//! Implements an interface for writing the Nix archive format (NAR).
//!
//! NAR files (and their hashed representations) are used in C++ Nix for
//! addressing fixed-output derivations and a variety of other things.
//!
//! NAR files can be output to any type that implements [`Write`], and content
//! can be read from any type that implementes [`BufRead`].
//!
//! Writing a single file might look like this:
//!
//! ```rust
//! # use std::io::BufReader;
//! # let some_file: Vec<u8> = vec![0, 1, 2, 3, 4];
//!
//! // Output location to write the NAR to.
//! let mut sink: Vec<u8> = Vec::new();
//!
//! // Instantiate writer for this output location.
//! let mut nar = nix_compat::nar::writer::open(&mut sink)?;
//!
//! // Acquire metadata for the single file to output, and pass it in a
//! // `BufRead`-implementing type.
//!
//! let executable = false;
//! let size = some_file.len() as u64;
//! let mut reader = BufReader::new(some_file.as_slice());
//! nar.file(executable, size, &mut reader)?;
//! # Ok::<(), std::io::Error>(())
//! ```
use std::io::{
self, BufRead,
ErrorKind::{InvalidInput, UnexpectedEof},
Write,
};
mod wire;
/// Convenience type alias for types implementing [`Write`].
pub type Writer<'a> = dyn Write + 'a;
/// Create a new NAR, writing the output to the specified writer.
pub fn open<'a, 'w: 'a>(writer: &'a mut Writer<'w>) -> io::Result<Node<'a, 'w>> {
let mut node = Node { writer };
node.write(&wire::TOK_NAR)?;
Ok(node)
}
/// Single node in a NAR file.
///
/// A NAR can be thought of as a tree of nodes represented by this type. Each
/// node can be a file, a symlink or a directory containing other nodes.
pub struct Node<'a, 'w: 'a> {
writer: &'a mut Writer<'w>,
}
impl<'a, 'w> Node<'a, 'w> {
fn write(&mut self, data: &[u8]) -> io::Result<()> {
self.writer.write_all(data)
}
fn pad(&mut self, n: u64) -> io::Result<()> {
match (n & 7) as usize {
0 => Ok(()),
n => self.write(&[0; 8][n..]),
}
}
/// Make this node a symlink.
pub fn symlink(mut self, target: &str) -> io::Result<()> {
debug_assert!(
target.len() <= wire::MAX_TARGET_LEN,
"target.len() > {}",
wire::MAX_TARGET_LEN
);
debug_assert!(
!target.contains('\0'),
"invalid target characters: {target:?}"
);
debug_assert!(!target.is_empty(), "empty target");
self.write(&wire::TOK_SYM)?;
self.write(&target.len().to_le_bytes())?;
self.write(target.as_bytes())?;
self.pad(target.len() as u64)?;
self.write(&wire::TOK_PAR)?;
Ok(())
}
/// Make this node a single file.
pub fn file(mut self, executable: bool, size: u64, reader: &mut dyn BufRead) -> io::Result<()> {
self.write(if executable {
&wire::TOK_EXE
} else {
&wire::TOK_REG
})?;
self.write(&size.to_le_bytes())?;
let mut need = size;
while need != 0 {
let data = reader.fill_buf()?;
if data.is_empty() {
return Err(UnexpectedEof.into());
}
let n = need.min(data.len() as u64) as usize;
self.write(&data[..n])?;
need -= n as u64;
reader.consume(n);
}
// bail if there's still data left in the passed reader.
// This uses the same code as [BufRead::has_data_left] (unstable).
if reader.fill_buf().map(|b| !b.is_empty())? {
return Err(io::Error::new(
InvalidInput,
"reader contained more data than specified size",
));
}
self.pad(size)?;
self.write(&wire::TOK_PAR)?;
Ok(())
}
/// Make this node a directory, the content of which is set using the
/// resulting [`Directory`] value.
pub fn directory(mut self) -> io::Result<Directory<'a, 'w>> {
self.write(&wire::TOK_DIR)?;
Ok(Directory::new(self))
}
}
#[cfg(debug_assertions)]
type Name = String;
#[cfg(not(debug_assertions))]
type Name = ();
fn into_name(_name: &str) -> Name {
#[cfg(debug_assertions)]
_name.to_owned()
}
/// Content of a NAR node that represents a directory.
pub struct Directory<'a, 'w> {
node: Node<'a, 'w>,
prev_name: Option<Name>,
}
impl<'a, 'w> Directory<'a, 'w> {
fn new(node: Node<'a, 'w>) -> Self {
Self {
node,
prev_name: None,
}
}
/// Add an entry to the directory.
///
/// The entry is simply another [`Node`], which can then be filled like the
/// root of a NAR (including, of course, by nesting directories).
pub fn entry(&mut self, name: &str) -> io::Result<Node<'_, 'w>> {
debug_assert!(
name.len() <= wire::MAX_NAME_LEN,
"name.len() > {}",
wire::MAX_NAME_LEN
);
debug_assert!(!["", ".", ".."].contains(&name), "invalid name: {name:?}");
debug_assert!(
!name.contains(['/', '\0']),
"invalid name characters: {name:?}"
);
match self.prev_name {
None => {
self.prev_name = Some(into_name(name));
}
Some(ref mut _prev_name) => {
#[cfg(debug_assertions)]
{
assert!(
&**_prev_name < name,
"misordered names: {_prev_name:?} >= {name:?}"
);
_prev_name.clear();
_prev_name.push_str(name);
}
self.node.write(&wire::TOK_PAR)?;
}
}
self.node.write(&wire::TOK_ENT)?;
self.node.write(&name.len().to_le_bytes())?;
self.node.write(name.as_bytes())?;
self.node.pad(name.len() as u64)?;
self.node.write(&wire::TOK_NOD)?;
Ok(Node {
writer: &mut *self.node.writer,
})
}
/// Close a directory and write terminators for the directory to the NAR.
///
/// **Important:** This *must* be called when all entries have been written
/// in a directory, otherwise the resulting NAR file will be invalid.
pub fn close(mut self) -> io::Result<()> {
if self.prev_name.is_some() {
self.node.write(&wire::TOK_PAR)?;
}
self.node.write(&wire::TOK_PAR)?;
Ok(())
}
}
