a(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(BLOB_A_NAME);
+
+    // peek at the file metadata
+    let metadata = fs::metadata(p).await.expect("must succeed");
+
+    assert!(metadata.is_file());
+    assert!(metadata.permissions().readonly());
+    assert_eq!(fixtures::BLOB_A.len() as u64, metadata.len());
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Ensure we can read a file at the root
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn read_file_at_root() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_blob_a(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(BLOB_A_NAME);
+
+    // read the file contents
+    let data = fs::read(p).await.expect("must succeed");
+
+    // ensure size and contents match
+    assert_eq!(fixtures::BLOB_A.len(), data.len());
+    assert_eq!(fixtures::BLOB_A.to_vec(), data);
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Ensure we can read a large file at the root
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn read_large_file_at_root() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_blob_b(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(BLOB_B_NAME);
+    {
+        // peek at the file metadata
+        let metadata = fs::metadata(&p).await.expect("must succeed");
+
+        assert!(metadata.is_file());
+        assert!(metadata.permissions().readonly());
+        assert_eq!(fixtures::BLOB_B.len() as u64, metadata.len());
+    }
+
+    // read the file contents
+    let data = fs::read(p).await.expect("must succeed");
+
+    // ensure size and contents match
+    assert_eq!(fixtures::BLOB_B.len(), data.len());
+    assert_eq!(fixtures::BLOB_B.to_vec(), data);
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Read the target of a symlink
+#[tokio::test]
+async fn symlink_readlink() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_symlink(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(SYMLINK_NAME);
+
+    let target = fs::read_link(&p).await.expect("must succeed");
+    assert_eq!(BLOB_A_NAME, target.to_str().unwrap());
+
+    // peek at the file metadata, which follows symlinks.
+    // this must fail, as we didn't populate the target.
+    let e = fs::metadata(&p).await.expect_err("must fail");
+    assert_eq!(std::io::ErrorKind::NotFound, e.kind());
+
+    // peeking at the file metadata without following symlinks will succeed.
+    let metadata = fs::symlink_metadata(&p).await.expect("must succeed");
+    assert!(metadata.is_symlink());
+
+    // reading from the symlink (which follows) will fail, because the target doesn't exist.
+    let e = fs::read(p).await.expect_err("must fail");
+    assert_eq!(std::io::ErrorKind::NotFound, e.kind());
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Read and stat a regular file through a symlink pointing to it.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn read_stat_through_symlink() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_blob_a(&blob_service, &directory_service, &path_info_service).await;
+    populate_symlink(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p_symlink = tmpdir.path().join(SYMLINK_NAME);
+    let p_blob = tmpdir.path().join(SYMLINK_NAME);
+
+    // peek at the file metadata, which follows symlinks.
+    // this must now return the same metadata as when statting at the target directly.
+    let metadata_symlink = fs::metadata(&p_symlink).await.expect("must succeed");
+    let metadata_blob = fs::metadata(&p_blob).await.expect("must succeed");
+    assert_eq!(metadata_blob.file_type(), metadata_symlink.file_type());
+    assert_eq!(metadata_blob.len(), metadata_symlink.len());
+
+    // reading from the symlink (which follows) will return the same data as if
+    // we were reading from the file directly.
+    assert_eq!(
+        fs::read(p_blob).await.expect("must succeed"),
+        fs::read(p_symlink).await.expect("must succeed"),
+    );
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Read a directory in the root, and validate some attributes.
+#[tokio::test]
+async fn read_stat_directory() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_directory_with_keep(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(DIRECTORY_WITH_KEEP_NAME);
+
+    // peek at the metadata of the directory
+    let metadata = fs::metadata(p).await.expect("must succeed");
+    assert!(metadata.is_dir());
+    assert!(metadata.permissions().readonly());
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+/// Read a blob inside a directory. This ensures we successfully populate directory data.
+async fn read_blob_inside_dir() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_directory_with_keep(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(DIRECTORY_WITH_KEEP_NAME).join(".keep");
+
+    // peek at metadata.
+    let metadata = fs::metadata(&p).await.expect("must succeed");
+    assert!(metadata.is_file());
+    assert!(metadata.permissions().readonly());
+
+    // read from it
+    let data = fs::read(&p).await.expect("must succeed");
+    assert_eq!(fixtures::EMPTY_BLOB_CONTENTS.to_vec(), data);
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+/// Read a blob inside a directory inside a directory. This ensures we properly
+/// populate directories as we traverse down the structure.
+async fn read_blob_deep_inside_dir() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_directory_complicated(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir
+        .path()
+        .join(DIRECTORY_COMPLICATED_NAME)
+        .join("keep")
+        .join(".keep");
+
+    // peek at metadata.
+    let metadata = fs::metadata(&p).await.expect("must succeed");
+    assert!(metadata.is_file());
+    assert!(metadata.permissions().readonly());
+
+    // read from it
+    let data = fs::read(&p).await.expect("must succeed");
+    assert_eq!(fixtures::EMPTY_BLOB_CONTENTS.to_vec(), data);
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Ensure readdir works.
+#[tokio::test]
+async fn readdir() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_directory_complicated(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(DIRECTORY_COMPLICATED_NAME);
+
+    {
+        // read_dir should succeed. Collect all elements
+        let elements: Vec<_> = ReadDirStream::new(fs::read_dir(p).await.expect("must succeed"))
+            .map(|e| e.expect("must not be err"))
+            .collect()
+            .await;
+
+        assert_eq!(3, elements.len(), "number of elements should be 3"); // rust skips . and ..
+
+        // We explicitly look at specific positions here, because we always emit
+        // them ordered.
+
+        // ".keep", 0 byte file.
+        let e = &elements[0];
+        assert_eq!(".keep", e.file_name());
+        assert!(e.file_type().await.expect("must succeed").is_file());
+        assert_eq!(0, e.metadata().await.expect("must succeed").len());
+
+        // "aa", symlink.
+        let e = &elements[1];
+        assert_eq!("aa", e.file_name());
+        assert!(e.file_type().await.expect("must succeed").is_symlink());
+
+        // "keep", directory
+        let e = &elements[2];
+        assert_eq!("keep", e.file_name());
+        assert!(e.file_type().await.expect("must succeed").is_dir());
+    }
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+#[tokio::test]
+/// Do a readdir deeper inside a directory, without doing readdir or stat in the parent directory.
+async fn readdir_deep() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_directory_complicated(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(DIRECTORY_COMPLICATED_NAME).join("keep");
+
+    {
+        // read_dir should succeed. Collect all elements
+        let elements: Vec<_> = ReadDirStream::new(fs::read_dir(p).await.expect("must succeed"))
+            .map(|e| e.expect("must not be err"))
+            .collect()
+            .await;
+
+        assert_eq!(1, elements.len(), "number of elements should be 1"); // rust skips . and ..
+
+        // ".keep", 0 byte file.
+        let e = &elements[0];
+        assert_eq!(".keep", e.file_name());
+        assert!(e.file_type().await.expect("must succeed").is_file());
+        assert_eq!(0, e.metadata().await.expect("must succeed").len());
+    }
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Check attributes match how they show up in /nix/store normally.
+#[tokio::test]
+async fn check_attributes() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_blob_a(&blob_service, &directory_service, &path_info_service).await;
+    populate_directory_with_keep(&blob_service, &directory_service, &path_info_service).await;
+    populate_symlink(&blob_service, &directory_service, &path_info_service).await;
+    populate_helloworld_blob(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p_file = tmpdir.path().join(BLOB_A_NAME);
+    let p_directory = tmpdir.path().join(DIRECTORY_WITH_KEEP_NAME);
+    let p_symlink = tmpdir.path().join(SYMLINK_NAME);
+    let p_executable_file = tmpdir.path().join(HELLOWORLD_BLOB_NAME);
+
+    // peek at metadata. We use symlink_metadata to ensure we don't traverse a symlink by accident.
+    let metadata_file = fs::symlink_metadata(&p_file).await.expect("must succeed");
+    let metadata_executable_file = fs::symlink_metadata(&p_executable_file)
+        .await
+        .expect("must succeed");
+    let metadata_directory = fs::symlink_metadata(&p_directory)
+        .await
+        .expect("must succeed");
+    let metadata_symlink = fs::symlink_metadata(&p_symlink)
+        .await
+        .expect("must succeed");
+
+    // modes should match. We & with 0o777 to remove any higher bits.
+    assert_eq!(0o444, metadata_file.mode() & 0o777);
+    assert_eq!(0o555, metadata_executable_file.mode() & 0o777);
+    assert_eq!(0o555, metadata_directory.mode() & 0o777);
+    assert_eq!(0o444, metadata_symlink.mode() & 0o777);
+
+    // files should have the correct filesize
+    assert_eq!(fixtures::BLOB_A.len() as u64, metadata_file.len());
+    // directories should have their "size" as filesize
+    assert_eq!(
+        { fixtures::DIRECTORY_WITH_KEEP.size() },
+        metadata_directory.size()
+    );
+
+    for metadata in &[&metadata_file, &metadata_directory, &metadata_symlink] {
+        // uid and gid should be 0.
+        assert_eq!(0, metadata.uid());
+        assert_eq!(0, metadata.gid());
+
+        // all times should be set to the unix epoch.
+        assert_eq!(0, metadata.atime());
+        assert_eq!(0, metadata.mtime());
+        assert_eq!(0, metadata.ctime());
+        // crtime seems MacOS only
+    }
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+#[tokio::test]
+/// Ensure we allocate the same inodes for the same directory contents.
+/// $DIRECTORY_COMPLICATED_NAME/keep contains the same data as $DIRECTORY_WITH_KEEP.
+async fn compare_inodes_directories() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_directory_with_keep(&blob_service, &directory_service, &path_info_service).await;
+    populate_directory_complicated(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p_dir_with_keep = tmpdir.path().join(DIRECTORY_WITH_KEEP_NAME);
+    let p_sibling_dir = tmpdir.path().join(DIRECTORY_COMPLICATED_NAME).join("keep");
+
+    // peek at metadata.
+    assert_eq!(
+        fs::metadata(p_dir_with_keep)
+            .await
+            .expect("must succeed")
+            .ino(),
+        fs::metadata(p_sibling_dir)
+            .await
+            .expect("must succeed")
+            .ino()
+    );
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Ensure we allocate the same inodes for the same directory contents.
+/// $DIRECTORY_COMPLICATED_NAME/keep/,keep contains the same data as $DIRECTORY_COMPLICATED_NAME/.keep
+#[tokio::test]
+async fn compare_inodes_files() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_directory_complicated(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p_keep1 = tmpdir.path().join(DIRECTORY_COMPLICATED_NAME).join(".keep");
+    let p_keep2 = tmpdir
+        .path()
+        .join(DIRECTORY_COMPLICATED_NAME)
+        .join("keep")
+        .join(".keep");
+
+    // peek at metadata.
+    assert_eq!(
+        fs::metadata(p_keep1).await.expect("must succeed").ino(),
+        fs::metadata(p_keep2).await.expect("must succeed").ino()
+    );
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Ensure we allocate the same inode for symlinks pointing to the same targets.
+/// $DIRECTORY_COMPLICATED_NAME/aa points to the same target as SYMLINK_NAME2.
+#[tokio::test]
+async fn compare_inodes_symlinks() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_directory_complicated(&blob_service, &directory_service, &path_info_service).await;
+    populate_symlink2(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p1 = tmpdir.path().join(DIRECTORY_COMPLICATED_NAME).join("aa");
+    let p2 = tmpdir.path().join(SYMLINK_NAME2);
+
+    // peek at metadata.
+    assert_eq!(
+        fs::symlink_metadata(p1).await.expect("must succeed").ino(),
+        fs::symlink_metadata(p2).await.expect("must succeed").ino()
+    );
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Check we match paths exactly.
+#[tokio::test]
+async fn read_wrong_paths_in_root() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_blob_a(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    // wrong name
+    assert!(
+        fs::metadata(tmpdir.path().join("00000000000000000000000000000000-tes"))
+            .await
+            .is_err()
+    );
+
+    // invalid hash
+    assert!(
+        fs::metadata(tmpdir.path().join("0000000000000000000000000000000-test"))
+            .await
+            .is_err()
+    );
+
+    // right name, must exist
+    assert!(
+        fs::metadata(tmpdir.path().join("00000000000000000000000000000000-test"))
+            .await
+            .is_ok()
+    );
+
+    // now wrong name with right hash still may not exist
+    assert!(
+        fs::metadata(tmpdir.path().join("00000000000000000000000000000000-tes"))
+            .await
+            .is_err()
+    );
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+/// Make sure writes are not allowed
+#[tokio::test]
+async fn disallow_writes() {
+    // https://plume.benboeckel.net/~/JustAnotherBlog/skipping-tests-in-rust
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(BLOB_A_NAME);
+    let e = fs::File::create(p).await.expect_err("must fail");
+
+    assert_eq!(Some(libc::EROFS), e.raw_os_error());
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+#[tokio::test]
+/// Ensure we get an IO error if the directory service does not have the Directory object.
+async fn missing_directory() {
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_pathinfo_without_directory(&blob_service, &directory_service, &path_info_service)
+        .await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(DIRECTORY_WITH_KEEP_NAME);
+
+    {
+        // `stat` on the path should succeed, because it doesn't trigger the directory request.
+        fs::metadata(&p).await.expect("must succeed");
+
+        // However, calling either `readdir` or `stat` on a child should fail with an IO error.
+        // It fails when trying to pull the first entry, because we don't implement opendir separately
+        ReadDirStream::new(fs::read_dir(&p).await.unwrap())
+            .next()
+            .await
+            .expect("must be some")
+            .expect_err("must be err");
+
+        // rust currently sets e.kind() to Uncategorized, which isn't very
+        // helpful, so we don't look at the error more closely than that..
+        fs::metadata(p.join(".keep")).await.expect_err("must fail");
+    }
+
+    fuse_daemon.unmount().expect("unmount");
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+/// Ensure we get an IO error if the blob service does not have the blob
+async fn missing_blob() {
+    if !std::path::Path::new("/dev/fuse").exists() {
+        eprintln!("skipping test");
+        return;
+    }
+    let tmpdir = TempDir::new().unwrap();
+
+    let (blob_service, directory_service, path_info_service) = gen_svcs();
+    populate_blob_a_without_blob(&blob_service, &directory_service, &path_info_service).await;
+
+    let mut fuse_daemon = do_mount(
+        blob_service,
+        directory_service,
+        path_info_service,
+        tmpdir.path(),
+        false,
+    )
+    .expect("must succeed");
+
+    let p = tmpdir.path().join(BLOB_A_NAME);
+
+    {
+        // `stat` on the blob should succeed, because it doesn't trigger a request to the blob service.
+        fs::metadata(&p).await.expect("must succeed");
+
+        // However, calling read on the blob should fail.
+        // rust currently sets e.kind() to Uncategorized, which isn't very
+        // helpful, so we don't look at the error more closely than that..
+        fs::read(p).await.expect_err("must fail");
+    }
+
+    fuse_daemon.unmount().expect("unmount");
+}
diff --git a/tvix/store/src/pathinfoservice/mod.rs b/tvix/store/src/pathinfoservice/mod.rs
index 5faa0900a0b0..3bd0ef206998 100644
--- a/tvix/store/src/pathinfoservice/mod.rs
+++ b/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 {