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>

1 files changed, 627 insertions, 0 deletions

diff --git a/tvix/castore/src/fs/mod.rs b/tvix/castore/src/fs/mod.rs
new file mode 100644
index 0000000000..9bd021cb09
--- /dev/null
+++ b/tvix/castore/src/fs/mod.rs
@@ -0,0 +1,627 @@
+mod file_attr;
+mod inode_tracker;
+mod inodes;
+mod root_nodes;
+
+#[cfg(feature = "fuse")]
+pub mod fuse;
+
+#[cfg(feature = "virtiofs")]
+pub mod virtiofs;
+
+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;
+use parking_lot::RwLock;
+use std::ops::Deref;
+use std::{
+    collections::HashMap,
+    io,
+    sync::atomic::AtomicU64,
+    sync::{atomic::Ordering, Arc},
+    time::Duration,
+};
+use tokio::{
+    io::{AsyncReadExt, AsyncSeekExt},
+    sync::mpsc,
+};
+use tracing::{debug, info_span, instrument, warn};
+
+pub use self::root_nodes::RootNodes;
+use self::{
+    file_attr::{gen_file_attr, ROOT_FILE_ATTR},
+    inode_tracker::InodeTracker,
+    inodes::{DirectoryInodeData, InodeData},
+};
+
+/// This implements a read-only FUSE filesystem for a tvix-store
+/// with the passed [BlobService], [DirectoryService] and [RootNodes].
+///
+/// Linux uses inodes in filesystems. When implementing FUSE, most calls are
+/// *for* a given inode.
+///
+/// This means, we need to have a stable mapping of inode numbers to the
+/// corresponding store nodes.
+///
+/// We internally delegate all inode allocation and state keeping to the
+/// inode tracker.
+/// We store a mapping from currently "explored" names in the root to their
+/// inode.
+///
+/// There's some places where inodes are allocated / data inserted into
+/// the inode tracker, if not allocated before already:
+///  - Processing a `lookup` request, either in the mount root, or somewhere
+///    deeper.
+///  - Processing a `readdir` request
+///
+///  Things pointing to the same contents get the same inodes, irrespective of
+///  their own location.
+///  This means:
+///  - Symlinks with the same target will get the same inode.
+///  - Regular/executable files with the same contents will get the same inode
+///  - Directories with the same contents will get the same inode.
+///
+/// Due to the above being valid across the whole store, and considering the
+/// 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,
+    root_nodes_provider: RN,
+
+    /// Whether to (try) listing elements in the root.
+    list_root: bool,
+
+    /// This maps a given basename in the root to the inode we allocated for the node.
+    root_nodes: RwLock<HashMap<Vec<u8>, u64>>,
+
+    /// This keeps track of inodes and data alongside them.
+    inode_tracker: RwLock<InodeTracker>,
+
+    /// This holds all open file handles
+    #[allow(clippy::type_complexity)]
+    file_handles: RwLock<HashMap<u64, Arc<tokio::sync::Mutex<Box<dyn BlobReader>>>>>,
+
+    next_file_handle: AtomicU64,
+
+    tokio_handle: tokio::runtime::Handle,
+}
+
+impl<BS, DS, RN> TvixStoreFs<BS, DS, RN>
+where
+    BS: Deref<Target = dyn BlobService> + Clone + Send,
+    DS: Deref<Target = dyn DirectoryService> + Clone + Send + 'static,
+    RN: RootNodes + Clone + 'static,
+{
+    pub fn new(
+        blob_service: BS,
+        directory_service: DS,
+        root_nodes_provider: RN,
+        list_root: bool,
+    ) -> Self {
+        Self {
+            blob_service,
+            directory_service,
+            root_nodes_provider,
+
+            list_root,
+
+            root_nodes: RwLock::new(HashMap::default()),
+            inode_tracker: RwLock::new(Default::default()),
+
+            file_handles: RwLock::new(Default::default()),
+            next_file_handle: AtomicU64::new(1),
+            tokio_handle: tokio::runtime::Handle::current(),
+        }
+    }
+
+    /// Retrieves the inode for a given root node basename, if present.
+    /// This obtains a read lock on self.root_nodes.
+    fn get_inode_for_root_name(&self, name: &[u8]) -> Option<u64> {
+        self.root_nodes.read().get(name).cloned()
+    }
+
+    /// For a given inode, look up the given directory behind it (from
+    /// self.inode_tracker), and return its children.
+    /// The inode_tracker MUST know about this inode already, and it MUST point
+    /// to a [InodeData::Directory].
+    /// It is ok if it's a [DirectoryInodeData::Sparse] - in that case, a lookup
+    /// in self.directory_service is performed, and self.inode_tracker is updated with the
+    /// [DirectoryInodeData::Populated].
+    #[instrument(skip(self), err)]
+    fn get_directory_children(&self, ino: u64) -> io::Result<(B3Digest, Vec<(u64, Node)>)> {
+        let data = self.inode_tracker.read().get(ino).unwrap();
+        match *data {
+            // if it's populated already, return children.
+            InodeData::Directory(DirectoryInodeData::Populated(
+                ref parent_digest,
+                ref children,
+            )) => Ok((parent_digest.clone(), children.clone())),
+            // if it's sparse, fetch data using directory_service, populate child nodes
+            // and update it in [self.inode_tracker].
+            InodeData::Directory(DirectoryInodeData::Sparse(ref parent_digest, _)) => {
+                let directory = self
+                    .tokio_handle
+                    .block_on(self.tokio_handle.spawn({
+                        let directory_service = self.directory_service.clone();
+                        let parent_digest = parent_digest.to_owned();
+                        async move { directory_service.get(&parent_digest).await }
+                    }))
+                    .unwrap()?
+                    .ok_or_else(|| {
+                        warn!(directory.digest=%parent_digest, "directory not found");
+                        // If the Directory can't be found, this is a hole, bail out.
+                        io::Error::from_raw_os_error(libc::EIO)
+                    })?;
+
+                // Turn the retrieved directory into a InodeData::Directory(DirectoryInodeData::Populated(..)),
+                // allocating inodes for the children on the way.
+                let children = {
+                    let mut inode_tracker = self.inode_tracker.write();
+
+                    let children: Vec<(u64, castorepb::node::Node)> = directory
+                        .nodes()
+                        .map(|child_node| {
+                            let child_ino = inode_tracker.put((&child_node).into());
+                            (child_ino, child_node)
+                        })
+                        .collect();
+
+                    // replace.
+                    inode_tracker.replace(
+                        ino,
+                        Arc::new(InodeData::Directory(DirectoryInodeData::Populated(
+                            parent_digest.clone(),
+                            children.clone(),
+                        ))),
+                    );
+
+                    children
+                };
+
+                Ok((parent_digest.clone(), children))
+            }
+            // if the parent inode was not a directory, this doesn't make sense
+            InodeData::Regular(..) | InodeData::Symlink(_) => {
+                Err(io::Error::from_raw_os_error(libc::ENOTDIR))
+            }
+        }
+    }
+
+    /// This will turn a lookup request for a name in the root to a ino and
+    /// [InodeData].
+    /// It will peek in [self.root_nodes], and then either look it up from
+    /// [self.inode_tracker],
+    /// or otherwise fetch from [self.root_nodes], and then insert into
+    /// [self.inode_tracker].
+    /// In the case the name can't be found, a libc::ENOENT is returned.
+    fn name_in_root_to_ino_and_data(
+        &self,
+        name: &std::ffi::CStr,
+    ) -> io::Result<(u64, Arc<InodeData>)> {
+        // Look up the inode for that root node.
+        // If there's one, [self.inode_tracker] MUST also contain the data,
+        // which we can then return.
+        if let Some(inode) = self.get_inode_for_root_name(name.to_bytes()) {
+            return Ok((
+                inode,
+                self.inode_tracker
+                    .read()
+                    .get(inode)
+                    .expect("must exist")
+                    .to_owned(),
+            ));
+        }
+
+        // We don't have it yet, look it up in [self.root_nodes].
+        match self.tokio_handle.block_on({
+            let root_nodes_provider = self.root_nodes_provider.clone();
+            async move { root_nodes_provider.get_by_basename(name.to_bytes()).await }
+        }) {
+            // if there was an error looking up the root node, propagate up an IO error.
+            Err(_e) => Err(io::Error::from_raw_os_error(libc::EIO)),
+            // the root node doesn't exist, so the file doesn't exist.
+            Ok(None) => Err(io::Error::from_raw_os_error(libc::ENOENT)),
+            // The root node does exist
+            Ok(Some(root_node)) => {
+                // The name must match what's passed in the lookup, otherwise this is also a ENOENT.
+                if root_node.get_name() != name.to_bytes() {
+                    debug!(root_node.name=?root_node.get_name(), found_node.name=%name.to_string_lossy(), "node name mismatch");
+                    return Err(io::Error::from_raw_os_error(libc::ENOENT));
+                }
+
+                // Let's check if someone else beat us to updating the inode tracker and
+                // root_nodes map. This avoids locking inode_tracker for writing.
+                if let Some(ino) = self.root_nodes.read().get(name.to_bytes()) {
+                    return Ok((
+                        *ino,
+                        self.inode_tracker.read().get(*ino).expect("must exist"),
+                    ));
+                }
+
+                // Only in case it doesn't, lock [self.root_nodes] and
+                // [self.inode_tracker] for writing.
+                let mut root_nodes = self.root_nodes.write();
+                let mut inode_tracker = self.inode_tracker.write();
+
+                // insert the (sparse) inode data and register in
+                // self.root_nodes.
+                let inode_data: InodeData = (&root_node).into();
+                let ino = inode_tracker.put(inode_data.clone());
+                root_nodes.insert(name.to_bytes().into(), ino);
+
+                Ok((ino, Arc::new(inode_data)))
+            }
+        }
+    }
+}
+
+impl<BS, DS, RN> FileSystem for TvixStoreFs<BS, DS, RN>
+where
+    BS: Deref<Target = dyn BlobService> + Clone + Send + 'static,
+    DS: Deref<Target = dyn DirectoryService> + Send + Clone + 'static,
+    RN: RootNodes + Clone + 'static,
+{
+    type Handle = u64;
+    type Inode = u64;
+
+    fn init(&self, _capable: FsOptions) -> io::Result<FsOptions> {
+        Ok(FsOptions::empty())
+    }
+
+    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
+    fn getattr(
+        &self,
+        _ctx: &Context,
+        inode: Self::Inode,
+        _handle: Option<Self::Handle>,
+    ) -> io::Result<(stat64, Duration)> {
+        if inode == ROOT_ID {
+            return Ok((ROOT_FILE_ATTR.into(), Duration::MAX));
+        }
+
+        match self.inode_tracker.read().get(inode) {
+            None => Err(io::Error::from_raw_os_error(libc::ENOENT)),
+            Some(node) => {
+                debug!(node = ?node, "found node");
+                Ok((gen_file_attr(&node, inode).into(), Duration::MAX))
+            }
+        }
+    }
+
+    #[tracing::instrument(skip_all, fields(rq.parent_inode = parent, rq.name = ?name))]
+    fn lookup(
+        &self,
+        _ctx: &Context,
+        parent: Self::Inode,
+        name: &std::ffi::CStr,
+    ) -> io::Result<fuse_backend_rs::api::filesystem::Entry> {
+        debug!("lookup");
+
+        // This goes from a parent inode to a node.
+        // - If the parent is [ROOT_ID], we need to check
+        //   [self.root_nodes] (fetching from a [RootNode] provider if needed)
+        // - Otherwise, lookup the parent in [self.inode_tracker] (which must be
+        //   a [InodeData::Directory]), and find the child with that name.
+        if parent == ROOT_ID {
+            let (ino, inode_data) = self.name_in_root_to_ino_and_data(name)?;
+
+            debug!(inode_data=?&inode_data, ino=ino, "Some");
+            return Ok(fuse_backend_rs::api::filesystem::Entry {
+                inode: ino,
+                attr: gen_file_attr(&inode_data, ino).into(),
+                attr_timeout: Duration::MAX,
+                entry_timeout: Duration::MAX,
+                ..Default::default()
+            });
+        }
+        // This is the "lookup for "a" inside inode 42.
+        // We already know that inode 42 must be a directory.
+        let (parent_digest, children) = self.get_directory_children(parent)?;
+
+        let span = info_span!("lookup", directory.digest = %parent_digest);
+        let _enter = span.enter();
+
+        // Search for that name in the list of children and return the FileAttrs.
+
+        // in the children, find the one with the desired name.
+        if let Some((child_ino, _)) = children.iter().find(|e| e.1.get_name() == name.to_bytes()) {
+            // lookup the child [InodeData] in [self.inode_tracker].
+            // We know the inodes for children have already been allocated.
+            let child_inode_data = self.inode_tracker.read().get(*child_ino).unwrap();
+
+            // Reply with the file attributes for the child.
+            // For child directories, we still have all data we need to reply.
+            Ok(fuse_backend_rs::api::filesystem::Entry {
+                inode: *child_ino,
+                attr: gen_file_attr(&child_inode_data, *child_ino).into(),
+                attr_timeout: Duration::MAX,
+                entry_timeout: Duration::MAX,
+                ..Default::default()
+            })
+        } else {
+            // Child not found, return ENOENT.
+            Err(io::Error::from_raw_os_error(libc::ENOENT))
+        }
+    }
+
+    // TODO: readdirplus?
+
+    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.offset = offset))]
+    fn readdir(
+        &self,
+        _ctx: &Context,
+        inode: Self::Inode,
+        _handle: Self::Handle,
+        _size: u32,
+        offset: u64,
+        add_entry: &mut dyn FnMut(fuse_backend_rs::api::filesystem::DirEntry) -> io::Result<usize>,
+    ) -> io::Result<()> {
+        debug!("readdir");
+
+        if inode == ROOT_ID {
+            if !self.list_root {
+                return Err(io::Error::from_raw_os_error(libc::EPERM)); // same error code as ipfs/kubo
+            } else {
+                let root_nodes_provider = self.root_nodes_provider.clone();
+                let (tx, mut rx) = mpsc::channel(16);
+
+                // This task will run in the background immediately and will exit
+                // after the stream ends or if we no longer want any more entries.
+                self.tokio_handle.spawn(async move {
+                    let mut stream = root_nodes_provider.list().skip(offset as usize).enumerate();
+                    while let Some(node) = stream.next().await {
+                        if tx.send(node).await.is_err() {
+                            // If we get a send error, it means the sync code
+                            // doesn't want any more entries.
+                            break;
+                        }
+                    }
+                });
+
+                while let Some((i, root_node)) = rx.blocking_recv() {
+                    let root_node = match root_node {
+                        Err(e) => {
+                            warn!("failed to retrieve pathinfo: {}", e);
+                            return Err(io::Error::from_raw_os_error(libc::EPERM));
+                        }
+                        Ok(root_node) => root_node,
+                    };
+
+                    let name = root_node.get_name();
+                    // obtain the inode, or allocate a new one.
+                    let ino = self.get_inode_for_root_name(name).unwrap_or_else(|| {
+                        // insert the (sparse) inode data and register in
+                        // self.root_nodes.
+                        let ino = self.inode_tracker.write().put((&root_node).into());
+                        self.root_nodes.write().insert(name.into(), ino);
+                        ino
+                    });
+
+                    let ty = match root_node {
+                        Node::Directory(_) => libc::S_IFDIR,
+                        Node::File(_) => libc::S_IFREG,
+                        Node::Symlink(_) => libc::S_IFLNK,
+                    };
+
+                    let written = add_entry(fuse_backend_rs::api::filesystem::DirEntry {
+                        ino,
+                        offset: offset + i as u64 + 1,
+                        type_: ty,
+                        name,
+                    })?;
+                    // If the buffer is full, add_entry will return `Ok(0)`.
+                    if written == 0 {
+                        break;
+                    }
+                }
+
+                return Ok(());
+            }
+        }
+
+        // lookup the children, or return an error if it's not a directory.
+        let (parent_digest, children) = self.get_directory_children(inode)?;
+
+        let span = info_span!("lookup", directory.digest = %parent_digest);
+        let _enter = span.enter();
+
+        for (i, (ino, child_node)) in children.iter().skip(offset as usize).enumerate() {
+            // the second parameter will become the "offset" parameter on the next call.
+            let written = add_entry(fuse_backend_rs::api::filesystem::DirEntry {
+                ino: *ino,
+                offset: offset + i as u64 + 1,
+                type_: match child_node {
+                    #[allow(clippy::unnecessary_cast)]
+                    // libc::S_IFDIR is u32 on Linux and u16 on MacOS
+                    Node::Directory(_) => libc::S_IFDIR as u32,
+                    #[allow(clippy::unnecessary_cast)]
+                    // libc::S_IFDIR is u32 on Linux and u16 on MacOS
+                    Node::File(_) => libc::S_IFREG as u32,
+                    #[allow(clippy::unnecessary_cast)]
+                    // libc::S_IFDIR is u32 on Linux and u16 on MacOS
+                    Node::Symlink(_) => libc::S_IFLNK as u32,
+                },
+                name: child_node.get_name(),
+            })?;
+            // If the buffer is full, add_entry will return `Ok(0)`.
+            if written == 0 {
+                break;
+            }
+        }
+
+        Ok(())
+    }
+
+    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
+    fn open(
+        &self,
+        _ctx: &Context,
+        inode: Self::Inode,
+        _flags: u32,
+        _fuse_flags: u32,
+    ) -> io::Result<(
+        Option<Self::Handle>,
+        fuse_backend_rs::api::filesystem::OpenOptions,
+    )> {
+        if inode == ROOT_ID {
+            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
+        }
+
+        // lookup the inode
+        match *self.inode_tracker.read().get(inode).unwrap() {
+            // read is invalid on non-files.
+            InodeData::Directory(..) | InodeData::Symlink(_) => {
+                warn!("is directory");
+                Err(io::Error::from_raw_os_error(libc::EISDIR))
+            }
+            InodeData::Regular(ref blob_digest, _blob_size, _) => {
+                let span = info_span!("read", blob.digest = %blob_digest);
+                let _enter = span.enter();
+
+                let blob_service = self.blob_service.clone();
+                let blob_digest = blob_digest.clone();
+
+                let task = self
+                    .tokio_handle
+                    .spawn(async move { blob_service.open_read(&blob_digest).await });
+
+                let blob_reader = self.tokio_handle.block_on(task).unwrap();
+
+                match blob_reader {
+                    Ok(None) => {
+                        warn!("blob not found");
+                        Err(io::Error::from_raw_os_error(libc::EIO))
+                    }
+                    Err(e) => {
+                        warn!(e=?e, "error opening blob");
+                        Err(io::Error::from_raw_os_error(libc::EIO))
+                    }
+                    Ok(Some(blob_reader)) => {
+                        // get a new file handle
+                        // TODO: this will overflow after 2**64 operations,
+                        // which is fine for now.
+                        // See https://cl.tvl.fyi/c/depot/+/8834/comment/a6684ce0_d72469d1
+                        // for the discussion on alternatives.
+                        let fh = self.next_file_handle.fetch_add(1, Ordering::SeqCst);
+
+                        debug!("add file handle {}", fh);
+                        self.file_handles
+                            .write()
+                            .insert(fh, Arc::new(tokio::sync::Mutex::new(blob_reader)));
+
+                        Ok((
+                            Some(fh),
+                            fuse_backend_rs::api::filesystem::OpenOptions::empty(),
+                        ))
+                    }
+                }
+            }
+        }
+    }
+
+    #[tracing::instrument(skip_all, fields(rq.inode = inode, fh = handle))]
+    fn release(
+        &self,
+        _ctx: &Context,
+        inode: Self::Inode,
+        _flags: u32,
+        handle: Self::Handle,
+        _flush: bool,
+        _flock_release: bool,
+        _lock_owner: Option<u64>,
+    ) -> io::Result<()> {
+        // remove and get ownership on the blob reader
+        match self.file_handles.write().remove(&handle) {
+            // drop it, which will close it.
+            Some(blob_reader) => drop(blob_reader),
+            None => {
+                // These might already be dropped if a read error occured.
+                debug!("file_handle {} not found", handle);
+            }
+        }
+
+        Ok(())
+    }
+
+    #[tracing::instrument(skip_all, fields(rq.inode = inode, rq.offset = offset, rq.size = size))]
+    fn read(
+        &self,
+        _ctx: &Context,
+        inode: Self::Inode,
+        handle: Self::Handle,
+        w: &mut dyn fuse_backend_rs::api::filesystem::ZeroCopyWriter,
+        size: u32,
+        offset: u64,
+        _lock_owner: Option<u64>,
+        _flags: u32,
+    ) -> io::Result<usize> {
+        debug!("read");
+
+        // We need to take out the blob reader from self.file_handles, so we can
+        // interact with it in the separate task.
+        // On success, we pass it back out of the task, so we can put it back in self.file_handles.
+        let blob_reader = match self.file_handles.read().get(&handle) {
+            Some(blob_reader) => blob_reader.clone(),
+            None => {
+                warn!("file handle {} unknown", handle);
+                return Err(io::Error::from_raw_os_error(libc::EIO));
+            }
+        };
+
+        let task = self.tokio_handle.spawn(async move {
+            let mut blob_reader = blob_reader.lock().await;
+
+            // seek to the offset specified, which is relative to the start of the file.
+            let resp = blob_reader.seek(io::SeekFrom::Start(offset)).await;
+
+            match resp {
+                Ok(pos) => {
+                    debug_assert_eq!(offset, pos);
+                }
+                Err(e) => {
+                    warn!("failed to seek to offset {}: {}", offset, e);
+                    return Err(io::Error::from_raw_os_error(libc::EIO));
+                }
+            }
+
+            // As written in the fuse docs, read should send exactly the number
+            // of bytes requested except on EOF or error.
+
+            let mut buf: Vec<u8> = Vec::with_capacity(size as usize);
+
+            // copy things from the internal buffer into buf to fill it till up until size
+            tokio::io::copy(&mut blob_reader.as_mut().take(size as u64), &mut buf).await?;
+
+            Ok(buf)
+        });
+
+        let buf = self.tokio_handle.block_on(task).unwrap()?;
+
+        w.write(&buf)
+    }
+
+    #[tracing::instrument(skip_all, fields(rq.inode = inode))]
+    fn readlink(&self, _ctx: &Context, inode: Self::Inode) -> io::Result<Vec<u8>> {
+        if inode == ROOT_ID {
+            return Err(io::Error::from_raw_os_error(libc::ENOSYS));
+        }
+
+        // lookup the inode
+        match *self.inode_tracker.read().get(inode).unwrap() {
+            InodeData::Directory(..) | InodeData::Regular(..) => {
+                Err(io::Error::from_raw_os_error(libc::EINVAL))
+            }
+            InodeData::Symlink(ref target) => Ok(target.to_vec()),
+        }
+    }
+}