feat(tvix/nar): init writer

Change-Id: I101f35840cb14c58af708c91e59f997f6d167f75
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7505
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

tvix/nar/src/lib.rs

@@ -0,0 +1,157 @@
+use std::io::{self, BufRead, ErrorKind::UnexpectedEof, Write};
+
+mod wire;
+
+pub type Writer<'a> = dyn Write + 'a;
+
+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)
+}
+
+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..]),
+        }
+    }
+
+    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(())
+    }
+
+    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);
+        }
+
+        self.pad(size)?;
+        self.write(&wire::TOK_PAR)?;
+
+        Ok(())
+    }
+
+    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()
+}
+
+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,
+        }
+    }
+
+    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,
+        })
+    }
+
+    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(())
+    }
+}