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diff --git a/tvix/nix-compat/src/nar/writer/sync.rs b/tvix/nix-compat/src/nar/writer/sync.rs
new file mode 100644
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+++ b/tvix/nix-compat/src/nar/writer/sync.rs
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+//! 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 super::wire;
+use bstr::ByteSlice;
+use std::io::{
+    self, BufRead,
+    ErrorKind::{InvalidInput, UnexpectedEof},
+    Write,
+};
+
+/// Convenience type alias for types implementing [`Write`].
+pub type Writer<'a> = dyn Write + Send + '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: &[u8]) -> io::Result<()> {
+        debug_assert!(
+            target.len() <= wire::MAX_TARGET_LEN,
+            "target.len() > {}",
+            wire::MAX_TARGET_LEN
+        );
+        debug_assert!(!target.is_empty(), "target is empty");
+        debug_assert!(!target.contains(&0), "target contains null byte");
+
+        self.write(&wire::TOK_SYM)?;
+        self.write(&target.len().to_le_bytes())?;
+        self.write(target)?;
+        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.
+    ///
+    /// It is the caller's responsibility to invoke [`Directory::close`],
+    /// or invalid archives will be produced silently.
+    pub fn directory(mut self) -> io::Result<Directory<'a, 'w>> {
+        self.write(&wire::TOK_DIR)?;
+        Ok(Directory::new(self))
+    }
+}
+
+#[cfg(debug_assertions)]
+type Name = Vec<u8>;
+#[cfg(not(debug_assertions))]
+type Name = ();
+
+fn into_name(_name: &[u8]) -> 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).
+    ///
+    /// It is the caller's responsibility to ensure that directory entries are
+    /// written in order of ascending name. If this is not ensured, this method
+    /// may panic or silently produce invalid archives.
+    pub fn entry(&mut self, name: &[u8]) -> io::Result<Node<'_, 'w>> {
+        debug_assert!(
+            name.len() <= wire::MAX_NAME_LEN,
+            "name.len() > {}",
+            wire::MAX_NAME_LEN
+        );
+        debug_assert!(!name.is_empty(), "name is empty");
+        debug_assert!(!name.contains(&0), "name contains null byte");
+        debug_assert!(!name.contains(&b'/'), "name contains {:?}", '/');
+        debug_assert!(name != b".", "name == {:?}", ".");
+        debug_assert!(name != b"..", "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.as_bstr(),
+                        name.as_bstr()
+                    );
+                    _prev_name.clear();
+                    _prev_name.extend_from_slice(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)?;
+        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(())
+    }
+}