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>

18 files changed, 257 insertions, 199 deletions

diff --git a/tvix/Cargo.lock b/tvix/Cargo.lock
index 297adf5c846a..2f04f037a78e 100644
--- a/tvix/Cargo.lock
+++ b/tvix/Cargo.lock
@@ -3108,9 +3108,12 @@ dependencies = [
  "bstr",
  "bytes",
  "data-encoding",
+ "fuse-backend-rs",
  "futures",
  "hex-literal",
  "lazy_static",
+ "libc",
+ "parking_lot 0.12.1",
  "pin-project-lite",
  "prost",
  "prost-build",
@@ -3128,6 +3131,12 @@ dependencies = [
  "tower",
  "tracing",
  "url",
+ "vhost",
+ "vhost-user-backend",
+ "virtio-bindings 0.2.1",
+ "virtio-queue",
+ "vm-memory",
+ "vmm-sys-util",
  "walkdir",
 ]
 
@@ -3233,12 +3242,10 @@ dependencies = [
  "clap",
  "count-write",
  "data-encoding",
- "fuse-backend-rs",
  "futures",
  "lazy_static",
  "libc",
  "nix-compat",
- "parking_lot 0.12.1",
  "pin-project-lite",
  "prost",
  "prost-build",
@@ -3261,12 +3268,6 @@ dependencies = [
  "tracing-subscriber",
  "tvix-castore",
  "url",
- "vhost",
- "vhost-user-backend",
- "virtio-bindings 0.2.1",
- "virtio-queue",
- "vm-memory",
- "vmm-sys-util",
  "walkdir",
  "xz2",
 ]
diff --git a/tvix/Cargo.nix b/tvix/Cargo.nix
index 2c9120376871..5bae02070bcb 100644
--- a/tvix/Cargo.nix
+++ b/tvix/Cargo.nix
@@ -9609,6 +9609,11 @@ rec {
             packageId = "data-encoding";
           }
           {
+            name = "fuse-backend-rs";
+            packageId = "fuse-backend-rs";
+            optional = true;
+          }
+          {
             name = "futures";
             packageId = "futures";
           }
@@ -9617,6 +9622,15 @@ rec {
             packageId = "lazy_static";
           }
           {
+            name = "libc";
+            packageId = "libc";
+            optional = true;
+          }
+          {
+            name = "parking_lot";
+            packageId = "parking_lot 0.12.1";
+          }
+          {
             name = "pin-project-lite";
             packageId = "pin-project-lite";
           }
@@ -9669,6 +9683,36 @@ rec {
             packageId = "url";
           }
           {
+            name = "vhost";
+            packageId = "vhost";
+            optional = true;
+          }
+          {
+            name = "vhost-user-backend";
+            packageId = "vhost-user-backend";
+            optional = true;
+          }
+          {
+            name = "virtio-bindings";
+            packageId = "virtio-bindings 0.2.1";
+            optional = true;
+          }
+          {
+            name = "virtio-queue";
+            packageId = "virtio-queue";
+            optional = true;
+          }
+          {
+            name = "vm-memory";
+            packageId = "vm-memory";
+            optional = true;
+          }
+          {
+            name = "vmm-sys-util";
+            packageId = "vmm-sys-util";
+            optional = true;
+          }
+          {
             name = "walkdir";
             packageId = "walkdir";
           }
@@ -9702,9 +9746,12 @@ rec {
           }
         ];
         features = {
+          "fs" = [ "dep:libc" "dep:fuse-backend-rs" ];
+          "fuse" = [ "fs" ];
           "tonic-reflection" = [ "dep:tonic-reflection" ];
+          "virtiofs" = [ "fs" "dep:vhost" "dep:vhost-user-backend" "dep:virtio-queue" "dep:vm-memory" "dep:vmm-sys-util" "dep:virtio-bindings" "fuse-backend-rs?/vhost-user-fs" "fuse-backend-rs?/virtiofs" ];
         };
-        resolvedDefaultFeatures = [ "default" "tonic-reflection" ];
+        resolvedDefaultFeatures = [ "default" "fs" "fuse" "tonic-reflection" "virtiofs" ];
       };
       "tvix-cli" = rec {
         crateName = "tvix-cli";
@@ -10111,11 +10158,6 @@ rec {
             packageId = "data-encoding";
           }
           {
-            name = "fuse-backend-rs";
-            packageId = "fuse-backend-rs";
-            optional = true;
-          }
-          {
             name = "futures";
             packageId = "futures";
           }
@@ -10124,20 +10166,11 @@ rec {
             packageId = "lazy_static";
           }
           {
-            name = "libc";
-            packageId = "libc";
-            optional = true;
-          }
-          {
             name = "nix-compat";
             packageId = "nix-compat";
             features = [ "async" ];
           }
           {
-            name = "parking_lot";
-            packageId = "parking_lot 0.12.1";
-          }
-          {
             name = "pin-project-lite";
             packageId = "pin-project-lite";
           }
@@ -10215,36 +10248,6 @@ rec {
             packageId = "url";
           }
           {
-            name = "vhost";
-            packageId = "vhost";
-            optional = true;
-          }
-          {
-            name = "vhost-user-backend";
-            packageId = "vhost-user-backend";
-            optional = true;
-          }
-          {
-            name = "virtio-bindings";
-            packageId = "virtio-bindings 0.2.1";
-            optional = true;
-          }
-          {
-            name = "virtio-queue";
-            packageId = "virtio-queue";
-            optional = true;
-          }
-          {
-            name = "vm-memory";
-            packageId = "vm-memory";
-            optional = true;
-          }
-          {
-            name = "vmm-sys-util";
-            packageId = "vmm-sys-util";
-            optional = true;
-          }
-          {
             name = "walkdir";
             packageId = "walkdir";
           }
@@ -10265,6 +10268,10 @@ rec {
         ];
         devDependencies = [
           {
+            name = "libc";
+            packageId = "libc";
+          }
+          {
             name = "tempfile";
             packageId = "tempfile";
           }
@@ -10279,12 +10286,11 @@ rec {
         ];
         features = {
           "default" = [ "fuse" "tonic-reflection" ];
-          "fs" = [ "dep:libc" "dep:fuse-backend-rs" ];
-          "fuse" = [ "fs" ];
+          "fuse" = [ "tvix-castore/fuse" ];
           "tonic-reflection" = [ "dep:tonic-reflection" "tvix-castore/tonic-reflection" ];
-          "virtiofs" = [ "fs" "dep:vhost" "dep:vhost-user-backend" "dep:virtio-queue" "dep:vm-memory" "dep:vmm-sys-util" "dep:virtio-bindings" "fuse-backend-rs?/vhost-user-fs" "fuse-backend-rs?/virtiofs" ];
+          "virtiofs" = [ "tvix-castore/virtiofs" ];
         };
-        resolvedDefaultFeatures = [ "default" "fs" "fuse" "tonic-reflection" "virtiofs" ];
+        resolvedDefaultFeatures = [ "default" "fuse" "tonic-reflection" "virtiofs" ];
       };
       "typenum" = rec {
         crateName = "typenum";
diff --git a/tvix/castore/Cargo.toml b/tvix/castore/Cargo.toml
index 2a421280b82b..0f0146972969 100644
--- a/tvix/castore/Cargo.toml
+++ b/tvix/castore/Cargo.toml
@@ -10,6 +10,7 @@ bytes = "1.4.0"
 data-encoding = "2.3.3"
 futures = "0.3.28"
 lazy_static = "1.4.0"
+parking_lot = "0.12.1"
 pin-project-lite = "0.2.13"
 prost = "0.12.1"
 sled = { version = "0.34.7" }
@@ -25,10 +26,42 @@ walkdir = "2.4.0"
 bstr = "1.6.0"
 async-tempfile = "0.4.0"
 
+[dependencies.fuse-backend-rs]
+optional = true
+version = "0.11.0"
+
+[dependencies.libc]
+optional = true
+version = "0.2.144"
+
 [dependencies.tonic-reflection]
 optional = true
 version = "0.10.2"
 
+[dependencies.vhost]
+optional = true
+version = "0.6"
+
+[dependencies.vhost-user-backend]
+optional = true
+version = "0.8"
+
+[dependencies.virtio-queue]
+optional = true
+version = "0.7"
+
+[dependencies.vm-memory]
+optional = true
+version = "0.10"
+
+[dependencies.vmm-sys-util]
+optional = true
+version = "0.11"
+
+[dependencies.virtio-bindings]
+optional = true
+version = "0.2.1"
+
 [build-dependencies]
 prost-build = "0.12.1"
 tonic-build = "0.10.2"
@@ -41,4 +74,17 @@ hex-literal = "0.4.1"
 
 [features]
 default = []
+fs = ["dep:libc", "dep:fuse-backend-rs"]
+virtiofs = [
+  "fs",
+  "dep:vhost",
+  "dep:vhost-user-backend",
+  "dep:virtio-queue",
+  "dep:vm-memory",
+  "dep:vmm-sys-util",
+  "dep:virtio-bindings",
+  "fuse-backend-rs?/vhost-user-fs", # impl FsCacheReqHandler for SlaveFsCacheReq
+  "fuse-backend-rs?/virtiofs",
+]
+fuse = ["fs"]
 tonic-reflection = ["dep:tonic-reflection"]
diff --git a/tvix/store/src/fs/file_attr.rs b/tvix/castore/src/fs/file_attr.rs
index ad41f036a253..ad41f036a253 100644
--- a/tvix/store/src/fs/file_attr.rs
+++ b/tvix/castore/src/fs/file_attr.rs
diff --git a/tvix/store/src/fs/fuse.rs b/tvix/castore/src/fs/fuse.rs
index 98793bf47d58..98793bf47d58 100644
--- a/tvix/store/src/fs/fuse.rs
+++ b/tvix/castore/src/fs/fuse.rs
diff --git a/tvix/store/src/fs/inode_tracker.rs b/tvix/castore/src/fs/inode_tracker.rs
index 3cabbbd247b5..4a8283b6b144 100644
--- a/tvix/store/src/fs/inode_tracker.rs
+++ b/tvix/castore/src/fs/inode_tracker.rs
@@ -1,7 +1,7 @@
 use std::{collections::HashMap, sync::Arc};
 
 use super::inodes::{DirectoryInodeData, InodeData};
-use tvix_castore::B3Digest;
+use crate::B3Digest;
 
 /// InodeTracker keeps track of inodes, stores data being these inodes and deals
 /// with inode allocation.
@@ -132,7 +132,7 @@ impl InodeTracker {
 
 #[cfg(test)]
 mod tests {
-    use crate::tests::fixtures;
+    use crate::fixtures;
 
     use super::InodeData;
     use super::InodeTracker;
diff --git a/tvix/store/src/fs/inodes.rs b/tvix/castore/src/fs/inodes.rs
index 4047199982b2..9131b703bae0 100644
--- a/tvix/store/src/fs/inodes.rs
+++ b/tvix/castore/src/fs/inodes.rs
@@ -1,7 +1,7 @@
 //! This module contains all the data structures used to track information
-//! about inodes, which present tvix-store nodes in a filesystem.
-use tvix_castore::proto as castorepb;
-use tvix_castore::B3Digest;
+//! about inodes, which present tvix-castore nodes in a filesystem.
+use crate::proto as castorepb;
+use crate::B3Digest;
 
 #[derive(Clone, Debug)]
 pub enum InodeData {
diff --git a/tvix/store/src/fs/mod.rs b/tvix/castore/src/fs/mod.rs
index c11bd0a44c7e..9bd021cb09fe 100644
--- a/tvix/store/src/fs/mod.rs
+++ b/tvix/castore/src/fs/mod.rs
@@ -9,9 +9,13 @@ pub mod fuse;
 #[cfg(feature = "virtiofs")]
 pub mod virtiofs;
 
-#[cfg(test)]
-mod tests;
-
+use crate::proto as castorepb;
+use crate::{
+    blobservice::{BlobReader, BlobService},
+    directoryservice::DirectoryService,
+    proto::{node::Node, NamedNode},
+    B3Digest,
+};
 use fuse_backend_rs::abi::fuse_abi::stat64;
 use fuse_backend_rs::api::filesystem::{Context, FileSystem, FsOptions, ROOT_ID};
 use futures::StreamExt;
@@ -29,15 +33,8 @@ use tokio::{
     sync::mpsc,
 };
 use tracing::{debug, info_span, instrument, warn};
-use tvix_castore::proto as castorepb;
-use tvix_castore::{
-    blobservice::{BlobReader, BlobService},
-    directoryservice::DirectoryService,
-    proto::{node::Node, NamedNode},
-    B3Digest,
-};
 
-use self::root_nodes::RootNodes;
+pub use self::root_nodes::RootNodes;
 use self::{
     file_attr::{gen_file_attr, ROOT_FILE_ATTR},
     inode_tracker::InodeTracker,
@@ -75,6 +72,7 @@ use self::{
 /// merkle structure is a DAG, not a tree, this also means we can't do "bucketed
 /// allocation", aka reserve Directory.size inodes for each directory node we
 /// explore.
+/// Tests for this live in the tvix-store crate.
 pub struct TvixStoreFs<BS, DS, RN> {
     blob_service: BS,
     directory_service: DS,
diff --git a/tvix/castore/src/fs/root_nodes.rs b/tvix/castore/src/fs/root_nodes.rs
new file mode 100644
index 000000000000..8d27b477ff69
--- /dev/null
+++ b/tvix/castore/src/fs/root_nodes.rs
@@ -0,0 +1,18 @@
+use std::pin::Pin;
+
+use crate::{proto::node::Node, Error};
+use futures::Stream;
+use tonic::async_trait;
+
+/// 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>>;
+}
diff --git a/tvix/store/src/fs/virtiofs.rs b/tvix/castore/src/fs/virtiofs.rs
index 846270d28568..846270d28568 100644
--- a/tvix/store/src/fs/virtiofs.rs
+++ b/tvix/castore/src/fs/virtiofs.rs
diff --git a/tvix/castore/src/lib.rs b/tvix/castore/src/lib.rs
index 8d3dc7b4c4a5..8da0edef786b 100644
--- a/tvix/castore/src/lib.rs
+++ b/tvix/castore/src/lib.rs
@@ -4,6 +4,10 @@ mod errors;
 pub mod blobservice;
 pub mod directoryservice;
 pub mod fixtures;
+
+#[cfg(feature = "fs")]
+pub mod fs;
+
 pub mod import;
 pub mod proto;
 pub mod tonic;
diff --git a/tvix/store/Cargo.toml b/tvix/store/Cargo.toml
index 044b305b96ae..0a8e690e8250 100644
--- a/tvix/store/Cargo.toml
+++ b/tvix/store/Cargo.toml
@@ -14,7 +14,6 @@ data-encoding = "2.3.3"
 futures = "0.3.28"
 lazy_static = "1.4.0"
 nix-compat = { path = "../nix-compat", features = ["async"] }
-parking_lot = "0.12.1"
 pin-project-lite = "0.2.13"
 prost = "0.12.1"
 sha2 = "0.10.6"
@@ -35,42 +34,10 @@ async-recursion = "1.0.5"
 reqwest = { version = "0.11.22", features = ["rustls-tls", "stream"], default-features = false }
 xz2 = "0.1.7"
 
-[dependencies.fuse-backend-rs]
-optional = true
-version = "0.11.0"
-
-[dependencies.vhost]
-optional = true
-version = "0.6"
-
-[dependencies.vhost-user-backend]
-optional = true
-version = "0.8"
-
-[dependencies.virtio-queue]
-optional = true
-version = "0.7"
-
-[dependencies.vm-memory]
-optional = true
-version = "0.10"
-
-[dependencies.vmm-sys-util]
-optional = true
-version = "0.11"
-
-[dependencies.virtio-bindings]
-optional = true
-version = "0.2.1"
-
 [dependencies.tonic-reflection]
 optional = true
 version = "0.10.2"
 
-[dependencies.libc]
-optional = true
-version = "0.2.144"
-
 [build-dependencies]
 prost-build = "0.12.1"
 tonic-build = "0.10.2"
@@ -80,19 +47,11 @@ test-case = "2.2.2"
 tempfile = "3.3.0"
 tokio-retry = "0.3.0"
 
+[dev-dependencies.libc]
+version = "0.2.144"
+
 [features]
 default = ["fuse", "tonic-reflection"]
-fs = ["dep:libc", "dep:fuse-backend-rs"]
-virtiofs = [
-  "fs",
-  "dep:vhost",
-  "dep:vhost-user-backend",
-  "dep:virtio-queue",
-  "dep:vm-memory",
-  "dep:vmm-sys-util",
-  "dep:virtio-bindings",
-  "fuse-backend-rs?/vhost-user-fs", # impl FsCacheReqHandler for SlaveFsCacheReq
-  "fuse-backend-rs?/virtiofs",
-]
-fuse = ["fs"]
+fuse = ["tvix-castore/fuse"]
+virtiofs = ["tvix-castore/virtiofs"]
 tonic-reflection = ["dep:tonic-reflection", "tvix-castore/tonic-reflection"]
diff --git a/tvix/store/src/bin/tvix-store.rs b/tvix/store/src/bin/tvix-store.rs
index e4f2e0801b81..bf59366eb34e 100644
--- a/tvix/store/src/bin/tvix-store.rs
+++ b/tvix/store/src/bin/tvix-store.rs
@@ -29,14 +29,14 @@ use tvix_store::proto::GRPCPathInfoServiceWrapper;
 use tvix_store::proto::NarInfo;
 use tvix_store::proto::PathInfo;
 
-#[cfg(feature = "fs")]
-use tvix_store::fs::TvixStoreFs;
+#[cfg(any(feature = "fuse", feature = "virtiofs"))]
+use tvix_store::pathinfoservice::make_fs;
 
 #[cfg(feature = "fuse")]
-use tvix_store::fs::fuse::FuseDaemon;
+use tvix_castore::fs::fuse::FuseDaemon;
 
 #[cfg(feature = "virtiofs")]
-use tvix_store::fs::virtiofs::start_virtiofs_daemon;
+use tvix_castore::fs::virtiofs::start_virtiofs_daemon;
 
 #[cfg(feature = "tonic-reflection")]
 use tvix_castore::proto::FILE_DESCRIPTOR_SET as CASTORE_FILE_DESCRIPTOR_SET;
@@ -365,7 +365,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
             let path_info_service: Arc<dyn PathInfoService> = path_info_service.into();
 
             let mut fuse_daemon = tokio::task::spawn_blocking(move || {
-                let f = TvixStoreFs::new(
+                let fs = make_fs(
                     blob_service,
                     directory_service,
                     path_info_service,
@@ -373,7 +373,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                 );
                 info!("mounting tvix-store on {:?}", &dest);
 
-                FuseDaemon::new(f, &dest, threads)
+                FuseDaemon::new(fs, &dest, threads)
             })
             .await??;
 
@@ -409,7 +409,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
             let path_info_service: Arc<dyn PathInfoService> = path_info_service.into();
 
             tokio::task::spawn_blocking(move || {
-                let fs = TvixStoreFs::new(
+                let fs = make_fs(
                     blob_service,
                     directory_service,
                     path_info_service,
diff --git a/tvix/store/src/fs/root_nodes.rs b/tvix/store/src/fs/root_nodes.rs
deleted file mode 100644
index e672c6e647f3..000000000000
--- a/tvix/store/src/fs/root_nodes.rs
+++ /dev/null
@@ -1,61 +0,0 @@
-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")
-            })
-        }))
-    }
-}
diff --git a/tvix/store/src/lib.rs b/tvix/store/src/lib.rs
index c59121453352..5b57781c4dab 100644
--- a/tvix/store/src/lib.rs
+++ b/tvix/store/src/lib.rs
@@ -1,6 +1,3 @@
-#[cfg(feature = "fs")]
-pub mod fs;
-
 pub mod nar;
 pub mod pathinfoservice;
 pub mod proto;
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/fs/tests.rs b/tvix/store/src/pathinfoservice/fs/tests.rs
index a3977c727505..d12ee2f2a03e 100644
--- a/tvix/store/src/fs/tests.rs
+++ b/tvix/store/src/pathinfoservice/fs/tests.rs
@@ -1,22 +1,22 @@
+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 tempfile::TempDir;
-
-use crate::fs::{fuse::FuseDaemon, TvixStoreFs};
-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 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";
@@ -44,7 +44,7 @@ fn do_mount<P: AsRef<Path>>(
     mountpoint: P,
     list_root: bool,
 ) -> io::Result<FuseDaemon> {
-    let fs = TvixStoreFs::new(
+    let fs = make_fs(
         blob_service,
         directory_service,
         path_info_service,
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 {