// SPDX-License-Identifier: AGPL-3.0-or-later
//! Contains the types used to implement [LocalFs].
use crate::{msg::*, server::FsProvider};

use btlib::{
    accessor::Accessor,
    bterr,
    crypto::{rand_vec, Creds, Decrypter, HashKind, Signer, SymKey},
    error::BtErr,
    AuthzAttrs, BlockAccessor, BlockError, BlockMeta, BlockMetaSecrets, BlockOpenOptions,
    BlockPath, BlockReader, DirEntry, Directory, Epoch, FileBlock, FlushMeta, IssuedProcRec,
    MetaAccess, MetaReader, Positioned, Principal, Principaled, ProcRec, Result, Split, TrySeek,
    ZeroExtendable,
};
use btserde::{read_from, write_to};
use core::future::Ready;
use log::{debug, error, warn};
use positioned_io::{ReadAt, Size};
use serde::{Deserialize, Serialize};
use std::{
    collections::hash_map::{self, HashMap},
    fmt::{Display, Formatter},
    fs::File,
    future::Future,
    io::{self, Seek, SeekFrom, Write as IoWrite},
    net::{IpAddr, Ipv6Addr},
    ops::{Deref, DerefMut},
    path::{Path, PathBuf},
    sync::{
        atomic::{AtomicU64, Ordering},
        Arc,
    },
    time::Duration,
};
use tokio::sync::{
    Mutex, MutexGuard, OwnedMutexGuard, OwnedRwLockReadGuard, RwLock, RwLockReadGuard,
    RwLockWriteGuard,
};
use zeroize::ZeroizeOnDrop;

pub use private::{Authorizer, AuthzContext, Error, LocalFs, ModeAuthorizer};

mod private {

    use super::*;

    type Inode = u64;
    type Handle = u64;

    /// Represents the errors which can occur in the [LocalFs] type.
    #[derive(Debug, Clone, PartialEq, Eq, Hash, Serialize, Deserialize)]
    pub enum Error {
        NotOpen(Inode),
        InvalidHandle { inode: Inode, handle: Handle },
        NoHandlesAvailable(Inode),
        InodeNotFound(Inode),
        ReadOnlyHandle(Handle),
        WrongOwner,
    }

    impl Display for Error {
        fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
            match self {
                Error::NotOpen(inode) => write!(f, "inode {inode} is not open"),
                Error::InvalidHandle { inode, handle } => {
                    write!(f, "invalid handle {handle} for inode {inode}")
                }
                Error::NoHandlesAvailable(inode) => {
                    write!(f, "no handles are available for inode {inode}")
                }
                Error::InodeNotFound(inode) => write!(f, "inode {inode} could not be found"),
                Error::ReadOnlyHandle(handle) => {
                    write!(f, "cannot mutably access read-only handle {handle}")
                }
                Error::WrongOwner => write!(f, "handle is not owned by the requestor"),
            }
        }
    }

    trait BlockPathExt {
        fn assert_eq(&self, other: &BlockPath) -> Result<()>;
    }

    impl BlockPathExt for BlockPath {
        fn assert_eq(&self, other: &BlockPath) -> Result<()> {
            if self != other {
                Err(Error::WrongOwner.into())
            } else {
                Ok(())
            }
        }
    }

    impl std::error::Error for Error {}

    /// This type provides context for an authorization decision as to whether a given process will
    /// be allowed to access a block.
    pub struct AuthzContext<'a> {
        /// The path from which this request was received.
        pub from: &'a BlockPath,
        /// The attributes of the principal whose access is being authorized.
        pub attrs: &'a AuthzAttrs,
        /// A reference to the metadata of a block, the access to which is being authorized.
        pub meta: &'a BlockMeta,
    }

    impl<'a> AuthzContext<'a> {
        fn new(from: &'a BlockPath, attrs: &'a AuthzAttrs, meta: &'a BlockMeta) -> Self {
            Self { from, attrs, meta }
        }
    }

    /// A trait for types which can render authorization decisions.
    pub trait Authorizer {
        /// Returns [Ok] if read authorization is granted, and [Err] otherwise.
        fn can_read(&self, ctx: &AuthzContext<'_>) -> io::Result<()>;
        /// Returns [Ok] if write authorization is granted, and [Err] otherwise.
        fn can_write(&self, ctx: &AuthzContext<'_>) -> io::Result<()>;
        /// Returns [Ok] if execute authorization is granted, and [Err] otherwise.
        fn can_exec(&self, ctx: &AuthzContext<'_>) -> io::Result<()>;
    }

    /// A particularly simple authorizer that just looks at the mode bits in the block metadata
    /// to make authorization decisions.
    pub struct ModeAuthorizer;

    impl ModeAuthorizer {
        fn authorize(mode: u32, mask: u32, denied_msg: &str) -> io::Result<()> {
            if (mode & mask) != 0 {
                Ok(())
            } else {
                Err(io::Error::new(io::ErrorKind::PermissionDenied, denied_msg))
            }
        }

        fn user_is_root(ctx: &AuthzContext<'_>) -> bool {
            ctx.attrs.uid == 0
        }
    }

    impl Authorizer for ModeAuthorizer {
        fn can_read(&self, ctx: &AuthzContext<'_>) -> io::Result<()> {
            if Self::user_is_root(ctx) {
                return Ok(());
            }
            let secrets = ctx.meta.body().secrets()?;
            let mask = (libc::S_IRUSR * (secrets.uid == ctx.attrs.uid) as u32)
                | (libc::S_IRGRP * (secrets.gid == ctx.attrs.gid) as u32)
                | libc::S_IROTH;
            Self::authorize(secrets.mode, mask, "read access denied")
        }

        fn can_write(&self, ctx: &AuthzContext<'_>) -> io::Result<()> {
            if Self::user_is_root(ctx) {
                return Ok(());
            }
            let secrets = ctx.meta.body().secrets()?;
            let mask = (libc::S_IWUSR * (secrets.uid == ctx.attrs.uid) as u32)
                | (libc::S_IWGRP * (secrets.gid == ctx.attrs.gid) as u32)
                | libc::S_IWOTH;
            Self::authorize(secrets.mode, mask, "write access denied")
        }

        fn can_exec(&self, ctx: &AuthzContext<'_>) -> io::Result<()> {
            if Self::user_is_root(ctx) {
                return Ok(());
            }
            let secrets = ctx.meta.body().secrets()?;
            let mask = (libc::S_IXUSR * (secrets.uid == ctx.attrs.uid) as u32)
                | (libc::S_IXGRP * (secrets.gid == ctx.attrs.gid) as u32)
                | libc::S_IXOTH;
            Self::authorize(secrets.mode, mask, "exec access denied")
        }
    }

    type EmptyAccessor = Option<Accessor<&'static [u8]>>;

    type HandleValueParts<'a> = (&'a Arc<Mutex<EmptyAccessor>>, &'a Arc<BlockPath>, Flags);

    struct HandleGuard<B: Size, G: DerefMut<Target = EmptyAccessor>> {
        guard: G,
        accessor: Option<Accessor<B>>,
        flags: Flags,
    }

    impl<B: Size, G: DerefMut<Target = EmptyAccessor>> HandleGuard<B, G> {
        fn new(flags: Flags, mut guard: G, block: B) -> Self {
            let accessor = guard
                .take()
                .map(move |accessor| Accessor::combine(accessor, block));
            Self {
                guard,
                accessor,
                flags,
            }
        }
    }

    impl<B: Size, G: DerefMut<Target = EmptyAccessor>> Drop for HandleGuard<B, G> {
        fn drop(&mut self) {
            *self.guard = self.accessor.take().map(|accessor| {
                let (accessor, _) = accessor.split();
                accessor
            });
        }
    }

    impl<B: Size, G: DerefMut<Target = EmptyAccessor>> Deref for HandleGuard<B, G> {
        type Target = Accessor<B>;
        fn deref(&self) -> &Self::Target {
            self.accessor.as_ref().unwrap()
        }
    }

    impl<B: Size, G: DerefMut<Target = EmptyAccessor>> DerefMut for HandleGuard<B, G> {
        fn deref_mut(&mut self) -> &mut Self::Target {
            self.accessor.as_mut().unwrap()
        }
    }

    enum HandleValue {
        File {
            accessor: Arc<Mutex<EmptyAccessor>>,
            owner: Arc<BlockPath>,
            flags: Flags,
        },
        Directory {
            accessor: Arc<Mutex<EmptyAccessor>>,
            owner: Arc<BlockPath>,
            flags: Flags,
            dir: Directory,
        },
    }

    impl HandleValue {
        fn new<T: Size>(accessor: Accessor<T>, owner: Arc<BlockPath>, flags: Flags) -> HandleValue {
            let (accessor, ..) = accessor.split();
            HandleValue::File {
                accessor: Arc::new(Mutex::new(Some(accessor))),
                owner,
                flags,
            }
        }

        fn flags(&self) -> Flags {
            match self {
                Self::File { flags, .. } => *flags,
                Self::Directory { flags, .. } => *flags,
            }
        }

        fn parts(&self) -> HandleValueParts<'_> {
            match self {
                Self::File {
                    accessor,
                    owner,
                    flags,
                } => (accessor, owner, *flags),
                Self::Directory {
                    accessor,
                    owner,
                    flags,
                    ..
                } => (accessor, owner, *flags),
            }
        }

        fn convert_to_dir<C: Signer + Principaled + Decrypter>(
            self,
            block: &mut FileBlock<C>,
        ) -> Result<HandleValue> {
            let (accessor, owner, flags) = match self {
                Self::File {
                    accessor,
                    owner,
                    flags,
                } => (accessor, owner, flags),
                Self::Directory {
                    accessor,
                    owner,
                    flags,
                    ..
                } => (accessor, owner, flags),
            };
            let accessor = Arc::try_unwrap(accessor).map_err(|_| {
                bterr!("LOGIC ERROR: accessor was still in use even though convert_to_dir owns it")
            })?;
            let accessor = accessor
                .into_inner()
                .ok_or_else(|| bterr!("LOGIC ERROR: accessor was not returned to mutex"))?;
            let mut accessor = Accessor::combine(accessor, block);
            let dir = accessor.read_dir()?;
            let (accessor, ..) = accessor.split();
            Ok(HandleValue::Directory {
                dir,
                accessor: Arc::new(Mutex::new(Some(accessor))),
                owner,
                flags,
            })
        }

        fn directory(&self) -> io::Result<&Directory> {
            match self {
                Self::Directory { dir, .. } => Ok(dir),
                _ => Err(io::Error::new(
                    io::ErrorKind::Other,
                    "handle is not for a directory",
                )),
            }
        }

        async fn lock(&self, from: &BlockPath) -> Result<(Flags, OwnedMutexGuard<EmptyAccessor>)> {
            let (mutex, owner, flags) = self.parts();
            owner.assert_eq(from)?;
            Ok((flags, mutex.clone().lock_owned().await))
        }

        async fn guard<'a, B: Size>(
            &'a self,
            from: &BlockPath,
            block: B,
        ) -> Result<HandleGuard<B, MutexGuard<'a, EmptyAccessor>>> {
            let (mutex, owner, flags) = self.parts();
            owner.assert_eq(from)?;
            let guard = mutex.lock().await;
            Ok(HandleGuard::new(flags, guard, block))
        }

        fn set_flags(&mut self, new_flags: Flags) {
            match self {
                Self::File { flags, .. } => *flags = new_flags,
                Self::Directory { flags, .. } => *flags = new_flags,
            }
        }
    }

    struct BlockGuard<B> {
        inner: B,
    }

    impl<B> BlockGuard<B> {
        fn new(inner: B) -> Self {
            Self { inner }
        }
    }

    impl<B: Deref<Target = InodeTableValue>> Deref for BlockGuard<B> {
        type Target = FileBlock<Arc<dyn Creds>>;
        fn deref(&self) -> &Self::Target {
            self.inner.block.get_ref()
        }
    }

    impl<B: DerefMut<Target = InodeTableValue>> DerefMut for BlockGuard<B> {
        fn deref_mut(&mut self) -> &mut Self::Target {
            self.inner.block.get_mut()
        }
    }

    impl<B: Deref<Target = InodeTableValue>> Size for BlockGuard<B> {
        fn size(&self) -> io::Result<Option<u64>> {
            self.inner.block.size()
        }
    }

    impl<B: Deref<Target = InodeTableValue>> ReadAt for BlockGuard<B> {
        fn read_at(&self, pos: u64, buf: &mut [u8]) -> io::Result<usize> {
            self.inner.block.get_ref().read_at(pos, buf)
        }
    }

    impl<B: Deref<Target = InodeTableValue>> AsRef<BlockMeta> for BlockGuard<B> {
        fn as_ref(&self) -> &BlockMeta {
            self.inner.block.as_ref()
        }
    }

    pub struct InodeTableValue {
        block: Accessor<FileBlock<Arc<dyn Creds>>>,
        handle_values: HashMap<Handle, HandleValue>,
        next_handle: Handle,
        lookup_counts: HashMap<Arc<BlockPath>, u64>,
    }

    impl InodeTableValue {
        fn new(
            block: Accessor<FileBlock<Arc<dyn Creds>>>,
            opener: Arc<BlockPath>,
        ) -> InodeTableValue {
            let mut lookup_counts = HashMap::with_capacity(1);
            lookup_counts.insert(opener, 1);
            Self {
                block,
                handle_values: HashMap::new(),
                next_handle: 1,
                lookup_counts,
            }
        }

        fn invalid_handle_err(handle: Handle) -> io::Error {
            io::Error::new(io::ErrorKind::Other, format!("invalid handle {handle}"))
        }

        fn value(&self, handle: Handle) -> io::Result<&HandleValue> {
            self.handle_values
                .get(&handle)
                .ok_or_else(|| Self::invalid_handle_err(handle))
        }

        fn block(&self) -> &FileBlock<Arc<dyn Creds>> {
            self.block.get_ref()
        }

        fn block_mut(&mut self) -> &mut FileBlock<Arc<dyn Creds>> {
            self.block.get_mut()
        }

        fn convert_to_dir(&mut self, handle: Handle) -> io::Result<()> {
            let value = self
                .handle_values
                .remove(&handle)
                .ok_or_else(|| Self::invalid_handle_err(handle))?;
            let block = self.block_mut();
            let value = value.convert_to_dir(block)?;
            self.handle_values.insert(handle, value);
            Ok(())
        }

        async fn handle_guard<'a>(
            &'a self,
            from: &BlockPath,
            handle: Handle,
        ) -> Result<HandleGuard<&FileBlock<Arc<dyn Creds>>, MutexGuard<'a, EmptyAccessor>>>
        {
            let value = self.value(handle)?;
            let block = self.block();
            value.guard(from, block).await
        }

        async fn handle_guard_owned(
            guard: OwnedRwLockReadGuard<Self>,
            from: &BlockPath,
            handle: Handle,
        ) -> Result<
            HandleGuard<BlockGuard<OwnedRwLockReadGuard<Self>>, OwnedMutexGuard<EmptyAccessor>>,
        > {
            let value = guard.value(handle)?;
            let (flags, mutex_guard) = value.lock(from).await?;
            let guard = BlockGuard::new(guard);
            Ok(HandleGuard::new(flags, mutex_guard, guard))
        }

        async fn handle_guard_mut<'a>(
            &'a mut self,
            from: &BlockPath,
            handle: Handle,
        ) -> Result<HandleGuard<&mut FileBlock<Arc<dyn Creds>>, MutexGuard<'a, EmptyAccessor>>>
        {
            let value = self
                .handle_values
                .get(&handle)
                .ok_or_else(|| Self::invalid_handle_err(handle))?;
            if !value.flags().writeable() {
                return Err(Error::ReadOnlyHandle(handle).into());
            }
            let inner = self.block.get_mut();
            value.guard(from, inner).await
        }

        fn new_handle(&mut self, owner: Arc<BlockPath>, flags: Flags) -> Result<Handle> {
            if self.handle_values.len() as u64 >= u64::MAX {
                return Err(bterr!("no handles are available"));
            }
            let mut handle_value = HandleValue::new(Accessor::new(self.block())?, owner, flags);
            loop {
                let handle = self.next_handle;
                self.next_handle = self.next_handle.wrapping_add(1);
                match self.handle_values.insert(handle, handle_value) {
                    Some(prev) => {
                        // We've wrapped around and this handle is already taken. Put the previous
                        // value back and try again.
                        handle_value = self.handle_values.insert(handle, prev).unwrap();
                    }
                    // We generated an unused handle. Return it.
                    None => return Ok(handle),
                }
            }
        }

        fn set_flags(&mut self, handle: Handle, flags: Flags) -> Result<()> {
            let handle_value = self
                .handle_values
                .get_mut(&handle)
                .ok_or_else(|| Self::invalid_handle_err(handle))?;
            handle_value.set_flags(flags);
            Ok(())
        }

        fn forget_handle(&mut self, handle: Handle) {
            self.handle_values.remove(&handle);
        }

        /// Increments the lookup count from the given path by 1.
        fn incr_lookup_count(&mut self, from: &Arc<BlockPath>) {
            match self.lookup_counts.entry(from.clone()) {
                hash_map::Entry::Occupied(mut entry) => {
                    // I don't want this to silently overflow.
                    let new_count = entry.get().checked_add(1).unwrap();
                    *entry.get_mut() = new_count;
                }
                hash_map::Entry::Vacant(entry) => {
                    entry.insert(1);
                }
            }
        }

        /// Decrements the lookup count from the given path by the given amount.
        fn decr_lookup_count(&mut self, from: Arc<BlockPath>, decr: u64) {
            match self.lookup_counts.entry(from) {
                hash_map::Entry::Occupied(mut entry) => {
                    let new_count = entry.get().saturating_sub(decr);
                    if new_count > 0 {
                        *entry.get_mut() = new_count;
                    } else {
                        entry.remove();
                    }
                }
                hash_map::Entry::Vacant(..) => (),
            }
        }

        fn total_lookup_count(&self) -> u64 {
            self.lookup_counts.values().sum()
        }
    }

    type InodeTable = HashMap<Inode, Arc<RwLock<InodeTableValue>>>;
    type OwnedTableLock = OwnedRwLockReadGuard<InodeTable>;
    type TableLock<'a> = RwLockReadGuard<'a, InodeTable>;

    struct TableGuard<G> {
        table_guard: G,
    }

    impl TableGuard<OwnedTableLock> {
        async fn new_owned(table: Arc<RwLock<InodeTable>>) -> TableGuard<OwnedTableLock> {
            let table_guard = table.read_owned().await;
            TableGuard { table_guard }
        }
    }

    impl<'a> TableGuard<TableLock<'a>> {
        async fn new(table: &'a RwLock<InodeTable>) -> TableGuard<TableLock<'a>> {
            let table_guard = table.read().await;
            TableGuard { table_guard }
        }
    }

    impl<G: Deref<Target = InodeTable>> TableGuard<G> {
        fn get_value(&self, inode: Inode) -> Result<&Arc<RwLock<InodeTableValue>>> {
            self.table_guard
                .get(&inode)
                .ok_or_else(|| bterr!(Error::NotOpen(inode)))
        }

        async fn read(&self, inode: Inode) -> Result<RwLockReadGuard<'_, InodeTableValue>> {
            let value = self.get_value(inode)?;
            Ok(value.read().await)
        }

        async fn write(&self, inode: Inode) -> Result<RwLockWriteGuard<'_, InodeTableValue>> {
            let value = self.get_value(inode)?;
            Ok(value.write().await)
        }
    }

    /// Structure for metadata about a blocktree.
    #[derive(Debug, Serialize, Deserialize, ZeroizeOnDrop)]
    struct Superblock {
        /// The generation number of the cluster this part of the blocktree is stored on.
        generation: u64,
        /// The next free inode available to the cluster.
        #[zeroize(skip)]
        next_inode: AtomicU64,
        /// The hash algorithm to use when computing inode paths.
        #[zeroize(skip)]
        inode_hash: HashKind,
        /// The key to use when hashing inodes to file paths.
        inode_key: Vec<u8>,
    }

    /// A struct for managing the part of a blocktree stored in the local filesystem.
    pub struct LocalFs<A> {
        /// The path to the directory in the local filesystem where this blocktree is located.
        path: PathBuf,
        /// A map from inode numbers to their reference counts.
        inodes: Arc<RwLock<InodeTable>>,
        /// An in-memory copy of the superblock.
        sb: Superblock,
        /// The credentials this blocktree instance will use for all cryptographic operations.
        creds: Arc<dyn Creds>,
        authorizer: A,
        root_principal: Principal,
    }

    impl<A> LocalFs<A> {
        /// The maximum number of directory entries that can be returned in any given call to
        /// `read_dir`.
        const READ_DIR_LIMIT: usize = 1024;
    }

    impl<A: Authorizer> LocalFs<A> {
        /// Creates a new empty blocktree at the given path.
        pub async fn new_empty(
            btdir: PathBuf,
            generation: u64,
            creds: Arc<dyn Creds>,
            authorizer: A,
        ) -> Result<LocalFs<A>> {
            let writecap = creds.writecap().ok_or(BlockError::MissingWritecap)?;
            let mut root_block_path = writecap.root_block_path();
            let root_principal = writecap.root_principal();

            // Initialize the superblock.
            let mut sb_block = Self::open_superblock(
                &btdir,
                creds.clone(),
                root_block_path.clone(),
                &root_principal,
                true,
            )?;
            let sb = Superblock {
                generation,
                next_inode: AtomicU64::new(SpecInodes::FirstFree.into()),
                inode_hash: HashKind::default(),
                inode_key: rand_vec(HashKind::default().len())?,
            };
            write_to(&sb, &mut sb_block)?;
            sb_block.mut_meta_body().access_secrets(|secrets| {
                secrets.block_id.generation = generation;
                secrets.block_id.inode = SpecInodes::Sb.into();
                secrets.mode = FileType::Reg.value() | 0o666;
                secrets.uid = 0;
                secrets.gid = 0;
                secrets.nlink = 1;
                Ok(())
            })?;
            sb_block.flush()?;

            // Initialize the root directory.
            let mut root_block = Self::open_block(
                &btdir,
                SpecInodes::RootDir.into(),
                creds.clone(),
                root_block_path.clone(),
                None,
                sb.inode_hash,
                &sb.inode_key,
            )?;
            write_to(&Directory::new(), &mut root_block)?;
            root_block.mut_meta_body().access_secrets(|secrets| {
                secrets.block_id.generation = generation;
                secrets.block_id.inode = SpecInodes::RootDir.into();
                secrets.mode = FileType::Dir.value() | 0o777;
                secrets.uid = 0;
                secrets.gid = 0;
                secrets.nlink = 1;
                Ok(())
            })?;
            root_block.flush()?;

            let fs = Self::new(
                btdir,
                sb,
                sb_block,
                root_block,
                creds,
                authorizer,
                root_principal,
            )?;
            let writecap = fs.creds.writecap().ok_or(BlockError::MissingWritecap)?;
            if fs.creds.principal() != fs.root_principal {
                let proc_rec = IssuedProcRec {
                    addr: IpAddr::V6(Ipv6Addr::LOCALHOST),
                    pub_creds: fs.creds.concrete_pub(),
                    writecap: writecap.to_owned(),
                    authz_attrs: AuthzAttrs {
                        uid: 0,
                        gid: 0,
                        supp_gids: Vec::new(),
                    },
                };
                root_block_path.push_component(fs.root_principal.to_string());
                fs.grant_access_to(
                    &Arc::new(root_block_path),
                    SpecInodes::RootDir.into(),
                    proc_rec,
                )
                .await?;
            }
            Ok(fs)
        }

        /// Opens an existing blocktree stored at the given path.
        pub fn new_existing(
            btdir: PathBuf,
            creds: Arc<dyn Creds>,
            authorizer: A,
        ) -> Result<LocalFs<A>> {
            let writecap = creds.writecap().ok_or(BlockError::MissingWritecap)?;
            let root_block_path = writecap.root_block_path();
            let root_principal = writecap.root_principal();
            let mut sb_block = Self::open_superblock(
                &btdir,
                creds.clone(),
                root_block_path.clone(),
                &root_principal,
                false,
            )?;
            let sb: Superblock = read_from(&mut sb_block)?;
            let root_block = Self::open_block(
                &btdir,
                SpecInodes::RootDir.into(),
                creds.clone(),
                root_block_path,
                None,
                sb.inode_hash,
                &sb.inode_key,
            )?;
            Self::new(
                btdir,
                sb,
                sb_block,
                root_block,
                creds,
                authorizer,
                root_principal,
            )
        }

        fn new(
            btdir: PathBuf,
            sb: Superblock,
            sb_block: Accessor<FileBlock<Arc<dyn Creds>>>,
            root_block: Accessor<FileBlock<Arc<dyn Creds>>>,
            creds: Arc<dyn Creds>,
            authorizer: A,
            root_principal: Principal,
        ) -> Result<LocalFs<A>> {
            let mut inodes = HashMap::with_capacity(1);
            let empty_path = Arc::new(BlockPath::default());
            inodes.insert(
                SpecInodes::Sb.into(),
                Arc::new(RwLock::new(InodeTableValue::new(
                    sb_block,
                    empty_path.clone(),
                ))),
            );
            inodes.insert(
                SpecInodes::RootDir.into(),
                Arc::new(RwLock::new(InodeTableValue::new(root_block, empty_path))),
            );
            Ok(LocalFs {
                path: btdir,
                inodes: Arc::new(RwLock::new(inodes)),
                sb,
                creds,
                authorizer,
                root_principal,
            })
        }

        fn make_path<P: AsRef<Path>>(parent: P, file_name: &str) -> PathBuf {
            let mut path =
                PathBuf::with_capacity(parent.as_ref().as_os_str().len() + 1 + file_name.len() + 1);
            path.push(parent);
            path.push(&file_name[..2]);
            path.push(&file_name[2..]);
            path
        }

        fn hex_encode(src: &[u8]) -> Result<String> {
            use std::fmt::Write;
            let mut string = String::with_capacity(2 * src.len());
            for byte in src.iter() {
                write!(string, "{byte:02x}")?;
            }
            Ok(string)
        }

        fn ensure_parent_created(path: &Path) -> Result<()> {
            let dir = path.ancestors().nth(1).unwrap();
            if let Err(err) = std::fs::create_dir(dir) {
                match err.kind() {
                    io::ErrorKind::AlreadyExists => Ok(()),
                    _ => Err(err.into()),
                }
            } else {
                Ok(())
            }
        }

        fn open_superblock<P: AsRef<Path>>(
            btdir: P,
            creds: Arc<dyn Creds>,
            block_path: BlockPath,
            root_principal: &Principal,
            create_new: bool,
        ) -> Result<Accessor<FileBlock<Arc<dyn Creds>>>> {
            const HASH: HashKind = HashKind::Sha2_256;
            let mut buf = [0u8; HASH.len()];
            HASH.digest(
                &mut buf,
                [
                    SpecInodes::Sb.value().to_le_bytes().as_slice(),
                    root_principal.as_slice(),
                ]
                .into_iter(),
            )?;
            let hex_str = Self::hex_encode(&buf)?;
            let path = Self::make_path(btdir, &hex_str);
            Self::ensure_parent_created(&path)?;
            let file = std::fs::OpenOptions::new()
                .read(true)
                .write(true)
                .create_new(create_new)
                .open(path)?;
            let block = BlockOpenOptions::new()
                .with_creds(creds)
                .with_encrypt(true)
                .with_inner(file)
                .with_block_path(block_path)
                .open()?;
            Ok(block)
        }

        /// Returns the path to the file storing the given inode's data.
        fn block_path<P: AsRef<Path>>(
            parent: P,
            inode: Inode,
            inode_hash: HashKind,
            inode_key: &[u8],
        ) -> Result<PathBuf> {
            let mut buf = vec![0u8; inode_hash.len()];
            inode_hash.digest(
                &mut buf,
                [inode.to_le_bytes().as_slice(), inode_key].into_iter(),
            )?;
            let hex_str = Self::hex_encode(&buf)?;
            Ok(Self::make_path(parent, &hex_str))
        }

        fn open_block<P: AsRef<Path>>(
            btdir: P,
            inode: Inode,
            creds: Arc<dyn Creds>,
            block_path: BlockPath,
            parent_key: Option<SymKey>,
            inode_hash: HashKind,
            inode_key: &[u8],
        ) -> Result<Accessor<FileBlock<Arc<dyn Creds>>>> {
            let path = Self::block_path(&btdir, inode, inode_hash, inode_key)?;
            Self::ensure_parent_created(&path)?;
            let file = std::fs::OpenOptions::new()
                .read(true)
                .write(true)
                .create(true)
                .open(path)?;
            Self::open_block_file(file, creds, block_path, parent_key)
        }

        fn open_block_file(
            file: File,
            creds: Arc<dyn Creds>,
            block_path: BlockPath,
            parent_key: Option<SymKey>,
        ) -> Result<Accessor<FileBlock<Arc<dyn Creds>>>> {
            let block = BlockOpenOptions::new()
                .with_creds(creds)
                .with_encrypt(true)
                .with_inner(file)
                .with_parent_key(parent_key)
                .with_block_path(block_path)
                .open()?;
            Ok(block)
        }

        async fn table_guard(&self) -> TableGuard<TableLock<'_>> {
            TableGuard::new(&self.inodes).await
        }

        async fn open_value(
            &self,
            from: Arc<BlockPath>,
            inode: Inode,
            block_path: BlockPath,
            parent_key: Option<SymKey>,
        ) -> Result<()> {
            let block = Self::open_block(
                &self.path,
                inode,
                self.creds.clone(),
                block_path,
                parent_key,
                self.sb.inode_hash,
                &self.sb.inode_key,
            )?;
            let value = Arc::new(RwLock::new(InodeTableValue::new(block, from)));
            let mut inodes = self.inodes.write().await;
            if inodes.insert(inode, value).is_some() {
                error!(
                    "LOGIC ERROR: open_value was called with inode {inode}, which is already open"
                );
            }
            Ok(())
        }

        /// Ensures that the given inode is open. If the inode is already open, then this method
        /// does nothing and returns the table guard which was used to check the status of the
        /// inode.
        /// ## Warning
        /// Because this method creates new table guards, no table guard must be alive when it's
        /// called. Otherwise a deadlock will occur.
        async fn ensure_open<'a>(
            &'a self,
            from: &Arc<BlockPath>,
            inode: Inode,
            block_path: BlockPath,
            parent_key: Option<SymKey>,
        ) -> Result<TableGuard<OwnedTableLock>> {
            {
                let table_guard = self.inodes.clone().read_owned().await;
                if table_guard.contains_key(&inode) {
                    return Ok(TableGuard { table_guard });
                }
            }
            self.open_value(from.clone(), inode, block_path, parent_key)
                .await?;
            Ok(TableGuard::new_owned(self.inodes.clone()).await)
        }

        fn delete_block_file(&self, inode: Inode) -> Result<()> {
            let mut path =
                Self::block_path(&self.path, inode, self.sb.inode_hash, &self.sb.inode_key)?;
            std::fs::remove_file(&path)?;
            path.pop();
            let mut contents = std::fs::read_dir(&path)?;
            if contents.next().is_none() {
                std::fs::remove_dir(&path)?;
            }
            Ok(())
        }

        async fn inode_forget<'a>(
            &self,
            from: Arc<BlockPath>,
            inode: Inode,
            count: u64,
        ) -> io::Result<()> {
            let mut inodes = self.inodes.write().await;
            let lookup_count = {
                let inode_lock = match inodes.get_mut(&inode) {
                    Some(inode_lock) => inode_lock,
                    None => {
                        warn!("an attempt was made to forget non-existent inode {inode}");
                        return Ok(());
                    }
                };
                let mut value = inode_lock.write().await;
                value.decr_lookup_count(from, count);
                value.total_lookup_count()
            };
            if 0 == lookup_count {
                let entry = Arc::try_unwrap(inodes.remove(&inode).unwrap())
                    .map_err(|_| bterr!("LOGIC ERROR: entry for inode {inode} was still in use while it was being forgotten"))?;
                let value = entry.into_inner();
                if value.block().meta_body().secrets()?.nlink == 0 {
                    self.delete_block_file(inode)?;
                }
            }
            Ok(())
        }

        /// Returns the next available inode and updates the superblock in one atomic operation.
        /// TODO: Obviously this strategy won't work when there are multiple servers in this
        /// generation.
        async fn next_inode(&self) -> Result<Inode> {
            let table_guard = self.table_guard().await;
            let mut value_guard = table_guard.write(SpecInodes::Sb.into()).await?;
            let mut block = &mut value_guard.block;
            // We don't need strict ordering because the lock on the inode table value is already
            // serializing access.
            let inode = self.sb.next_inode.fetch_add(1, Ordering::Relaxed);
            block.rewind()?;
            write_to(&self.sb, &mut block)?;
            block.flush()?;
            Ok(inode)
        }

        fn attr_timeout(&self) -> Duration {
            Duration::from_secs(5)
        }

        fn entry_timeout(&self) -> Duration {
            Duration::from_secs(5)
        }

        fn unsupported_flag_err(flag: &str) -> btlib::Error {
            bterr!("unsupported flag: {flag}")
        }

        fn bt_entry(&self, attr: BlockMetaSecrets) -> crate::msg::Entry {
            crate::msg::Entry {
                attr,
                attr_timeout: self.attr_timeout(),
                entry_timeout: self.entry_timeout(),
            }
        }

        /// Grants the given credentials access to the directory this instance is responsible for.
        ///
        /// ## Warning
        /// This method calls `self.authz_attrs`, so the same consideration for avoiding deadlock
        /// apply to this method as well. See the documentation of `self.authz_attrs` for details.
        async fn grant_access_to(
            &self,
            from: &Arc<BlockPath>,
            inode: Inode,
            proc_rec: IssuedProcRec,
        ) -> Result<()> {
            let authz_attrs = self.authz_attrs(from).await?;
            let principal = proc_rec.pub_creds.principal();
            let (next_inode, parent_key) = {
                let table_guard = self.table_guard().await;

                let next_inode = if inode == SpecInodes::RootDir.value() {
                    // If the inode is for the root directory we need to add a readcap for the
                    // superblock.
                    let mut value_guard = table_guard.write(SpecInodes::Sb.into()).await?;
                    let mut block = &mut value_guard.block;
                    let next_inode = self.sb.next_inode.fetch_add(1, Ordering::Relaxed);
                    block.rewind()?;
                    write_to(&self.sb, &mut block)?;
                    block.mut_meta_body().add_readcap_for(&proc_rec.pub_creds)?;
                    block.flush()?;

                    Ok(next_inode)
                } else {
                    self.next_inode().await
                }?;

                let parent_key = {
                    let mut value_guard = table_guard.write(inode).await?;
                    let block = &mut value_guard.block;
                    self.authorizer.can_write(&AuthzContext::new(
                        from,
                        &authz_attrs,
                        block.meta(),
                    ))?;
                    block.mut_meta_body().add_readcap_for(&proc_rec.pub_creds)?;

                    let mut dir = block.read_dir()?;
                    let proc_rec_name = principal.to_string();
                    dir.add_file(proc_rec_name, next_inode)?;
                    // Note that write_dir calls flush, which also ensures metadata is written to
                    // disk.
                    block.write_dir(&dir)?;

                    block.meta_body().block_key()?.clone()
                };

                (next_inode, parent_key)
            };

            let self_writecap = self.creds.writecap().ok_or(BlockError::MissingWritecap)?;
            let self_bind_path = Arc::new(self_writecap.bind_path());
            let bind_path = proc_rec.writecap.bind_path();
            self.open_value(
                self_bind_path.clone(),
                next_inode,
                bind_path,
                Some(parent_key),
            )
            .await?;
            {
                let table_guard = self.table_guard().await;
                let mut value_guard = table_guard.write(next_inode).await?;
                let block = &mut value_guard.block;
                block.write_proc_rec(&ProcRec::Valid(proc_rec))?;
                block.mut_meta_body().access_secrets(|secrets| {
                    secrets.nlink += 1;
                    Ok(())
                })?;
            };
            // We must ensure the reference count for the inode is decremented, otherwise the table
            // entry will never be freed.
            self.inode_forget(self_bind_path, next_inode, 1).await?;
            Ok(())
        }

        async fn lookup_inode_in<'a>(
            table_guard: &'a TableGuard<TableLock<'a>>,
            parent: Inode,
            name: &str,
        ) -> Result<Inode> {
            let mut value_guard = table_guard.write(parent).await?;
            let dir = value_guard.block.read_dir()?;
            dir.entry(name)
                .ok_or_else(|| io::Error::from_raw_os_error(libc::ENOENT).into())
                .map(|e| e.inode())
        }

        /// Returns a pair of inodes, where the first inode is the inode referred to by the given
        /// path, and the second is the parent inode.
        async fn lookup_inode<'a, 'b, I: Iterator<Item = &'a str>>(
            table_guard: &'b TableGuard<TableLock<'b>>,
            components: I,
        ) -> Result<(Inode, Option<Inode>)> {
            const ROOT: Inode = SpecInodes::RootDir as Inode;
            let mut parent = None;
            let mut inode = ROOT;
            for component in components {
                parent = Some(inode);
                inode = Self::lookup_inode_in(table_guard, inode, component).await?;
            }
            Ok((inode, parent))
        }

        /// Retrieves the authorization attributes for the principal identified by the given path.
        /// If the principal is not associated with a valid process record, then an [Err] is
        /// returned.
        /// ## Warning
        /// If this method is called while a lock for any component on the given path is held, then
        /// a deadlock may occur. It's safest to call this method when _no_ locks are held.
        async fn authz_attrs(&self, from: &Arc<BlockPath>) -> Result<AuthzAttrs> {
            let writecap = self.creds.writecap().ok_or(BlockError::MissingWritecap)?;
            let root_principal = writecap.root_principal();
            let from_principal = from.components().last().map_or_else(
                || Err(bterr!("path from which message was received was empty")),
                Principal::try_from,
            )?;
            if root_principal == from_principal {
                // Now I am become root, the destroyer of files.
                return Ok(AuthzAttrs {
                    uid: 0,
                    gid: 0,
                    supp_gids: Vec::new(),
                });
            }
            let local_root = writecap.path();
            let relative = from.relative_to(local_root)?;
            let (inode, parent_key) = {
                let table_guard = self.table_guard().await;
                let (inode, parent) =
                    Self::lookup_inode(&table_guard, relative.components()).await?;
                let parent_key = if let Some(parent) = parent {
                    let value_guard = table_guard.read(parent).await?;
                    Some(value_guard.block.meta_body().block_key()?.clone())
                } else {
                    None
                };
                (inode, parent_key)
            };
            let proc_rec = {
                let table_guard = self
                    .ensure_open(from, inode, from.as_ref().to_owned(), parent_key)
                    .await?;
                let mut value_guard = table_guard.write(inode).await?;
                value_guard.block.read_proc_rec()?
            };
            let proc_rec = proc_rec.validate()?;
            Ok(proc_rec.authz_attrs)
        }

        /// Decrements the link count of the given inode.
        ///
        /// If the link count goes to zero, the inode's file in the local filesystem is deleted.
        /// The `block_path` parameter is the path to the block identified by the inode.
        /// The `parent_key` is the block key of the inode's parent.
        async fn decr_link_count<G>(
            &self,
            inode: Inode,
            block_path: BlockPath,
            parent_key: SymKey,
            table_guard: &TableGuard<G>,
        ) -> Result<()>
        where
            G: Deref<Target = InodeTable>,
        {
            fn decr_nlink(secrets: &mut BlockMetaSecrets) -> Result<u32> {
                secrets.nlink -= 1;
                Ok(secrets.nlink)
            }
            let delete = match table_guard.write(inode).await {
                Ok(mut value) => {
                    value
                        .block_mut()
                        .mut_meta_body()
                        .access_secrets(decr_nlink)?;
                    value.block_mut().flush_meta()?;
                    // Since this block was already open, a client is keeping it alive. When they
                    // choose to forget this inode it will be deleted. Thus we return false here.
                    false
                }
                Err(err) => {
                    if let Some(Error::NotOpen(_)) = err.downcast_ref::<Error>() {
                        // It may be tempting to drop the table_guard here, but if this were done then
                        // another this block file could be opened concurrently.
                        let mut block = Self::open_block(
                            &self.path,
                            inode,
                            self.creds.clone(),
                            block_path,
                            Some(parent_key),
                            self.sb.inode_hash,
                            &self.sb.inode_key,
                        )?;
                        let nlink = block.mut_meta_body().access_secrets(decr_nlink)?;
                        if nlink > 0 {
                            block.flush_meta()?;
                            false
                        } else {
                            true
                        }
                    } else {
                        return Err(err);
                    }
                }
            };
            if delete {
                self.delete_block_file(inode)?;
            }
            Ok(())
        }
    }

    unsafe impl<A: Sync> Sync for LocalFs<A> {}

    /// An owned guard which allows read access to file data.
    pub struct BufGuard {
        offset: u64,
        size: u64,
        // Note that handle must come before _table to ensure the guards are dropped in the correct
        // order.
        handle: HandleGuard<
            BlockGuard<OwnedRwLockReadGuard<InodeTableValue>>,
            OwnedMutexGuard<EmptyAccessor>,
        >,
        _table: OwnedTableLock,
    }

    impl BufGuard {
        async fn new(
            table: Arc<RwLock<InodeTable>>,
            from: &BlockPath,
            inode: Inode,
            handle: Handle,
            offset: u64,
            mut size: u64,
        ) -> Result<BufGuard> {
            let table = table.read_owned().await;
            let entry = table.get(&inode).ok_or(Error::NotOpen(inode))?;
            let inode_guard = {
                let inode_guard = entry.clone().read_owned().await;
                let total_size = inode_guard.block().meta_body().secrets()?.size;
                size = size.min(total_size.saturating_sub(offset));
                let mut handle_guard = inode_guard.handle_guard(from, handle).await?;
                handle_guard.flags.assert_readable()?;
                let pos = handle_guard.pos() as u64;
                if offset != pos {
                    if let Err(err) = handle_guard.try_seek(SeekFrom::Start(offset)) {
                        //  An error with `ErrorKind::Unsupported` means that the `SectoredBuf`
                        // has unflushed data and it needs exclusive access to the block to
                        // perform this seek because this data needs to be written.
                        if let io::ErrorKind::Unsupported = err.kind() {
                            None
                        } else {
                            return Err(err.into());
                        }
                    } else {
                        drop(handle_guard);
                        Some(inode_guard)
                    }
                } else {
                    drop(handle_guard);
                    Some(inode_guard)
                }
            };
            let inode_guard = match inode_guard {
                Some(inode_guard) => inode_guard,
                None => {
                    {
                        let mut inode_guard = entry.write().await;
                        let mut handle_guard = inode_guard.handle_guard_mut(from, handle).await?;
                        handle_guard.seek(SeekFrom::Start(offset))?;
                    }
                    entry.clone().read_owned().await
                }
            };
            let handle = InodeTableValue::handle_guard_owned(inode_guard, from, handle).await?;
            Ok(BufGuard {
                handle,
                _table: table,
                offset,
                size,
            })
        }
    }

    impl Deref for BufGuard {
        type Target = [u8];
        fn deref(&self) -> &Self::Target {
            self.handle.get_buf(self.offset, self.size).unwrap()
        }
    }

    impl<A: 'static + Authorizer + Send + Sync> FsProvider for LocalFs<A> {
        type LookupFut<'c> = impl 'c + Send + Future<Output = Result<LookupReply>>;
        fn lookup<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Lookup<'c>) -> Self::LookupFut<'c> {
            async move {
                let Lookup { parent, name, .. } = msg;
                debug!("lookup: parent {parent}, {:?}", name);
                let authz_attrs = self.authz_attrs(from).await?;

                let (dir, block_path, parent_key) = {
                    let table_guard = self.table_guard().await;
                    let mut value_guard = table_guard.write(parent).await?;
                    let parent_block = &mut value_guard.block;
                    self.authorizer.can_exec(&AuthzContext::new(
                        from,
                        &authz_attrs,
                        parent_block.meta(),
                    ))?;
                    let dir = parent_block.read_dir()?;
                    let meta_body = parent_block.meta_body();
                    let block_path = meta_body.path().to_owned();
                    let parent_key = meta_body.block_key()?.clone();
                    (dir, block_path, parent_key)
                };

                let entry = dir
                    .entry(name)
                    .ok_or_else(|| io::Error::from_raw_os_error(libc::ENOENT))?;
                let inode = entry.inode();
                let stat = {
                    let table_guard = self
                        .ensure_open(from, inode, block_path, Some(parent_key))
                        .await?;
                    let mut value_guard = table_guard.write(inode).await?;
                    let stat = value_guard.block.meta_body().secrets()?.to_owned();
                    value_guard.incr_lookup_count(from);
                    stat
                };
                let entry = self.bt_entry(stat);
                let reply = LookupReply {
                    inode,
                    generation: self.sb.generation,
                    entry,
                };
                Ok(reply)
            }
        }

        type CreateFut<'c> = impl 'c + Send + Future<Output = Result<CreateReply>>;
        fn create<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Create<'c>) -> Self::CreateFut<'c> {
            async move {
                let Create {
                    parent,
                    name,
                    flags,
                    mode,
                    umask,
                } = msg;
                debug!("create: parent {parent}, name {:?}", name);
                let authz_attrs = self.authz_attrs(from).await?;

                let name = msg.name.to_owned();

                // Add a directory entry to the parent for the new inode.
                let (inode, mut block_path, parent_key) = {
                    let table_guard = self.table_guard().await;
                    let mut value_guard = table_guard.write(parent).await?;
                    let block = &mut value_guard.block;

                    self.authorizer.can_write(&AuthzContext::new(
                        from,
                        &authz_attrs,
                        block.meta(),
                    ))?;

                    let mut dir = block.read_dir()?;
                    if dir.contains_entry(&name) {
                        return Err(io::Error::from_raw_os_error(libc::EEXIST).into());
                    }

                    // Reserve a free inode.
                    let inode = self.next_inode().await?;
                    dir.add_file(name.clone(), inode)?;
                    block.write_dir(&dir)?;

                    let meta_body = block.meta_body();
                    let block_path = meta_body.path().clone();
                    let parent_key = meta_body.block_key()?.clone();

                    (inode, block_path, parent_key)
                };
                block_path.push_component(name);

                let (handle, stat) = {
                    let table_guard = self
                        .ensure_open(from, inode, block_path, Some(parent_key))
                        .await?;
                    let mut value_guard = table_guard.write(inode).await?;
                    let handle =
                        value_guard.new_handle(from.clone(), FlagValue::ReadWrite.into())?;
                    let stat = {
                        let mut block = value_guard.handle_guard_mut(from, handle).await?;
                        let stat = block.mut_meta_body().access_secrets(|secrets| {
                            secrets.block_id.generation = self.sb.generation;
                            secrets.block_id.inode = inode;
                            secrets.mode = mode & !umask;
                            if flags.directory() {
                                secrets.mode |= FileType::Dir.value();
                            } else {
                                secrets.mode |= FileType::Reg.value();
                            }
                            secrets.uid = authz_attrs.uid;
                            secrets.gid = authz_attrs.gid;
                            let now = Epoch::now();
                            secrets.atime = now;
                            secrets.ctime = now;
                            secrets.mtime = now;
                            secrets.nlink = 1;
                            Ok(secrets.to_owned())
                        })?;
                        if flags.directory() {
                            // Note that write_dir flushes data after writing, including
                            // metadata.
                            block.write_dir(&Directory::new())?;
                        } else {
                            block.flush_meta()?;
                        }
                        stat
                    };
                    if flags.directory() {
                        value_guard.convert_to_dir(handle)?;
                    }
                    value_guard.set_flags(handle, flags)?;
                    (handle, stat)
                };

                Ok(CreateReply {
                    inode,
                    handle,
                    entry: self.bt_entry(stat),
                })
            }
        }

        type OpenFut<'c> = impl 'c + Send + Future<Output = Result<OpenReply>>;
        fn open<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Open) -> Self::OpenFut<'c> {
            async move {
                let Open { inode, flags } = msg;
                debug!("open: inode {inode}, flags {flags}");
                if flags.value() & libc::O_APPEND != 0 {
                    return Err(Self::unsupported_flag_err("O_APPEND"));
                }
                if flags.value() & libc::O_CLOEXEC != 0 {
                    return Err(Self::unsupported_flag_err("O_CLOEXEC"));
                }
                let authz_attrs = self.authz_attrs(from).await?;
                let handle = {
                    let table_guard = self.table_guard().await;
                    let mut value = table_guard.write(inode).await?;
                    let handle = value.new_handle(from.clone(), flags)?;
                    {
                        let block = {
                            let result = value.handle_guard(from, handle).await;
                            match result {
                                Ok(block) => block,
                                Err(ref err) => {
                                    let message = err.to_string();
                                    drop(result);
                                    value.forget_handle(handle);
                                    return Err(bterr!(message));
                                }
                            }
                        };
                        let ctx = AuthzContext::new(from, &authz_attrs, block.meta());
                        if flags.readable() {
                            if let Err(err) = self.authorizer.can_read(&ctx) {
                                drop(block);
                                value.forget_handle(handle);
                                return Err(err.into());
                            }
                        }
                        if flags.writeable() {
                            if let Err(err) = self.authorizer.can_write(&ctx) {
                                drop(block);
                                value.forget_handle(handle);
                                return Err(err.into());
                            }
                        }
                    }
                    if flags.directory() {
                        if let Err(err) = value.convert_to_dir(handle) {
                            value.forget_handle(handle);
                            return Err(err.into());
                        }
                    }
                    if let Err(err) = value.set_flags(handle, flags) {
                        value.forget_handle(handle);
                        return Err(err);
                    }
                    handle
                };
                Ok(OpenReply { handle })
            }
        }

        type ReadGuard = BufGuard;
        type ReadFut<'c> = impl 'c + Send + Future<Output = Result<Self::ReadGuard>>;
        fn read<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Read) -> Self::ReadFut<'c> {
            async move {
                let Read {
                    inode,
                    handle,
                    offset,
                    size,
                } = msg;
                debug!("read: inode {inode}, handle {handle}, offset {offset}, size {size}");
                BufGuard::new(self.inodes.clone(), from, inode, handle, offset, size).await
            }
        }

        type WriteFut<'r> = impl 'r + Send + Future<Output = Result<WriteReply>>;
        fn write<'c>(
            &'c self,
            from: &'c Arc<BlockPath>,
            write: Write<&'c [u8]>,
        ) -> Self::WriteFut<'c> {
            async move {
                let Write {
                    inode,
                    handle,
                    offset,
                    mut data,
                } = write;
                debug!("write: inode {inode}, handle {handle}, offset {offset}");
                let table_guard = self.table_guard().await;
                let mut value_guard = table_guard.write(inode).await?;
                let mut block = value_guard.handle_guard_mut(from, handle).await?;
                block.flags.assert_writeable()?;
                let pos = block.pos() as u64;
                if offset != pos {
                    block.seek(SeekFrom::Start(offset))?;
                }
                let written = std::io::copy(&mut data, block.deref_mut())?;
                Ok(WriteReply { written })
            }
        }

        type FlushFut<'c> = impl 'c + Send + Future<Output = Result<()>>;
        fn flush<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Flush) -> Self::FlushFut<'c> {
            async move {
                let Flush { inode, handle } = msg;
                debug!("flush: inode {inode}, handle {handle}");
                let table_guard = self.table_guard().await;
                let mut value_guard = table_guard.write(inode).await?;
                let mut handle_guard = match value_guard.handle_guard_mut(from, handle).await {
                    Ok(guard) => guard,
                    Err(err) => match err.downcast_ref::<Error>() {
                        Some(Error::ReadOnlyHandle(..)) => {
                            // We ignore attempts to flush read-only handles.
                            return Ok(());
                        }
                        _ => return Err(err),
                    },
                };
                handle_guard.flush()?;
                Ok(())
            }
        }

        type ReadDirFut<'c> = impl 'c + Send + Future<Output = Result<ReadDirReply>>;
        fn read_dir<'c>(&'c self, from: &'c Arc<BlockPath>, msg: ReadDir) -> Self::ReadDirFut<'c> {
            async move {
                let ReadDir {
                    inode,
                    handle,
                    limit,
                    state,
                } = msg;
                debug!("read_dir: inode {inode}, handle {handle}, state {state}");
                let table_guard = self.table_guard().await;
                let value = table_guard.read(inode).await?;
                let handle_value = value
                    .value(handle)
                    .map_err(|_| bterr!(Error::InvalidHandle { handle, inode }))?;
                let (_, owner, flags) = handle_value.parts();
                flags.assert_readable()?;
                owner.assert_eq(from)?;
                let dir = handle_value.directory()?;
                let state: usize = state.try_into()?;
                let server_limit = Self::READ_DIR_LIMIT.min(dir.num_entries() - state);
                let entries_len = if limit > 0 {
                    server_limit.min(limit as usize)
                } else {
                    server_limit
                };
                let pairs = dir
                    .entries()
                    .skip(state)
                    .take(entries_len)
                    .map(|(name, entry)| (name.to_owned(), entry.to_owned()));
                let mut entries = Vec::with_capacity(entries_len);
                entries.extend(pairs);
                Ok(ReadDirReply {
                    entries,
                    new_state: (state + entries_len) as u64,
                })
            }
        }

        type LinkFut<'c> = impl 'c + Send + Future<Output = Result<LinkReply>>;
        fn link<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Link<'c>) -> Self::LinkFut<'c> {
            async move {
                let Link {
                    inode,
                    new_parent,
                    name,
                } = msg;
                debug!("link: inode {inode}, new_parent {new_parent}, name '{name}'");
                let authz_attrs = self.authz_attrs(from).await?;
                let table_guard = self.table_guard().await;
                let mut value_guard = table_guard.write(new_parent).await?;
                let parent_block = &mut value_guard.block;
                self.authorizer.can_write(&AuthzContext::new(
                    from,
                    &authz_attrs,
                    parent_block.meta(),
                ))?;

                let mut dir = parent_block.read_dir()?;
                if let Some(old_entry) = dir.entry(name) {
                    let parent_meta = parent_block.meta_body();
                    let mut block_path = parent_meta.path().to_owned();
                    block_path.push_component(name);
                    let parent_key = parent_meta.block_key()?.clone();
                    self.decr_link_count(old_entry.inode(), block_path, parent_key, &table_guard)
                        .await?;
                }

                let attr = {
                    let table_guard = self.table_guard().await;
                    let mut value = table_guard.write(inode).await?;
                    let block = value.block_mut();
                    let meta = block.mut_meta_body();
                    let attr = meta.access_secrets(|secrets| {
                        secrets.nlink += 1;
                        Ok(secrets.to_owned())
                    })?;
                    block.flush_meta()?;
                    value.incr_lookup_count(from);
                    attr
                };
                let file_type = FileType::from_value(attr.mode)?;
                let entry = match file_type {
                    FileType::Reg => DirEntry::File(inode),
                    FileType::Dir => DirEntry::Directory(inode),
                };
                dir.insert_entry(name.to_owned(), entry);
                parent_block.write_dir(&dir)?;
                let entry = self.bt_entry(attr);
                Ok(LinkReply { entry })
            }
        }

        type UnlinkFut<'c> = impl 'c + Send + Future<Output = Result<()>>;
        fn unlink<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Unlink<'c>) -> Self::UnlinkFut<'c> {
            async move {
                let Unlink { parent, name } = msg;
                debug!("unlink: parent {parent}, name {name}");
                let authz_attrs = self.authz_attrs(from).await?;
                let table_guard = self.table_guard().await;
                let (block_path, inode, parent_key) = {
                    let mut value_guard = table_guard.write(parent).await?;
                    let parent_block = &mut value_guard.block;

                    self.authorizer.can_write(&AuthzContext::new(
                        from,
                        &authz_attrs,
                        parent_block.meta(),
                    ))?;

                    let mut dir = parent_block.read_dir()?;
                    let entry = match dir.remove_entry(name) {
                        None => return Err(io::Error::from_raw_os_error(libc::ENOENT).into()),
                        Some(entry) => entry,
                    };
                    let inode = entry.inode();
                    parent_block.write_dir(&dir)?;

                    let meta_body = parent_block.meta_body();
                    let mut block_path = meta_body.path().clone();
                    block_path.push_component(name.to_owned());
                    let parent_key = meta_body.block_key()?.clone();
                    (block_path, inode, parent_key)
                };

                self.decr_link_count(inode, block_path, parent_key, &table_guard)
                    .await
            }
        }

        type ReadMetaFut<'c> = impl 'c + Send + Future<Output = Result<ReadMetaReply>>;
        fn read_meta<'c>(
            &'c self,
            from: &'c Arc<BlockPath>,
            msg: ReadMeta,
        ) -> Self::ReadMetaFut<'c> {
            async move {
                let ReadMeta { inode, handle } = msg;
                debug!("read_meta: inode {inode}, handle {:?}", handle);
                let table_guard = self.table_guard().await;
                let value_guard = table_guard.read(inode).await?;
                let attrs = if let Some(handle) = handle {
                    let block = value_guard.handle_guard(from, handle).await?;
                    block.meta_body().secrets()?.to_owned()
                } else {
                    value_guard.block().meta_body().secrets()?.to_owned()
                };
                debug!("read_meta attrs: {:?}", attrs);
                let reply = ReadMetaReply {
                    attrs,
                    valid_for: self.attr_timeout(),
                };
                Ok(reply)
            }
        }

        type WriteMetaFut<'c> = impl 'c + Send + Future<Output = Result<WriteMetaReply>>;
        fn write_meta<'c>(
            &'c self,
            from: &'c Arc<BlockPath>,
            msg: WriteMeta,
        ) -> Self::WriteMetaFut<'c> {
            async move {
                let WriteMeta {
                    inode,
                    handle,
                    attrs,
                    attrs_set,
                } = msg;
                debug!("write_meta: inode {inode}, handle {:?}", handle);
                let authz_attrs = self.authz_attrs(from).await?;
                let cb = |block: &mut FileBlock<Arc<dyn Creds>>| {
                    self.authorizer.can_write(&AuthzContext::new(
                        from,
                        &authz_attrs,
                        block.meta(),
                    ))?;
                    let attrs = block.mut_meta_body().access_secrets(|secrets| {
                        if attrs_set.mode() {
                            secrets.mode = attrs.mode;
                        }
                        if attrs_set.uid() {
                            secrets.uid = attrs.uid;
                        }
                        if attrs_set.gid() {
                            secrets.gid = attrs.gid;
                        }
                        if attrs_set.atime() {
                            secrets.atime = attrs.atime;
                        }
                        if attrs_set.mtime() {
                            secrets.mtime = attrs.mtime;
                        }
                        if attrs_set.ctime() {
                            secrets.ctime = attrs.ctime;
                        }
                        for (key, value) in attrs.tags.into_iter() {
                            secrets.tags.insert(key, value);
                        }
                        Ok(secrets.to_owned())
                    })?;
                    block.flush_meta()?;
                    Ok::<_, btlib::Error>(attrs)
                };
                let attrs = {
                    let table_guard = self.table_guard().await;
                    let mut value_guard = table_guard.write(inode).await?;
                    if let Some(handle) = handle {
                        let mut block = value_guard.handle_guard_mut(from, handle).await?;
                        block.flags.assert_writeable()?;
                        cb(block.get_mut())
                    } else {
                        cb(value_guard.block.get_mut())
                    }
                }?;
                Ok(WriteMetaReply {
                    attrs,
                    valid_for: self.attr_timeout(),
                })
            }
        }

        type AllocateFut<'c> = impl 'c + Send + Future<Output = Result<()>>;
        fn allocate<'c>(
            &'c self,
            from: &'c Arc<BlockPath>,
            msg: Allocate,
        ) -> Self::AllocateFut<'c> {
            async move {
                let Allocate {
                    inode,
                    handle,
                    offset,
                    size,
                } = msg;
                debug!(
                    "allocate: inode {inode}, handle {handle}, offset {:?}, size {size}",
                    offset
                );
                let table_guard = self.table_guard().await;
                let mut value_guard = table_guard.write(inode).await?;
                let mut block = value_guard.handle_guard_mut(from, handle).await?;
                let curr_size = block.meta_body().secrets()?.size;
                if let Some(offset) = offset {
                    if curr_size != offset {
                        return Err(bterr!("only allocations at the end of files are supported"));
                    }
                }
                let new_size = curr_size.max(size);
                if new_size > curr_size {
                    block.zero_extend(new_size - curr_size)?;
                }
                Ok(())
            }
        }

        type CloseFut<'c> = impl 'c + Send + Future<Output = Result<()>>;
        fn close<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Close) -> Self::CloseFut<'c> {
            async move {
                let Close { inode, handle } = msg;
                debug!("close: inode {inode}, handle {handle}");
                let table_guard = self.table_guard().await;
                let mut value = table_guard.write(inode).await?;
                match value.handle_guard_mut(from, handle).await {
                    Ok(mut block) => {
                        block.flush()?;
                        block.flush_meta()?;
                    }
                    Err(err) => match err.downcast_ref::<Error>() {
                        // If the cause of the error is that the handle is read-only, then it is
                        // not actually an error.
                        Some(Error::ReadOnlyHandle(_)) => (),
                        _ => return Err(err),
                    },
                };
                value.forget_handle(handle);
                Ok(())
            }
        }

        type ForgetFut<'c> = impl 'c + Send + Future<Output = Result<()>>;
        fn forget<'c>(&'c self, from: &'c Arc<BlockPath>, msg: Forget) -> Self::ForgetFut<'c> {
            async move {
                let Forget { inode, count } = msg;
                debug!("forget: inode {inode}, count {count}");
                self.inode_forget(from.clone(), inode, count).await.bterr()
            }
        }

        type LockFut<'c> = Ready<Result<()>>;
        fn lock<'c>(&'c self, _from: &'c Arc<BlockPath>, _msg: Lock) -> Self::LockFut<'c> {
            todo!();
        }

        type UnlockFut<'c> = Ready<Result<()>>;
        fn unlock<'c>(&'c self, _from: &'c Arc<BlockPath>, _msg: Unlock) -> Self::UnlockFut<'c> {
            todo!();
        }

        type AddReacapFut<'c> = impl 'c + Send + Future<Output = Result<()>>;
        fn add_readcap<'c>(
            &'c self,
            from: &'c Arc<BlockPath>,
            msg: AddReadcap,
        ) -> Self::AddReacapFut<'c> {
            async move {
                let AddReadcap {
                    inode,
                    handle,
                    pub_creds,
                } = msg;
                debug!("add_readcap: inode {inode}, handle {handle}");
                let table_guard = self.table_guard().await;
                let mut value_guard = table_guard.write(inode).await?;
                let mut block = value_guard.handle_guard_mut(from, handle).await?;
                block.mut_meta_body().add_readcap_for(pub_creds)
            }
        }

        type GrantAccessFut<'c> = impl 'c + Send + Future<Output = Result<()>>;
        fn grant_access<'c>(
            &'c self,
            from: &'c Arc<BlockPath>,
            msg: GrantAccess,
        ) -> Self::GrantAccessFut<'c> {
            let GrantAccess { inode, record } = msg;
            debug!("grant_access: inode {inode}, record {:?}", record);
            self.grant_access_to(from, inode, record)
        }
    }
}