//! 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, BufRead,
ErrorKind::{InvalidData, UnexpectedEof},
Read, Write,
};
#[cfg(not(debug_assertions))]
use std::marker::PhantomData;
// Required reading for understanding this module.
use crate::nar::wire;
mod read;
#[cfg(test)]
mod test;
pub type Reader<'a> = dyn BufRead + Send + 'a;
struct ArchiveReader<'a, 'r> {
inner: &'a mut Reader<'r>,
/// In debug mode, also track when we need to abandon this archive reader.
/// The archive reader must be abandoned when:
/// * An error is encountered at any point
/// * A file or directory reader is dropped before being read entirely.
/// All of these checks vanish in release mode.
status: ArchiveReaderStatus<'a>,
}
macro_rules! try_or_poison {
($it:expr, $ex:expr) => {
match $ex {
Ok(x) => x,
Err(e) => {
$it.status.poison();
return Err(e.into());
}
}
};
}
/// 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(ArchiveReader {
inner: reader,
status: ArchiveReaderStatus::top(),
})
}
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(mut reader: ArchiveReader<'a, 'r>) -> io::Result<Self> {
Ok(match read::tag(reader.inner)? {
wire::Node::Sym => {
let target =
try_or_poison!(reader, read::bytes(reader.inner, wire::MAX_TARGET_LEN));
if target.is_empty() || target.contains(&0) {
reader.status.poison();
return Err(InvalidData.into());
}
try_or_poison!(reader, read::token(reader.inner, &wire::TOK_PAR));
reader.status.ready_parent(); // Immediately allow reading from parent again
Node::Symlink { target }
}
tag @ (wire::Node::Reg | wire::Node::Exe) => {
let len = try_or_poison!(&mut reader, read::u64(reader.inner));
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.
pub struct FileReader<'a, 'r> {
reader: ArchiveReader<'a, '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(mut reader: ArchiveReader<'a, '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.inner, &wire::TOK_PAR)?;
reader.status.ready_parent();
}
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 FileReader<'_, '_> {
/// Equivalent to [BufRead::fill_buf]
///
/// We can't directly implement [BufRead], because [FileReader::consume] needs
/// to perform fallible I/O.
pub fn fill_buf(&mut self) -> io::Result<&[u8]> {
if self.is_empty() {
return Ok(&[]);
}
self.reader.check_correct();
let mut buf = try_or_poison!(self.reader, self.reader.inner.fill_buf());
if buf.is_empty() {
self.reader.status.poison();
return Err(UnexpectedEof.into());
}
if buf.len() as u64 > self.len {
buf = &buf[..self.len as usize];
}
Ok(buf)
}
/// Analogous to [BufRead::consume], differing only in that it needs
/// to perform I/O in order to read padding and terminators.
pub fn consume(&mut self, n: usize) -> io::Result<()> {
if n == 0 {
return Ok(());
}
self.reader.check_correct();
self.len = self
.len
.checked_sub(n as u64)
.expect("consumed bytes past EOF");
self.reader.inner.consume(n);
if self.is_empty() {
self.finish()?;
}
Ok(())
}
/// Copy the (remaining) contents of the file into `dst`.
pub fn copy(&mut self, mut dst: impl Write) -> io::Result<()> {
while !self.is_empty() {
let buf = self.fill_buf()?;
let n = try_or_poison!(self.reader, dst.write(buf));
self.consume(n)?;
}
Ok(())
}
}
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);
}
self.reader.check_correct();
if buf.len() as u64 > self.len {
buf = &mut buf[..self.len as usize];
}
let n = try_or_poison!(self.reader, self.reader.inner.read(buf));
self.len -= n as u64;
if n == 0 {
self.reader.status.poison();
return Err(UnexpectedEof.into());
}
if self.is_empty() {
self.finish()?;
}
Ok(n)
}
}
impl FileReader<'_, '_> {
/// 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.
fn finish(&mut self) -> io::Result<()> {
let pad = (self.pad & 7) as usize;
if pad != 0 {
let mut buf = [0; 8];
try_or_poison!(self.reader, self.reader.inner.read_exact(&mut buf[pad..]));
if buf != [0; 8] {
self.reader.status.poison();
return Err(InvalidData.into());
}
}
try_or_poison!(self.reader, read::token(self.reader.inner, &wire::TOK_PAR));
// Done with reading this file, allow going back up the chain of readers
self.reader.status.ready_parent();
Ok(())
}
}
/// 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: ArchiveReader<'a, '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: ArchiveReader<'a, '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].
#[allow(clippy::should_implement_trait)]
pub fn next(&mut self) -> io::Result<Option<Entry<'_, 'r>>> {
self.reader.check_correct();
// COME FROM the previous iteration: if we've already read an entry,
// read its terminating TOK_PAR here.
if self.prev_name.is_some() {
try_or_poison!(self.reader, read::token(self.reader.inner, &wire::TOK_PAR));
}
// Determine if there are more entries to follow
if let wire::Entry::None = try_or_poison!(self.reader, read::tag(self.reader.inner)) {
// We've reached the end of this directory.
self.reader.status.ready_parent();
return Ok(None);
}
let name = try_or_poison!(
self.reader,
read::bytes(self.reader.inner, wire::MAX_NAME_LEN)
);
if name.is_empty()
|| name.contains(&0)
|| name.contains(&b'/')
|| name == b"."
|| name == b".."
{
self.reader.status.poison();
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 {
self.reader.status.poison();
return Err(InvalidData.into());
}
name[..].clone_into(prev_name);
}
}
try_or_poison!(self.reader, read::token(self.reader.inner, &wire::TOK_NOD));
Ok(Some(Entry {
name,
// Don't need to worry about poisoning here: Node::new will do it for us if needed
node: Node::new(self.reader.child())?,
}))
}
}
/// We use a stack of statuses to:
/// * Share poisoned state across all objects from the same underlying reader,
/// so we can check they are abandoned when an error occurs
/// * Make sure only the most recently created object is read from, and is fully exhausted
/// before anything it was created from is used again.
enum ArchiveReaderStatus<'a> {
#[cfg(not(debug_assertions))]
None(PhantomData<&'a ()>),
#[cfg(debug_assertions)]
StackTop { poisoned: bool, ready: bool },
#[cfg(debug_assertions)]
StackChild {
poisoned: &'a mut bool,
parent_ready: &'a mut bool,
ready: bool,
},
}
impl ArchiveReaderStatus<'_> {
fn top() -> Self {
#[cfg(debug_assertions)]
{
ArchiveReaderStatus::StackTop {
poisoned: false,
ready: true,
}
}
#[cfg(not(debug_assertions))]
ArchiveReaderStatus::None(PhantomData)
}
/// Poison all the objects sharing the same reader, to be used when an error occurs
fn poison(&mut self) {
match self {
#[cfg(not(debug_assertions))]
ArchiveReaderStatus::None(_) => {}
#[cfg(debug_assertions)]
ArchiveReaderStatus::StackTop { poisoned: x, .. } => *x = true,
#[cfg(debug_assertions)]
ArchiveReaderStatus::StackChild { poisoned: x, .. } => **x = true,
}
}
/// Mark the parent as ready, allowing it to be used again and preventing this reference to the reader being used again.
fn ready_parent(&mut self) {
match self {
#[cfg(not(debug_assertions))]
ArchiveReaderStatus::None(_) => {}
#[cfg(debug_assertions)]
ArchiveReaderStatus::StackTop { ready, .. } => {
*ready = false;
}
#[cfg(debug_assertions)]
ArchiveReaderStatus::StackChild {
ready,
parent_ready,
..
} => {
*ready = false;
**parent_ready = true;
}
};
}
fn poisoned(&self) -> bool {
match self {
#[cfg(not(debug_assertions))]
ArchiveReaderStatus::None(_) => false,
#[cfg(debug_assertions)]
ArchiveReaderStatus::StackTop { poisoned, .. } => *poisoned,
#[cfg(debug_assertions)]
ArchiveReaderStatus::StackChild { poisoned, .. } => **poisoned,
}
}
fn ready(&self) -> bool {
match self {
#[cfg(not(debug_assertions))]
ArchiveReaderStatus::None(_) => true,
#[cfg(debug_assertions)]
ArchiveReaderStatus::StackTop { ready, .. } => *ready,
#[cfg(debug_assertions)]
ArchiveReaderStatus::StackChild { ready, .. } => *ready,
}
}
}
impl<'a, 'r> ArchiveReader<'a, 'r> {
/// Create a new child reader from this one.
/// In debug mode, this reader will panic if called before the new child is exhausted / calls `ready_parent`
fn child(&mut self) -> ArchiveReader<'_, 'r> {
ArchiveReader {
inner: self.inner,
#[cfg(not(debug_assertions))]
status: ArchiveReaderStatus::None(PhantomData),
#[cfg(debug_assertions)]
status: match &mut self.status {
ArchiveReaderStatus::StackTop { poisoned, ready } => {
*ready = false;
ArchiveReaderStatus::StackChild {
poisoned,
parent_ready: ready,
ready: true,
}
}
ArchiveReaderStatus::StackChild {
poisoned, ready, ..
} => {
*ready = false;
ArchiveReaderStatus::StackChild {
poisoned,
parent_ready: ready,
ready: true,
}
}
},
}
}
/// Check the reader is in the correct status.
/// Only does anything when debug assertions are on.
#[inline(always)]
fn check_correct(&self) {
assert!(
!self.status.poisoned(),
"Archive reader used after it was meant to be abandoned!"
);
assert!(
self.status.ready(),
"Non-ready archive reader used! (Should've been reading from something else)"
);
}
}
