diff options
Diffstat (limited to 'tvix/nix-compat/src/nar/wire')
| -rw-r--r-- | tvix/nix-compat/src/nar/wire/mod.rs | 132 | ||||
| -rw-r--r-- | tvix/nix-compat/src/nar/wire/tag.rs | 165 |
2 files changed, 297 insertions, 0 deletions
diff --git a/tvix/nix-compat/src/nar/wire/mod.rs b/tvix/nix-compat/src/nar/wire/mod.rs new file mode 100644 index 000000000000..fc8b8d3510aa --- /dev/null +++ b/tvix/nix-compat/src/nar/wire/mod.rs @@ -0,0 +1,132 @@ +//! NAR wire format, without I/O details, since those differ between +//! the synchronous and asynchronous implementations. +//! +//! The wire format is an S-expression format, encoded onto the wire +//! using simple encoding rules. +//! +//! # Encoding +//! +//! Lengths are represented as 64-bit unsigned integers in little-endian +//! format. Byte strings, including file contents and syntactic strings +//! part of the grammar, are prefixed by their 64-bit length, and padded +//! to 8-byte (64-bit) alignment with zero bytes. The zero-length string +//! is therefore encoded as eight zero bytes representing its length. +//! +//! # Grammar +//! +//! The NAR grammar is as follows: +//! ```plain +//! archive ::= "nix-archive-1" node +//! +//! node ::= "(" "type" "symlink" "target" string ")" +//! ||= "(" "type" "regular" ("executable" "")? "contents" string ")" +//! ||= "(" "type" "directory" entry* ")" +//! +//! entry ::= "entry" "(" "name" string "node" node ")" +//! ``` +//! +//! We rewrite it to pull together the purely syntactic elements into +//! unified tokens, producing an equivalent grammar that can be parsed +//! and serialized more elegantly: +//! ```plain +//! archive ::= TOK_NAR node +//! node ::= TOK_SYM string TOK_PAR +//! ||= (TOK_REG | TOK_EXE) string TOK_PAR +//! ||= TOK_DIR entry* TOK_PAR +//! +//! entry ::= TOK_ENT string TOK_NOD node TOK_PAR +//! +//! TOK_NAR ::= "nix-archive-1" "(" "type" +//! TOK_SYM ::= "symlink" "target" +//! TOK_REG ::= "regular" "contents" +//! TOK_EXE ::= "regular" "executable" "" +//! TOK_DIR ::= "directory" +//! TOK_ENT ::= "entry" "(" "name" +//! TOK_NOD ::= "node" "(" "type" +//! TOK_PAR ::= ")" +//! ``` +//! +//! # Restrictions +//! +//! NOTE: These restrictions are not (and cannot be) enforced by this module, +//! but must be enforced by its consumers, [super::reader] and [super::writer]. +//! +//! Directory entry names cannot have the reserved names `.` and `..`, nor contain +//! forward slashes. They must appear in strictly ascending lexicographic order +//! within a directory, and can be at most [MAX_NAME_LEN] bytes in length. +//! +//! Symlink targets can be at most [MAX_TARGET_LEN] bytes in length. +//! +//! Neither is permitted to be empty, or contain null bytes. + +// These values are the standard Linux length limits +/// Maximum length of a directory entry name +pub const MAX_NAME_LEN: usize = 255; +/// Maximum length of a symlink target +pub const MAX_TARGET_LEN: usize = 4095; + +#[cfg(test)] +fn token(xs: &[&str]) -> Vec<u8> { + let mut out = vec![]; + for x in xs { + let len = x.len() as u64; + out.extend_from_slice(&len.to_le_bytes()); + out.extend_from_slice(x.as_bytes()); + + let n = x.len() & 7; + if n != 0 { + const ZERO: [u8; 8] = [0; 8]; + out.extend_from_slice(&ZERO[n..]); + } + } + out +} + +pub const TOK_NAR: [u8; 56] = *b"\x0d\0\0\0\0\0\0\0nix-archive-1\0\0\0\x01\0\0\0\0\0\0\0(\0\0\0\0\0\0\0\x04\0\0\0\0\0\0\0type\0\0\0\0"; +pub const TOK_SYM: [u8; 32] = *b"\x07\0\0\0\0\0\0\0symlink\0\x06\0\0\0\0\0\0\0target\0\0"; +pub const TOK_REG: [u8; 32] = *b"\x07\0\0\0\0\0\0\0regular\0\x08\0\0\0\0\0\0\0contents"; +pub const TOK_EXE: [u8; 64] = *b"\x07\0\0\0\0\0\0\0regular\0\x0a\0\0\0\0\0\0\0executable\0\0\0\0\0\0\0\0\0\0\0\0\0\0\x08\0\0\0\0\0\0\0contents"; +pub const TOK_DIR: [u8; 24] = *b"\x09\0\0\0\0\0\0\0directory\0\0\0\0\0\0\0"; +pub const TOK_ENT: [u8; 48] = *b"\x05\0\0\0\0\0\0\0entry\0\0\0\x01\0\0\0\0\0\0\0(\0\0\0\0\0\0\0\x04\0\0\0\0\0\0\0name\0\0\0\0"; +pub const TOK_NOD: [u8; 48] = *b"\x04\0\0\0\0\0\0\0node\0\0\0\0\x01\0\0\0\0\0\0\0(\0\0\0\0\0\0\0\x04\0\0\0\0\0\0\0type\0\0\0\0"; +pub const TOK_PAR: [u8; 16] = *b"\x01\0\0\0\0\0\0\0)\0\0\0\0\0\0\0"; + +#[test] +fn tokens() { + let cases: &[(&[u8], &[&str])] = &[ + (&TOK_NAR, &["nix-archive-1", "(", "type"]), + (&TOK_SYM, &["symlink", "target"]), + (&TOK_REG, &["regular", "contents"]), + (&TOK_EXE, &["regular", "executable", "", "contents"]), + (&TOK_DIR, &["directory"]), + (&TOK_ENT, &["entry", "(", "name"]), + (&TOK_NOD, &["node", "(", "type"]), + (&TOK_PAR, &[")"]), + ]; + + for &(tok, xs) in cases { + assert_eq!(tok, token(xs)); + } +} + +pub use tag::Tag; +mod tag; + +tag::make! { + /// These are the node tokens, succeeding [TOK_NAR] or [TOK_NOD], + /// and preceding the next variable-length element. + pub enum Node[16] { + Sym = TOK_SYM, + Reg = TOK_REG, + Exe = TOK_EXE, + Dir = TOK_DIR, + } + + /// Directory entry or terminator + pub enum Entry[0] { + /// End of directory + None = TOK_PAR, + /// Directory entry, which must be followed by [Node] + Some = TOK_ENT, + } +} diff --git a/tvix/nix-compat/src/nar/wire/tag.rs b/tvix/nix-compat/src/nar/wire/tag.rs new file mode 100644 index 000000000000..55ce4b4a2a87 --- /dev/null +++ b/tvix/nix-compat/src/nar/wire/tag.rs @@ -0,0 +1,165 @@ +/// A type implementing Tag represents a static hash set of byte strings, +/// with a very simple perfect hash function: every element has a unique +/// discriminant at a common byte offset. The values of the type represent +/// the members by this single discriminant byte; they are indices into the +/// hash set. +pub trait Tag: Sized { + /// Discriminant offset + const OFF: usize; + /// Minimum variant length + const MIN: usize; + + /// Minimal suitably sized buffer for reading the wire representation + /// HACK: This is a workaround for const generics limitations. + type Buf: AsMut<[u8]> + Send; + + /// Make an instance of [Self::Buf] + fn make_buf() -> Self::Buf; + + /// Convert a discriminant into the corresponding variant + fn from_u8(x: u8) -> Option<Self>; + + /// Convert a variant back into the wire representation + fn as_bytes(&self) -> &'static [u8]; +} + +/// Generate an enum implementing [Tag], enforcing at compile time that +/// the discriminant values are distinct. +macro_rules! make { + ( + $( + $(#[doc = $doc:expr])* + $vis:vis enum $Enum:ident[$off:expr] { + $( + $(#[doc = $var_doc:expr])* + $Var:ident = $TOK:ident, + )+ + } + )* + ) => { + $( + $(#[doc = $doc])* + #[derive(Debug, PartialEq, Eq)] + #[repr(u8)] + $vis enum $Enum { + $( + $(#[doc = $var_doc])* + $Var = $TOK[$Enum::OFF] + ),+ + } + + impl Tag for $Enum { + /// Discriminant offset + const OFF: usize = $off; + /// Minimum variant length + const MIN: usize = tag::min_of(&[$($TOK.len()),+]); + + /// Minimal suitably sized buffer for reading the wire representation + type Buf = [u8; tag::buf_of(&[$($TOK.len()),+])]; + + /// Make an instance of [Self::Buf] + #[inline(always)] + fn make_buf() -> Self::Buf { + [0u8; tag::buf_of(&[$($TOK.len()),+])] + } + + /// Convert a discriminant into the corresponding variant + #[inline(always)] + fn from_u8(x: u8) -> Option<Self> { + #[allow(non_upper_case_globals)] + mod __variant { + $( + pub const $Var: u8 = super::$Enum::$Var as u8; + )+ + } + + match x { + $(__variant::$Var => Some(Self::$Var),)+ + _ => None + } + } + + /// Convert a variant back into the wire representation + #[inline(always)] + fn as_bytes(&self) -> &'static [u8] { + match self { + $(Self::$Var => &$TOK,)+ + } + } + } + )* + }; +} + +pub(crate) use make; + +#[cfg(test)] +mod test { + use super::super::tag::{self, Tag}; + + const TOK_A: [u8; 3] = [0xed, 0xef, 0x1c]; + const TOK_B: [u8; 3] = [0xed, 0xf0, 0x1c]; + + const OFFSET: usize = 1; + + make! { + enum Token[OFFSET] { + A = TOK_A, + B = TOK_B, + } + } + + #[test] + fn example() { + assert_eq!(Token::from_u8(0xed), None); + + let tag = Token::from_u8(0xef).unwrap(); + assert_eq!(tag.as_bytes(), &TOK_A[..]); + + let tag = Token::from_u8(0xf0).unwrap(); + assert_eq!(tag.as_bytes(), &TOK_B[..]); + } +} + +// The following functions are written somewhat unusually, +// since they're const functions that cannot use iterators. + +/// Maximum element of a slice +const fn max_of(mut xs: &[usize]) -> usize { + let mut y = usize::MIN; + while let &[x, ref tail @ ..] = xs { + y = if x > y { x } else { y }; + xs = tail; + } + y +} + +/// Minimum element of a slice +pub const fn min_of(mut xs: &[usize]) -> usize { + let mut y = usize::MAX; + while let &[x, ref tail @ ..] = xs { + y = if x < y { x } else { y }; + xs = tail; + } + y +} + +/// Minimum buffer size to contain either of `0..Tag::MIN` and `Tag::MIN..` +/// at a particular time, for all possible tag wire representations, given +/// the sizes of all wire representations. +/// +/// # Example +/// +/// ```plain +/// OFF = 16 +/// MIN = 24 +/// MAX = 64 +/// +/// BUF = max(MIN, MAX-MIN) +/// = max(24, 64-24) +/// = max(24, 40) +/// = 40 +/// ``` +pub const fn buf_of(xs: &[usize]) -> usize { + max_of(&[min_of(xs), max_of(xs) - min_of(xs)]) +} |