refactor(tvix/store/fs): Separate FUSE and filesystem code r/6622

In prepration for adding virtiofs support, I thought it would make sense
to split out the filesystem implementation from FUSE itself.

The `fs` module holds the tvix-store filesystem implemetation and the
`fuse` module holds the code to spawn a FUSE daemon backed by multiple
threads.

Change-Id: I8c58447b8c3aa016a613068f8e7ec166554e237c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9343
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: Connor Brewster <cbrewster@hey.com>

1 files changed, 660 insertions, 0 deletions

diff --git a/tvix/store/src/fs/mod.rs b/tvix/store/src/fs/mod.rs
new file mode 100644
index 0000000000..48e6054063
--- /dev/null
+++ b/tvix/store/src/fs/mod.rs
@@ -0,0 +1,660 @@
+mod file_attr;
+mod inode_tracker;
+mod inodes;
+
+#[cfg(feature = "fuse")]
+pub mod fuse;
+
+#[cfg(test)]
+mod tests;
+
+use crate::{
+    blobservice::{BlobReader, BlobService},
+    directoryservice::DirectoryService,
+    pathinfoservice::PathInfoService,
+    proto::{node::Node, NamedNode},
+    B3Digest, Error,
+};
+use fuse_backend_rs::api::filesystem::{Context, FileSystem, FsOptions, ROOT_ID};
+use nix_compat::store_path::StorePath;
+use parking_lot::RwLock;
+use std::{
+    collections::HashMap,
+    io,
+    str::FromStr,
+    sync::atomic::AtomicU64,
+    sync::{atomic::Ordering, Arc},
+    time::Duration,
+};
+use tokio::io::{AsyncBufReadExt, AsyncSeekExt};
+use tracing::{debug, info_span, warn};
+
+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 [PathInfoService].
+///
+/// We don't allow listing on the root mountpoint (inode 0).
+/// In the future, this might be made configurable once a listing method is
+/// added to [self.path_info_service], and then show all store paths in that
+/// store.
+///
+/// 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, and store the currently "explored" store paths together with
+/// root inode of the root.
+///
+/// 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 PathInfo.
+pub struct TvixStoreFs {
+    blob_service: Arc<dyn BlobService>,
+    directory_service: Arc<dyn DirectoryService>,
+    path_info_service: Arc<dyn PathInfoService>,
+
+    /// Whether to (try) listing elements in the root.
+    list_root: bool,
+
+    /// This maps a given StorePath to the inode we allocated for the root inode.
+    store_paths: RwLock<HashMap<StorePath, u64>>,
+
+    /// This keeps track of inodes and data alongside them.
+    inode_tracker: RwLock<InodeTracker>,
+
+    /// This holds all open file handles
+    file_handles: RwLock<HashMap<u64, Arc<tokio::sync::Mutex<Box<dyn BlobReader>>>>>,
+
+    next_file_handle: AtomicU64,
+
+    tokio_handle: tokio::runtime::Handle,
+}
+
+impl TvixStoreFs {
+    pub fn new(
+        blob_service: Arc<dyn BlobService>,
+        directory_service: Arc<dyn DirectoryService>,
+        path_info_service: Arc<dyn PathInfoService>,
+        list_root: bool,
+    ) -> Self {
+        Self {
+            blob_service,
+            directory_service,
+            path_info_service,
+
+            list_root,
+
+            store_paths: 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(),
+        }
+    }
+
+    /// This will turn a lookup request for [std::ffi::OsStr] in the root to
+    /// a ino and [InodeData].
+    /// It will peek in [self.store_paths], and then either look it up from
+    /// [self.inode_tracker],
+    /// or otherwise fetch from [self.path_info_service], and then insert into
+    /// [self.inode_tracker].
+    fn name_in_root_to_ino_and_data(
+        &self,
+        name: &std::ffi::CStr,
+    ) -> Result<Option<(u64, Arc<InodeData>)>, Error> {
+        // parse the name into a [StorePath].
+        let store_path = if let Some(name) = name.to_str().ok() {
+            match StorePath::from_str(name) {
+                Ok(store_path) => store_path,
+                Err(e) => {
+                    debug!(e=?e, "unable to parse as store path");
+                    // This is not an error, but a "ENOENT", as someone can stat
+                    // a file inside the root that's no valid store path
+                    return Ok(None);
+                }
+            }
+        } else {
+            debug!("{name:?} is not a valid utf-8 string");
+            // same here.
+            return Ok(None);
+        };
+
+        let ino = {
+            // This extra scope makes sure we drop the read lock
+            // immediately after reading, to prevent deadlocks.
+            let store_paths = self.store_paths.read();
+            store_paths.get(&store_path).cloned()
+        };
+
+        if let Some(ino) = ino {
+            // If we already have that store path, lookup the inode from
+            // self.store_paths and then get the data from [self.inode_tracker],
+            // which in the case of a [InodeData::Directory] will be fully
+            // populated.
+            Ok(Some((
+                ino,
+                self.inode_tracker.read().get(ino).expect("must exist"),
+            )))
+        } else {
+            // If we don't have it, look it up in PathInfoService.
+            match self.path_info_service.get(store_path.digest)? {
+                // the pathinfo doesn't exist, so the file doesn't exist.
+                None => Ok(None),
+                Some(path_info) => {
+                    // The pathinfo does exist, so there must be a root node
+                    let root_node = path_info.node.unwrap().node.unwrap();
+
+                    // The name must match what's passed in the lookup, otherwise we return nothing.
+                    if root_node.get_name() != store_path.to_string().as_bytes() {
+                        return Ok(None);
+                    }
+
+                    // Let's check if someone else beat us to updating the inode tracker and
+                    // store_paths map.
+                    let mut store_paths = self.store_paths.write();
+                    if let Some(ino) = store_paths.get(&store_path).cloned() {
+                        return Ok(Some((
+                            ino,
+                            self.inode_tracker.read().get(ino).expect("must exist"),
+                        )));
+                    }
+
+                    // insert the (sparse) inode data and register in
+                    // self.store_paths.
+                    // FUTUREWORK: change put to return the data after
+                    // inserting, so we don't need to lookup a second
+                    // time?
+                    let (ino, inode) = {
+                        let mut inode_tracker = self.inode_tracker.write();
+                        let ino = inode_tracker.put((&root_node).into());
+                        (ino, inode_tracker.get(ino).unwrap())
+                    };
+                    store_paths.insert(store_path, ino);
+
+                    Ok(Some((ino, inode)))
+                }
+            }
+        }
+    }
+
+    /// This will lookup a directory by digest, and will turn it into a
+    /// [InodeData::Directory(DirectoryInodeData::Populated(..))].
+    /// This is both used to initially insert the root node of a store path,
+    /// as well as when looking up an intermediate DirectoryNode.
+    fn fetch_directory_inode_data(&self, directory_digest: &B3Digest) -> Result<InodeData, Error> {
+        match self.directory_service.get(directory_digest) {
+            Err(e) => {
+                warn!(e = e.to_string(), directory.digest=%directory_digest, "failed to get directory");
+                Err(e)
+            }
+            // If the Directory can't be found, this is a hole, bail out.
+            Ok(None) => {
+                tracing::error!(directory.digest=%directory_digest, "directory not found in directory service");
+                Err(Error::StorageError(format!(
+                    "directory {} not found",
+                    directory_digest
+                )))
+            }
+            Ok(Some(directory)) => Ok(directory.into()),
+        }
+    }
+}
+
+impl FileSystem for TvixStoreFs {
+    type Inode = u64;
+    type Handle = 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<(libc::stat64, Duration)> {
+        if inode == ROOT_ID {
+            return Ok((ROOT_FILE_ATTR.into(), Duration::MAX));
+        }
+
+        match self.inode_tracker.read().get(inode) {
+            None => return 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.store_paths] (fetching from PathInfoService 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 {
+            return match self.name_in_root_to_ino_and_data(name) {
+                Err(e) => {
+                    warn!("{}", e);
+                    Err(io::Error::from_raw_os_error(libc::ENOENT))
+                }
+                Ok(None) => Err(io::Error::from_raw_os_error(libc::ENOENT)),
+                Ok(Some((ino, inode_data))) => {
+                    debug!(inode_data=?&inode_data, ino=ino, "Some");
+                    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.
+        // It might not be populated yet, so if it isn't, we do (by
+        // fetching from [self.directory_service]), and save the result in
+        // [self.inode_tracker].
+        // Now it for sure is populated, so we search for that name in the
+        // list of children and return the FileAttrs.
+
+        // TODO: Reduce the critical section of this write lock.
+        let mut inode_tracker = self.inode_tracker.write();
+        let parent_data = inode_tracker.get(parent).unwrap();
+        let parent_data = match *parent_data {
+            InodeData::Regular(..) | InodeData::Symlink(_) => {
+                // if the parent inode was not a directory, this doesn't make sense
+                return Err(io::Error::from_raw_os_error(libc::ENOTDIR));
+            }
+            InodeData::Directory(DirectoryInodeData::Sparse(ref parent_digest, _)) => {
+                match self.fetch_directory_inode_data(parent_digest) {
+                    Ok(new_data) => {
+                        // update data in [self.inode_tracker] with populated variant.
+                        // FUTUREWORK: change put to return the data after
+                        // inserting, so we don't need to lookup a second
+                        // time?
+                        let ino = inode_tracker.put(new_data);
+                        inode_tracker.get(ino).unwrap()
+                    }
+                    Err(_e) => {
+                        return Err(io::Error::from_raw_os_error(libc::EIO));
+                    }
+                }
+            }
+            InodeData::Directory(DirectoryInodeData::Populated(..)) => parent_data,
+        };
+
+        // now parent_data can only be a [InodeData::Directory(DirectoryInodeData::Populated(..))].
+        let (parent_digest, children) = if let InodeData::Directory(
+            DirectoryInodeData::Populated(ref parent_digest, ref children),
+        ) = *parent_data
+        {
+            (parent_digest, children)
+        } else {
+            panic!("unexpected type")
+        };
+        let span = info_span!("lookup", directory.digest = %parent_digest);
+        let _enter = span.enter();
+
+        // 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 = inode_tracker.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 {
+                for (i, path_info) in self
+                    .path_info_service
+                    .list()
+                    .skip(offset as usize)
+                    .enumerate()
+                {
+                    let path_info = match path_info {
+                        Err(e) => {
+                            warn!("failed to retrieve pathinfo: {}", e);
+                            return Err(io::Error::from_raw_os_error(libc::EPERM));
+                        }
+                        Ok(path_info) => path_info,
+                    };
+
+                    // We know the root node exists and the store_path can be parsed because clients MUST validate.
+                    let root_node = path_info.node.unwrap().node.unwrap();
+                    let store_path = StorePath::from_bytes(root_node.get_name()).unwrap();
+
+                    let ino = {
+                        // This extra scope makes sure we drop the read lock
+                        // immediately after reading, to prevent deadlocks.
+                        let store_paths = self.store_paths.read();
+                        store_paths.get(&store_path).cloned()
+                    };
+                    let ino = match ino {
+                        Some(ino) => ino,
+                        None => {
+                            // insert the (sparse) inode data and register in
+                            // self.store_paths.
+                            let ino = self.inode_tracker.write().put((&root_node).into());
+                            self.store_paths.write().insert(store_path.clone(), 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: store_path.to_string().as_bytes(),
+                    })?;
+                    // If the buffer is full, add_entry will return `Ok(0)`.
+                    if written == 0 {
+                        break;
+                    }
+                }
+                return Ok(());
+            }
+        }
+
+        // lookup the inode data.
+        let mut inode_tracker = self.inode_tracker.write();
+        let dir_inode_data = inode_tracker.get(inode).unwrap();
+        let dir_inode_data = match *dir_inode_data {
+            InodeData::Regular(..) | InodeData::Symlink(..) => {
+                warn!("Not a directory");
+                return Err(io::Error::from_raw_os_error(libc::ENOTDIR));
+            }
+            InodeData::Directory(DirectoryInodeData::Sparse(ref directory_digest, _)) => {
+                match self.fetch_directory_inode_data(directory_digest) {
+                    Ok(new_data) => {
+                        // update data in [self.inode_tracker] with populated variant.
+                        // FUTUREWORK: change put to return the data after
+                        // inserting, so we don't need to lookup a second
+                        // time?
+                        let ino = inode_tracker.put(new_data.clone());
+                        inode_tracker.get(ino).unwrap()
+                    }
+                    Err(_e) => {
+                        return Err(io::Error::from_raw_os_error(libc::EIO));
+                    }
+                }
+            }
+            InodeData::Directory(DirectoryInodeData::Populated(..)) => dir_inode_data,
+        };
+
+        // now parent_data can only be InodeData::Directory(DirectoryInodeData::Populated(..))
+        if let InodeData::Directory(DirectoryInodeData::Populated(ref _digest, ref children)) =
+            *dir_inode_data
+        {
+            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 {
+                        Node::Directory(_) => libc::S_IFDIR,
+                        Node::File(_) => libc::S_IFREG,
+                        Node::Symlink(_) => libc::S_IFLNK,
+                    },
+                    name: child_node.get_name(),
+                })?;
+                // If the buffer is full, add_entry will return `Ok(0)`.
+                if written == 0 {
+                    break;
+                }
+            }
+        } else {
+            panic!("unexpected type")
+        }
+
+        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");
+                return 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");
+                        return Err(io::Error::from_raw_os_error(libc::EIO));
+                    }
+                    Err(e) => {
+                        warn!(e=?e, "error opening blob");
+                        return 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 as u64)).await;
+
+            match resp {
+                Ok(pos) => {
+                    debug_assert_eq!(offset as u64, 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);
+
+            while (buf.len() as u64) < size as u64 {
+                let int_buf = blob_reader.fill_buf().await?;
+                // copy things from the internal buffer into buf to fill it till up until size
+
+                // an empty buffer signals we reached EOF.
+                if int_buf.is_empty() {
+                    break;
+                }
+
+                // calculate how many bytes we can read from int_buf.
+                // It's either all of int_buf, or the number of bytes missing in buf to reach size.
+                let len_to_copy = std::cmp::min(int_buf.len(), size as usize - buf.len());
+
+                // copy these bytes into our buffer
+                buf.extend_from_slice(&int_buf[..len_to_copy]);
+                // and consume them in the buffered reader.
+                blob_reader.consume(len_to_copy);
+            }
+
+            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()),
+        }
+    }
+}