Blocktree/crates/btlib/src/test_helpers.rs

// SPDX-License-Identifier: AGPL-3.0-or-later
/// Test data and functions to help with testing.
use btserde::{Error, Result};
use fuse_backend_rs::{
    api::filesystem::{ZeroCopyReader, ZeroCopyWriter},
    file_buf::FileVolatileSlice,
    file_traits::FileReadWriteVolatile,
};
use std::{
    cell::RefCell,
    fmt::Write as FmtWrite,
    fs::File,
    io::{Cursor, Write},
};
use vm_memory::bytes::Bytes;

use super::*;
use crate::{crypto::*, sectored_buf::SectoredBuf, Cursor as PioCursor};

/// A smaller default sector size for use in tests.
pub const SECTOR_SZ_DEFAULT: usize = 4096;

pub const PRINCIPAL: [u8; 32] = [
    0x75, 0x28, 0xA9, 0xE0, 0x9D, 0x24, 0xBA, 0xB3, 0x79, 0x56, 0x15, 0x68, 0xFD, 0xA4, 0xE2, 0xA4,
    0xCF, 0xB2, 0xC0, 0xE3, 0x96, 0xAE, 0xA2, 0x6E, 0x45, 0x15, 0x50, 0xED, 0xA6, 0xBE, 0x6D, 0xEC,
];

pub const PRINCIPAL2: [u8; 32] = [
    0x6E, 0x8A, 0x76, 0x1D, 0xC7, 0x3B, 0x50, 0x90, 0x51, 0x9E, 0x73, 0x9B, 0xEE, 0x7B, 0x02, 0xF9,
    0xB9, 0x17, 0x7C, 0xF6, 0xBB, 0xC8, 0xD5, 0x30, 0xBF, 0x2A, 0xB4, 0xDE, 0x1B, 0x38, 0xCC, 0xF6,
];

pub const SIGNATURE: [u8; 384] = [
    0x12, 0xB1, 0x09, 0x2F, 0x2B, 0x3C, 0x53, 0xE8, 0x1B, 0x2B, 0x6A, 0xE7, 0x97, 0x42, 0x9D, 0x83,
    0x71, 0x75, 0x65, 0x25, 0xFD, 0xB0, 0x0E, 0x2F, 0xAB, 0x53, 0xB7, 0x03, 0x03, 0x39, 0xEE, 0xE9,
    0x15, 0x19, 0xAB, 0x1A, 0xF1, 0x2C, 0x3C, 0xAB, 0x5C, 0x02, 0xEA, 0xD6, 0xF6, 0x94, 0x36, 0x80,
    0x5F, 0xD1, 0x8E, 0xC4, 0xB9, 0xB5, 0x04, 0x73, 0xBB, 0x77, 0x2C, 0x9C, 0xAF, 0xE6, 0x38, 0xB7,
    0xD8, 0xC1, 0xA3, 0x1B, 0xAD, 0xFB, 0xB2, 0x5E, 0x77, 0xBD, 0x79, 0xDC, 0x4E, 0xEC, 0xEA, 0xFA,
    0x80, 0x9F, 0x12, 0x60, 0xC4, 0xB3, 0x71, 0xAF, 0x1C, 0x28, 0xEE, 0x91, 0x1D, 0x6A, 0x69, 0x31,
    0xA4, 0x5A, 0xAA, 0xBE, 0x0B, 0x20, 0x6A, 0xE1, 0x61, 0x0E, 0x0B, 0x9F, 0x60, 0x73, 0xBE, 0x29,
    0x1F, 0x11, 0x0A, 0x8F, 0x42, 0x8F, 0x80, 0xC1, 0x06, 0x40, 0x29, 0xE9, 0xF1, 0x74, 0x44, 0x69,
    0xCC, 0xD3, 0x05, 0x22, 0x24, 0x87, 0x94, 0x49, 0x4B, 0x5B, 0x62, 0x48, 0x01, 0x7B, 0x05, 0x28,
    0xBE, 0x4D, 0x50, 0x06, 0xE5, 0x67, 0xA8, 0xFB, 0x9A, 0x19, 0x52, 0x57, 0x4B, 0xC4, 0xBB, 0x6A,
    0xF5, 0x0D, 0x67, 0xD0, 0x08, 0x93, 0x81, 0xD2, 0x9E, 0xE2, 0xC8, 0xFA, 0x25, 0xEE, 0x9D, 0xC5,
    0xD8, 0xF4, 0xD5, 0x17, 0xE4, 0xEF, 0xFC, 0x09, 0xB0, 0x09, 0xCA, 0xCA, 0x71, 0x36, 0x61, 0xBE,
    0xC1, 0xD0, 0x53, 0x6D, 0x0C, 0x0A, 0x4D, 0xF1, 0xE0, 0xAB, 0x2C, 0x89, 0x98, 0x81, 0xED, 0x25,
    0xF8, 0x81, 0x8E, 0x2C, 0x46, 0x74, 0x57, 0xCF, 0x5B, 0xE2, 0x14, 0xA5, 0xBF, 0x56, 0xED, 0xD3,
    0x11, 0xE7, 0xB4, 0x8D, 0x89, 0x3A, 0xB2, 0x78, 0xF1, 0xA9, 0x82, 0x9D, 0x3B, 0xE5, 0x6B, 0xD2,
    0xA5, 0xB6, 0xFB, 0x71, 0x0F, 0x98, 0x52, 0x54, 0x1E, 0x98, 0xD2, 0x3B, 0x78, 0x51, 0xD1, 0xE2,
    0x57, 0xAE, 0xB3, 0x34, 0x0E, 0x20, 0x26, 0xCF, 0x4B, 0x1F, 0x12, 0xCA, 0xB1, 0xD5, 0x67, 0x55,
    0xF9, 0xE2, 0xA9, 0x7B, 0xAE, 0x35, 0x35, 0x67, 0xA3, 0xCA, 0xBD, 0xF2, 0x41, 0x4E, 0x3F, 0x7E,
    0x20, 0x72, 0xC2, 0xCC, 0xE4, 0x90, 0x00, 0x04, 0x5D, 0xF7, 0xD9, 0xC7, 0x0C, 0x2C, 0xF8, 0x80,
    0xCB, 0xF2, 0x3E, 0x1A, 0xE9, 0x43, 0x23, 0xAF, 0x62, 0x57, 0xD1, 0x4C, 0xFD, 0x05, 0xB6, 0xB6,
    0x86, 0xB9, 0x86, 0x40, 0xC2, 0x56, 0x45, 0x52, 0xE2, 0x93, 0x62, 0x65, 0x23, 0xE7, 0x1D, 0x89,
    0xCA, 0x40, 0x23, 0x44, 0xDB, 0x56, 0x90, 0x4A, 0x0E, 0xB1, 0xB1, 0xE1, 0x9B, 0x5A, 0x5D, 0x53,
    0xEF, 0x4E, 0xE1, 0x0E, 0x12, 0xA9, 0x7C, 0x11, 0x0B, 0x6A, 0x4C, 0x3A, 0x8F, 0x2B, 0x9F, 0xC6,
    0x42, 0xD7, 0xFE, 0x7D, 0x10, 0x02, 0xBD, 0x31, 0x21, 0xCD, 0xD1, 0x32, 0x04, 0x22, 0xE2, 0x36,
];

/// The key of the parent of test blocks.
pub static PARENT_KEY: SymKey = {
    let key = [
        0x35, 0x3D, 0x8C, 0x95, 0x6C, 0x8D, 0xE6, 0xC0, 0xB0, 0xD5, 0x1C, 0xE9, 0x94, 0xB0, 0x58,
        0xD3, 0x80, 0x46, 0x12, 0x1C, 0xF3, 0x9B, 0x8A, 0xEC, 0x38, 0xD5, 0x8B, 0x05, 0x92, 0x8D,
        0xA1, 0x18,
    ];
    let iv = [
        0xDA, 0x96, 0x0E, 0xF5, 0x00, 0xDB, 0xC8, 0x4E, 0xFB, 0x83, 0x59, 0x99, 0x6C, 0x18, 0xD1,
        0x27,
    ];
    SymKey::Aes256Cbc { key, iv }
};

lazy_static! {
    static ref INHERIT: Ciphertext<SymKey> = PARENT_KEY
        .ser_encrypt(&BLOCK_KEY)
        .expect("failed to encrypt block key");
}

/// The key used to encrypt test blocks.
pub static BLOCK_KEY: SymKey = {
    let key = [
        0xB2, 0xB3, 0xDA, 0x5A, 0x1A, 0xF6, 0xB3, 0x78, 0x30, 0xAB, 0x1D, 0x33, 0x33, 0xE7, 0xE3,
        0x5B, 0xBB, 0xF9, 0xFE, 0xD0, 0xC1, 0xF7, 0x90, 0x34, 0x69, 0xB7, 0xE7, 0xC6, 0x1C, 0x46,
        0x85, 0x48,
    ];
    let iv = [
        0xEC, 0x19, 0x59, 0x3A, 0x1D, 0x1E, 0x4A, 0x58, 0x66, 0xC1, 0xD1, 0x9A, 0x61, 0x6E, 0xBA,
        0x16,
    ];
    SymKey::Aes256Cbc { key, iv }
};

lazy_static! {
    pub static ref ROOT_CREDS: ConcreteCreds =
        ConcreteCreds::generate().expect("root cred generation failed");
    pub static ref NODE_CREDS: ConcreteCreds = {
        let mut node_creds = ConcreteCreds::generate().expect("node cred generation failed");
        let expires = Epoch::now() + Duration::from_secs(3600);
        let writecap = ROOT_CREDS
            .issue_writecap(node_creds.principal(), vec![], expires)
            .expect("failed to issue writecap to test_helpers::NODE_CREDS");
        node_creds.set_writecap(writecap);
        node_creds
    };
}

pub fn node_creds() -> &'static ConcreteCreds {
    &NODE_CREDS
}

pub fn root_creds() -> &'static ConcreteCreds {
    &ROOT_CREDS
}

/// Converts the given error to a serde_block_tree error by turning it into a message.
fn convert_err<E: Display>(err: E) -> Error {
    Error::Message(err.to_string())
}

pub fn make_principal() -> Principal {
    Principal(VarHash::Sha2_256(PRINCIPAL.into()))
}

pub fn make_path_with_root(root: Principal, rel_components: Vec<&str>) -> BlockPath {
    let mut components = Vec::with_capacity(rel_components.len());
    for component in rel_components {
        components.push(component.to_string());
    }
    BlockPath::new(root, components)
}

pub fn make_path(rel_components: Vec<&str>) -> BlockPath {
    make_path_with_root(make_principal(), rel_components)
}

pub fn make_writecap_and_creds(rel_components: Vec<&str>) -> (Writecap, impl Creds) {
    let (root_writecap, root_key) = make_self_signed_writecap();
    let issued_to = Principal(VarHash::Sha2_256(PRINCIPAL.into()));
    (
        make_writecap_trusted_by(root_writecap, &root_key, issued_to, rel_components),
        root_key,
    )
}

pub fn make_writecap(rel_components: Vec<&str>) -> Writecap {
    let (writecap, ..) = make_writecap_and_creds(rel_components);
    writecap
}

pub fn make_writecap_trusted_by<C: Creds>(
    next: Writecap,
    trusting_creds: &C,
    issued_to: Principal,
    path_components: Vec<&str>,
) -> Writecap {
    let hour_hence = Epoch::now() + Duration::from_secs(3600);
    let mut writecap = Writecap {
        body: WritecapBody {
            issued_to,
            path: make_path_with_root(next.body.path.root().clone(), path_components),
            expires: hour_hence,
            signing_key: trusting_creds.public_sign().clone(),
        },
        signature: Signature::empty(Sign::RSA_PSS_3072_SHA_256),
        next: Some(Box::from(next)),
    };
    trusting_creds
        .sign_writecap(&mut writecap)
        .map_err(convert_err)
        .expect("failed to sign writecap");
    writecap
}

pub fn make_key_pair() -> impl Creds {
    NODE_CREDS.clone()
}

pub fn make_self_signed_writecap() -> (Writecap, impl Creds) {
    let key = make_key_pair();
    (make_self_signed_writecap_with(&key), key)
}

pub fn make_self_signed_writecap_with<C: Creds>(key: &C) -> Writecap {
    let root_principal = key.principal();
    let hour_hence = Epoch::now() + Duration::from_secs(3600);
    let mut writecap = Writecap {
        body: WritecapBody {
            issued_to: root_principal.clone(),
            path: make_path_with_root(root_principal, vec![]),
            expires: hour_hence,
            signing_key: key.public_sign().clone(),
        },
        signature: Signature::empty(Sign::RSA_PSS_3072_SHA_256),
        next: None,
    };
    key.sign_writecap(&mut writecap)
        .map_err(convert_err)
        .expect("failed to sign writecap");
    writecap
}

pub fn make_block_with() -> SectoredBuf<SecretStream<PioCursor<impl Block>>> {
    let block_key = SymKey::generate(SymKeyKind::default()).unwrap();
    // Notice that the writecap path contains the block path. If this were not the case, the block
    // would be invalid.
    let (writecap, creds) = make_writecap_and_creds(vec!["apps"]);
    let root_writecap = writecap.next.as_ref().unwrap();
    let path = make_path_with_root(root_writecap.body.issued_to.clone(), vec!["apps", "verse"]);
    let header = BlockMetaBody::new(&creds).unwrap();
    let sig = Signature::copy_from(Sign::RSA_PSS_3072_SHA_256, &SIGNATURE);
    let mut stream = BlockStream::new(
        SectoredCursor::new(Vec::new(), SECTOR_SZ_DEFAULT).require_sect_sz(false),
        creds,
        None,
        path,
    )
    .expect("create block stream failed");
    stream.meta.body = header;
    stream.meta.sig = sig;
    let stream = MerkleStream::new(stream).expect("create merkle stream failed");
    let stream = SecretStream::new(block_key)
        .try_compose(PioCursor::new(stream))
        .expect("create secret stream failed");
    SectoredBuf::new(stream).expect("failed to compose with SectoredBuf")
}

/// This function can be run as a test to write a new RSA key pair, as two Rust arrays,
/// out to a file.
#[allow(dead_code)]
fn write_test_keys() -> Result<()> {
    write_rsa_keys_to_file("test_keys.rs")
}

struct NamedSlice<'a> {
    name: &'a str,
    slice: &'a [u8],
}

impl<'a> NamedSlice<'a> {
    fn new(name: &'a str, slice: &'a [u8]) -> NamedSlice<'a> {
        NamedSlice { name, slice }
    }
}

fn write_rsa_keys_to_file(path: &str) -> Result<()> {
    use openssl::rsa::Rsa;
    let rsa = Rsa::generate(3072).map_err(convert_err)?;
    let public_der = rsa.public_key_to_der().map_err(convert_err)?;
    let private_der = rsa.private_key_to_der().map_err(convert_err)?;
    let slices = [
        NamedSlice::new("PUBLIC", public_der.as_slice()),
        NamedSlice::new("PRIVATE", private_der.as_slice()),
    ];
    write_to_file(path, slices.into_iter())
}

fn write_to_file<'a, I: Iterator<Item = NamedSlice<'a>>>(path: &str, slices: I) -> Result<()> {
    let path = std::path::Path::new(path);
    let mut file = File::create(path).map_err(Error::Io)?;
    for NamedSlice { name, slice } in slices {
        write_slice(&mut file, name, slice)?;
    }
    Ok(())
}

fn write_slice<W: Write>(output: &mut W, name: &str, slice: &[u8]) -> Result<()> {
    const LINE_LEN: usize = 100;
    let mut line = String::with_capacity(LINE_LEN);
    write!(line, "    ").map_err(Error::Format)?;
    writeln!(output, "pub const {}: [u8; {}] = [", name, slice.len()).map_err(Error::Io)?;
    for byte in slice {
        if line.len() + 6 > LINE_LEN {
            writeln!(output, "{}", line).map_err(Error::Io)?;
            line.clear();
            write!(line, "    ").map_err(Error::Format)?;
        }
        write!(line, "0x{:02X?}, ", byte).map_err(Error::Format)?;
    }
    writeln!(output, "{}", line).map_err(Error::Io)?;
    writeln!(output, "];").map_err(Error::Io)?;
    writeln!(output).map_err(Error::Io)?;
    Ok(())
}

/// A naive randomizer implementation that is intended only for testing.
pub struct Randomizer {
    state: [u8; Self::HASH.len()],
    buf: [u8; Self::HASH.len()],
}

impl Randomizer {
    pub const HASH: HashKind = HashKind::Sha2_256;

    pub fn new(seed: [u8; Self::HASH.len()]) -> Randomizer {
        Randomizer {
            state: seed,
            buf: [0u8; Self::HASH.len()],
        }
    }
}

impl Iterator for Randomizer {
    type Item = usize;

    fn next(&mut self) -> Option<Self::Item> {
        const BYTES: usize = usize::BITS as usize / 8;
        Self::HASH
            .digest(&mut self.buf, std::iter::once(self.state.as_slice()))
            .expect("digest failed");
        self.state.copy_from_slice(&self.buf);
        let int_bytes = self.buf.as_slice()[..BYTES]
            .try_into()
            .expect("failed to convert to array");
        Some(usize::from_ne_bytes(int_bytes))
    }
}

pub fn write_sector(mut buf: &mut [u8], sect_index: usize) {
    let data = (sect_index + 1).to_ne_bytes();
    for chunk in std::iter::repeat(data).take(buf.len() / data.len()) {
        (&mut buf[..chunk.len()]).copy_from_slice(chunk.as_slice());
        buf = &mut buf[chunk.len()..];
    }
}

pub fn write_fill<W: Write>(mut write: W, sect_sz: usize, sect_ct: usize) {
    let mut buf = vec![0u8; sect_sz];
    for sect_index in 0..sect_ct {
        write_sector(&mut buf, sect_index);
        write.write_all(&mut buf).expect("write failed");
    }
    write.flush().expect("flush failed");
}

pub fn read_check<R: Read>(mut read: R, sect_sz: usize, sect_ct: usize) {
    let mut actual = vec![0u8; sect_sz];
    let mut expected = vec![0u8; sect_sz];
    for sect_index in 0..sect_ct {
        write_sector(&mut expected, sect_index);
        read.read_exact(&mut actual).expect("read failed");
        assert_eq!(expected, actual);
    }
}

#[allow(dead_code)]
pub fn write_indices<W: Write + Seek, I: Iterator<Item = usize>>(
    mut write: W,
    sect_sz: usize,
    indices: I,
) {
    let mut buf = vec![0u8; sect_sz];
    for sect_index in indices {
        let offset = sect_index * sect_sz;
        write
            .seek(SeekFrom::Start(offset as u64))
            .expect("seek failed");
        write_sector(&mut buf, sect_index);
        write.write_all(&mut buf).expect("write failed");
    }
    write.flush().expect("flush failed");
}

#[allow(dead_code)]
pub fn read_indices<R: Read + Seek, I: Iterator<Item = usize>>(
    mut read: R,
    sect_sz: usize,
    indices: I,
) {
    let mut actual = vec![0u8; sect_sz];
    let mut expected = vec![0u8; sect_sz];
    for sect_index in indices {
        let offset = sect_index * sect_sz;
        read.seek(SeekFrom::Start(offset as u64))
            .expect("seek failed");
        write_sector(&mut expected, sect_index);
        read.read_exact(&mut actual).expect("read failed");
        assert_eq!(expected, actual);
    }
}

#[allow(dead_code)]
pub fn random_indices<'a>(
    rando: &'a mut Randomizer,
    sect_ct: usize,
) -> impl Iterator<Item = usize> + 'a {
    rando.take(sect_ct).map(move |e| e % sect_ct)
}

/// Returns an integer array starting at the given value and increasing by one for each subsequent
/// entry.
pub const fn integer_array<const N: usize>(start: u8) -> [u8; N] {
    let mut array = [0u8; N];
    let mut k = 0usize;
    while k < N {
        array[k] = start.wrapping_add(k as u8);
        k += 1;
    }
    array
}

/// Trait for types which can be referenced as slices and which support conversion from `Vec<u8>`.
pub trait FromVec: AsRef<[u8]> {
    fn from_vec(vec: Vec<u8>) -> Self;
}

impl FromVec for Vec<u8> {
    fn from_vec(vec: Vec<u8>) -> Self {
        vec
    }
}

impl<const N: usize> FromVec for [u8; N] {
    fn from_vec(vec: Vec<u8>) -> Self {
        assert_eq!(N, vec.len());
        let mut buf = [0u8; N];
        buf.copy_from_slice(&vec);
        buf
    }
}

/// Module containing functions for serializing and deserializing buffers in `SectoredCursor<T>`.
mod serde_cursor {
    use super::FromVec;
    use serde::{Deserialize, Deserializer, Serialize, Serializer};
    use std::{
        cell::RefCell,
        io::{Cursor, Read},
        result::Result,
    };

    pub fn serialize<T: AsRef<[u8]>, S: Serializer>(
        cursor: &RefCell<Cursor<T>>,
        ser: S,
    ) -> Result<S::Ok, S::Error> {
        let mut cursor = cursor.borrow_mut();
        let pos = cursor.position();
        cursor.set_position(0);
        let mut data = Vec::new();
        cursor
            .read_to_end(&mut data)
            .expect("reading from cursor failed");
        cursor.set_position(pos);
        data.serialize(ser)
    }

    pub fn deserialize<'de, T: FromVec, D: Deserializer<'de>>(
        de: D,
    ) -> Result<RefCell<Cursor<T>>, D::Error> {
        let data = Vec::<u8>::deserialize(de)?;
        Ok(RefCell::new(Cursor::new(T::from_vec(data))))
    }
}

/// A wrapper for `Cursor<T>` which implements additional traits.
#[derive(Debug, PartialEq, Serialize, Deserialize)]
pub struct BtCursor<T: FromVec> {
    #[serde(with = "serde_cursor")]
    cursor: RefCell<Cursor<T>>,
}

impl<T: FromVec> BtCursor<T> {
    pub fn new(inner: T) -> BtCursor<T> {
        BtCursor {
            cursor: RefCell::new(Cursor::new(inner)),
        }
    }
}

impl<T: FromVec> Decompose<T> for BtCursor<T> {
    fn into_inner(self) -> T {
        self.cursor.into_inner().into_inner()
    }
}

impl<T: FromVec + AsMut<[u8]>> BtCursor<T> {
    /// Access the data in this cursor as a mutable slice of bytes. The callback which access the
    /// bytes is expected to return the number of bytes to advance the cursor position by.
    fn access_advance<F: FnOnce(&mut [u8]) -> u64>(&mut self, limit: usize, accessor: F) -> u64 {
        let mut cursor = self.cursor.borrow_mut();
        let pos = cursor.position();
        let byte_slice = cursor.get_mut().as_mut();
        let max = byte_slice.len().max(limit);
        let pos_usize: usize = pos.try_into().unwrap();
        let byte_slice = &mut byte_slice[pos_usize..max];
        let advance_by = accessor(byte_slice);
        cursor.set_position(pos + advance_by);
        advance_by
    }

    /// Gives a `accessor` access to a slice containing all of the data in this cursor.
    fn access_whole<U, F: FnOnce(&mut [u8]) -> U>(&mut self, accessor: F) -> U {
        let mut cursor = self.cursor.borrow_mut();
        let byte_slice = cursor.get_mut().as_mut();
        accessor(byte_slice)
    }

    fn write_from_impl(
        &mut self,
        f: &mut dyn FileReadWriteVolatile,
        count: usize,
        off: u64,
    ) -> io::Result<usize> {
        let read = self.access_advance(count, |byte_slice| {
            let slice = unsafe {
                FileVolatileSlice::from_raw_ptr(byte_slice.as_mut_ptr(), byte_slice.len())
            };
            let read = f.read_at_volatile(slice, off).expect("read failed");
            read.try_into().unwrap()
        });
        Ok(read.try_into().unwrap())
    }
}

impl Write for BtCursor<Vec<u8>> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.cursor.get_mut().write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        self.cursor.get_mut().flush()
    }
}

impl<const N: usize> Write for BtCursor<[u8; N]> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.cursor.get_mut().write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        self.cursor.get_mut().flush()
    }
}

impl WriteAt for BtCursor<Vec<u8>> {
    fn write_at(&mut self, pos: u64, buf: &[u8]) -> io::Result<usize> {
        let pos: usize = pos.try_into().box_err()?;
        let vec = self.cursor.get_mut().get_mut();
        let end = pos + buf.len();
        if end > vec.len() {
            vec.resize(end, 0);
        }
        let slice = &mut vec[pos..end];
        slice.copy_from_slice(buf);
        Ok(end - pos)
    }

    fn flush(&mut self) -> io::Result<()> {
        Ok(())
    }
}

impl<const N: usize> WriteAt for BtCursor<[u8; N]> {
    fn write_at(&mut self, pos: u64, buf: &[u8]) -> io::Result<usize> {
        let pos: usize = pos.try_into().box_err()?;
        let slice = self.cursor.get_mut().get_mut().as_mut();
        let end = pos + buf.len();
        let slice = &mut slice[pos..end];
        slice.copy_from_slice(buf);
        Ok(end - pos)
    }

    fn flush(&mut self) -> io::Result<()> {
        Ok(())
    }
}

impl<T: FromVec> Read for BtCursor<T> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        self.cursor.get_mut().read(buf)
    }
}

impl<T: FromVec> ReadAt for BtCursor<T> {
    fn read_at(&self, pos: u64, buf: &mut [u8]) -> io::Result<usize> {
        let pos: usize = pos.try_into().box_err()?;
        let borrow = self.cursor.try_borrow().box_err()?;
        let slice: &[u8] = borrow.get_ref().as_ref();
        let end = slice.len().min(pos + buf.len());
        let buf = &mut buf[..(end - pos)];
        buf.copy_from_slice(&slice[pos..end]);
        Ok(end - pos)
    }
}

impl<T: FromVec> Seek for BtCursor<T> {
    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
        self.cursor.get_mut().seek(pos)
    }
}

impl WriteInteg for BtCursor<Vec<u8>> {
    fn flush_integ(&mut self, _: &[u8]) -> io::Result<()> {
        Ok(())
    }
}

impl<const N: usize> WriteInteg for BtCursor<[u8; N]> {
    fn flush_integ(&mut self, _: &[u8]) -> io::Result<()> {
        Ok(())
    }
}

impl<T: AsMut<[u8]> + FromVec> FileReadWriteVolatile for BtCursor<T> {
    fn read_volatile(
        &mut self,
        slice: fuse_backend_rs::file_buf::FileVolatileSlice,
    ) -> io::Result<usize> {
        let read = self.access_advance(usize::MAX, |byte_slice| {
            let written = slice.as_volatile_slice().write(byte_slice, 0).unwrap();
            written.try_into().unwrap()
        });
        Ok(read.try_into().unwrap())
    }

    fn write_volatile(
        &mut self,
        slice: fuse_backend_rs::file_buf::FileVolatileSlice,
    ) -> io::Result<usize> {
        let written = self.access_advance(usize::MAX, |byte_slice| {
            let read = slice.as_volatile_slice().read(byte_slice, 0).unwrap();
            read.try_into().unwrap()
        });
        Ok(written.try_into().unwrap())
    }

    fn read_at_volatile(
        &mut self,
        slice: fuse_backend_rs::file_buf::FileVolatileSlice,
        offset: u64,
    ) -> io::Result<usize> {
        Ok(self.access_whole(|byte_slice| {
            let offset = offset.try_into().unwrap();
            let byte_slice = &mut byte_slice[offset..];
            slice.as_volatile_slice().write(byte_slice, 0).unwrap()
        }))
    }

    fn write_at_volatile(
        &mut self,
        slice: fuse_backend_rs::file_buf::FileVolatileSlice,
        offset: u64,
    ) -> io::Result<usize> {
        Ok(self.access_whole(|byte_slice| {
            let offset = offset.try_into().unwrap();
            let byte_slice = &mut byte_slice[offset..];
            slice.as_volatile_slice().read(byte_slice, 0).unwrap()
        }))
    }
}

impl<T: AsMut<[u8]> + FromVec> ZeroCopyReader for BtCursor<T> {
    fn read_to(
        &mut self,
        f: &mut dyn FileReadWriteVolatile,
        count: usize,
        off: u64,
    ) -> io::Result<usize> {
        let written = self.access_advance(count, |byte_slice| {
            // Safety: The mut borrow of cursor is guaranteed to be the only one in existence
            // because this method takes &mut self.
            let slice = unsafe {
                FileVolatileSlice::from_raw_ptr(byte_slice.as_mut_ptr(), byte_slice.len())
            };
            let written = f.write_at_volatile(slice, off).expect("write failed");
            let written_u64: u64 = written.try_into().unwrap();
            written_u64
        });
        Ok(written.try_into().unwrap())
    }
}

impl ZeroCopyWriter for BtCursor<Vec<u8>> {
    fn write_from(
        &mut self,
        f: &mut dyn FileReadWriteVolatile,
        count: usize,
        off: u64,
    ) -> io::Result<usize> {
        self.write_from_impl(f, count, off)
    }
}

impl<const N: usize> ZeroCopyWriter for BtCursor<[u8; N]> {
    fn write_from(
        &mut self,
        f: &mut dyn FileReadWriteVolatile,
        count: usize,
        off: u64,
    ) -> io::Result<usize> {
        self.write_from_impl(f, count, off)
    }
}

impl<T: FromVec> Size for BtCursor<T> {
    fn size(&self) -> io::Result<Option<u64>> {
        let borrow = self.cursor.borrow();
        let slice: &[u8] = borrow.get_ref().as_ref();
        Ok(Some(slice.len() as u64))
    }
}

#[derive(Debug, PartialEq, Serialize, Deserialize)]
pub struct SectoredCursor<T: FromVec> {
    cursor: BtCursor<T>,
    sect_sz: usize,
    meta: BlockMeta,
    require_sect_sz: bool,
}

impl<T: FromVec> SectoredCursor<T> {
    pub fn new(inner: T, sect_sz: usize) -> SectoredCursor<T> {
        SectoredCursor {
            cursor: BtCursor::new(inner),
            sect_sz,
            meta: BlockMeta::new(&*NODE_CREDS).unwrap(),
            require_sect_sz: true,
        }
    }

    pub fn require_sect_sz(mut self, require: bool) -> Self {
        self.require_sect_sz = require;
        self
    }
}

impl<T: FromVec> Sectored for SectoredCursor<T> {
    fn sector_sz(&self) -> usize {
        self.sect_sz
    }
}

impl<T: FromVec> Decompose<T> for SectoredCursor<T> {
    fn into_inner(self) -> T {
        self.cursor.into_inner()
    }
}

impl Write for SectoredCursor<Vec<u8>> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        if self.require_sect_sz {
            self.assert_sector_sz(buf.len())?;
        }
        self.cursor.write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        Write::flush(&mut self.cursor)
    }
}

impl<const N: usize> Write for SectoredCursor<[u8; N]> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        if self.require_sect_sz {
            self.assert_sector_sz(buf.len())?;
        }
        self.cursor.write(buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        Write::flush(&mut self.cursor)
    }
}

impl WriteAt for SectoredCursor<Vec<u8>> {
    fn write_at(&mut self, pos: u64, buf: &[u8]) -> io::Result<usize> {
        self.cursor.write_at(pos, buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        Write::flush(&mut self.cursor)
    }
}

impl<const N: usize> WriteAt for SectoredCursor<[u8; N]> {
    fn write_at(&mut self, pos: u64, buf: &[u8]) -> io::Result<usize> {
        self.cursor.write_at(pos, buf)
    }

    fn flush(&mut self) -> io::Result<()> {
        Write::flush(&mut self.cursor)
    }
}

impl<T: FromVec> SectoredCursor<T> {
    fn flush_integ_impl(&mut self, integrity: &[u8]) -> io::Result<()> {
        let dest = self
            .meta
            .body
            .integrity
            .get_or_insert_with(<VarHash as Default>::default);
        dest.as_mut_slice().copy_from_slice(integrity);
        Ok(())
    }
}

impl WriteInteg for SectoredCursor<Vec<u8>> {
    fn flush_integ(&mut self, integrity: &[u8]) -> io::Result<()> {
        self.flush_integ_impl(integrity)
    }
}

impl<const N: usize> WriteInteg for SectoredCursor<[u8; N]> {
    fn flush_integ(&mut self, integrity: &[u8]) -> io::Result<()> {
        self.flush_integ_impl(integrity)
    }
}

impl<T: FromVec> Read for SectoredCursor<T> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        if self.require_sect_sz {
            self.assert_sector_sz(buf.len())?;
        }
        self.cursor.read(buf)
    }
}

impl<T: FromVec> ReadAt for SectoredCursor<T> {
    fn read_at(&self, pos: u64, buf: &mut [u8]) -> io::Result<usize> {
        if self.require_sect_sz {
            self.assert_sector_sz(buf.len())?;
        }
        self.cursor.read_at(pos, buf)
    }
}

impl<T: FromVec> Seek for SectoredCursor<T> {
    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
        self.cursor.seek(pos)
    }
}

impl<T: FromVec> AsRef<BlockMeta> for SectoredCursor<T> {
    fn as_ref(&self) -> &BlockMeta {
        &self.meta
    }
}

impl<T: FromVec> AsMut<BlockMeta> for SectoredCursor<T> {
    fn as_mut(&mut self) -> &mut BlockMeta {
        &mut self.meta
    }
}

impl<T: FromVec> Size for SectoredCursor<T> {
    fn size(&self) -> io::Result<Option<u64>> {
        self.cursor.size()
    }
}