//! Parser for the Nix archive format, aka NAR.
//!
//! NAR files (and their hashed representations) are used in C++ Nix for
//! a variety of things, including addressing fixed-output derivations
//! and transferring store paths between Nix stores.
use std::io::{
self,
ErrorKind::{InvalidData, UnexpectedEof},
Read,
};
// Required reading for understanding this module.
use crate::nar::wire;
mod read;
pub type Reader<'a> = dyn Read + Send + 'a;
/// Start reading a NAR file from `reader`.
pub fn open<'a, 'r>(reader: &'a mut Reader<'r>) -> io::Result<Node<'a, 'r>> {
read::token(reader, &wire::TOK_NAR)?;
Node::new(reader)
}
pub enum Node<'a, 'r> {
Symlink {
target: Vec<u8>,
},
File {
executable: bool,
reader: FileReader<'a, 'r>,
},
Directory(DirReader<'a, 'r>),
}
impl<'a, 'r> Node<'a, 'r> {
/// Start reading a [Node], matching the next [wire::Node].
///
/// Reading the terminating [wire::TOK_PAR] is done immediately for [Node::Symlink],
/// but is otherwise left to [DirReader] or [FileReader].
fn new(reader: &'a mut Reader<'r>) -> io::Result<Self> {
Ok(match read::tag(reader)? {
wire::Node::Sym => {
let target = read::bytes(reader, wire::MAX_TARGET_LEN)?;
if target.is_empty() || target.contains(&0) {
return Err(InvalidData.into());
}
read::token(reader, &wire::TOK_PAR)?;
Node::Symlink { target }
}
tag @ (wire::Node::Reg | wire::Node::Exe) => {
let len = read::u64(reader)?;
Node::File {
executable: tag == wire::Node::Exe,
reader: FileReader::new(reader, len)?,
}
}
wire::Node::Dir => Node::Directory(DirReader::new(reader)),
})
}
}
/// File contents, readable through the [Read] trait.
///
/// It comes with some caveats:
/// * You must always read the entire file, unless you intend to abandon the entire archive reader.
/// * You must abandon the entire archive reader upon the first error.
///
/// It's fine to read exactly `reader.len()` bytes without ever seeing an explicit EOF.
///
/// TODO(edef): enforce these in `#[cfg(debug_assertions)]`
pub struct FileReader<'a, 'r> {
reader: &'a mut Reader<'r>,
len: u64,
/// Truncated original file length for padding computation.
/// We only care about the 3 least significant bits; semantically, this is a u3.
pad: u8,
}
impl<'a, 'r> FileReader<'a, 'r> {
/// Instantiate a new reader, starting after [wire::TOK_REG] or [wire::TOK_EXE].
/// We handle the terminating [wire::TOK_PAR] on semantic EOF.
fn new(reader: &'a mut Reader<'r>, len: u64) -> io::Result<Self> {
// For zero-length files, we have to read the terminating TOK_PAR
// immediately, since FileReader::read may never be called; we've
// already reached semantic EOF by definition.
if len == 0 {
read::token(reader, &wire::TOK_PAR)?;
}
Ok(Self {
reader,
len,
pad: len as u8,
})
}
pub fn is_empty(&self) -> bool {
self.len == 0
}
pub fn len(&self) -> u64 {
self.len
}
}
impl Read for FileReader<'_, '_> {
fn read(&mut self, mut buf: &mut [u8]) -> io::Result<usize> {
if buf.is_empty() || self.is_empty() {
return Ok(0);
}
if buf.len() as u64 > self.len {
buf = &mut buf[..self.len as usize];
}
let n = self.reader.read(buf)?;
self.len -= n as u64;
if n == 0 {
return Err(UnexpectedEof.into());
}
// If we've reached semantic EOF, consume and verify the padding and terminating TOK_PAR.
// Files are padded to 64 bits (8 bytes), just like any other byte string in the wire format.
if self.is_empty() {
let pad = (self.pad & 7) as usize;
if pad != 0 {
let mut buf = [0; 8];
self.reader.read_exact(&mut buf[pad..])?;
if buf != [0; 8] {
return Err(InvalidData.into());
}
}
read::token(self.reader, &wire::TOK_PAR)?;
}
Ok(n)
}
}
/// A directory iterator, yielding a sequence of [Node]s.
/// It must be fully consumed before reading further from the [DirReader] that produced it, if any.
pub struct DirReader<'a, 'r> {
reader: &'a mut Reader<'r>,
/// Previous directory entry name.
/// We have to hang onto this to enforce name monotonicity.
prev_name: Option<Vec<u8>>,
}
pub struct Entry<'a, 'r> {
pub name: Vec<u8>,
pub node: Node<'a, 'r>,
}
impl<'a, 'r> DirReader<'a, 'r> {
fn new(reader: &'a mut Reader<'r>) -> Self {
Self {
reader,
prev_name: None,
}
}
/// Read the next [Entry] from the directory.
///
/// We explicitly don't implement [Iterator], since treating this as
/// a regular Rust iterator will surely lead you astray.
///
/// * You must always consume the entire iterator, unless you abandon the entire archive reader.
/// * You must abandon the entire archive reader on the first error.
/// * You must abandon the directory reader upon the first [None].
/// * Even if you know the amount of elements up front, you must keep reading until you encounter [None].
///
/// TODO(edef): enforce these in `#[cfg(debug_assertions)]`
#[allow(clippy::should_implement_trait)]
pub fn next(&mut self) -> io::Result<Option<Entry>> {
// COME FROM the previous iteration: if we've already read an entry,
// read its terminating TOK_PAR here.
if self.prev_name.is_some() {
read::token(self.reader, &wire::TOK_PAR)?;
}
// Determine if there are more entries to follow
if let wire::Entry::None = read::tag(self.reader)? {
// We've reached the end of this directory.
return Ok(None);
}
let name = read::bytes(self.reader, wire::MAX_NAME_LEN)?;
if name.is_empty()
|| name.contains(&0)
|| name.contains(&b'/')
|| name == b"."
|| name == b".."
{
return Err(InvalidData.into());
}
// Enforce strict monotonicity of directory entry names.
match &mut self.prev_name {
None => {
self.prev_name = Some(name.clone());
}
Some(prev_name) => {
if *prev_name >= name {
return Err(InvalidData.into());
}
name[..].clone_into(prev_name);
}
}
read::token(self.reader, &wire::TOK_NOD)?;
Ok(Some(Entry {
name,
node: Node::new(&mut self.reader)?,
}))
}
}
