refactor(tvix): introduce nix-compat crate r/5788

Move nixbase32 and store_path into this.

This allows //tvix/cli to not pull in //tvix/store for now.

Change-Id: Id3a32867205d95794bc0d33b21d4cb3d9bafd02a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7964
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

3 files changed, 367 insertions, 0 deletions

diff --git a/tvix/nix-compat/src/lib.rs b/tvix/nix-compat/src/lib.rs
new file mode 100644
index 000000000000..6ca48e9ef035
--- /dev/null
+++ b/tvix/nix-compat/src/lib.rs
@@ -0,0 +1,2 @@
+pub mod nixbase32;
+pub mod store_path;
diff --git a/tvix/nix-compat/src/nixbase32.rs b/tvix/nix-compat/src/nixbase32.rs
new file mode 100644
index 000000000000..39aa4f1d5461
--- /dev/null
+++ b/tvix/nix-compat/src/nixbase32.rs
@@ -0,0 +1,167 @@
+//! Implements the slightly odd "base32" encoding that's used in Nix.
+//!
+//! Nix uses a custom alphabet. Contrary to other implementations (RFC4648),
+//! encoding to "nix base32" doesn't use any padding, and reads in characters
+//! in reverse order.
+//!
+//! This is also the main reason why we can't use `data_encoding::Encoding` -
+//! it gets things wrong if there normally would be a need for padding.
+
+use std::fmt::Write;
+
+use thiserror::Error;
+
+const ALPHABET: &'static [u8; 32] = b"0123456789abcdfghijklmnpqrsvwxyz";
+
+/// Errors that can occur while decoding nixbase32-encoded data.
+#[derive(Debug, Eq, PartialEq, Error)]
+pub enum Nixbase32DecodeError {
+    #[error("character {0:x} not in alphabet")]
+    CharacterNotInAlphabet(u8),
+    #[error("nonzero carry")]
+    NonzeroCarry(),
+}
+
+/// Returns encoded input
+pub fn encode(input: &[u8]) -> String {
+    let output_len = encode_len(input.len());
+    let mut output = String::with_capacity(output_len);
+
+    if output_len > 0 {
+        for n in (0..=output_len - 1).rev() {
+            let b = n * 5; // bit offset within the entire input
+            let i = b / 8; // input byte index
+            let j = b % 8; // bit offset within that input byte
+
+            let mut c = input[i] >> j;
+            if i + 1 < input.len() {
+                // we want to right shift, and discard shifted out bits (unchecked)
+                // To do this without panicing, we need to do the shifting in u16
+                // and convert back to u8 afterwards.
+                c |= ((input[i + 1] as u16) << 8 - j as u16) as u8
+            }
+
+            output
+                .write_char(ALPHABET[(c & 0x1f) as usize] as char)
+                .unwrap();
+        }
+    }
+
+    output
+}
+
+/// This maps a nixbase32-encoded character to its binary representation, which
+/// is also the index of the character in the alphabet.
+fn decode_char(encoded_char: &u8) -> Option<u8> {
+    Some(match encoded_char {
+        b'0'..=b'9' => encoded_char - b'0',
+        b'a'..=b'd' => encoded_char - b'a' + 10_u8,
+        b'f'..=b'n' => encoded_char - b'f' + 14_u8,
+        b'p'..=b's' => encoded_char - b'p' + 23_u8,
+        b'v'..=b'z' => encoded_char - b'v' + 27_u8,
+        _ => return None,
+    })
+}
+
+/// Returns decoded input
+pub fn decode(input: &[u8]) -> Result<Vec<u8>, Nixbase32DecodeError> {
+    let output_len = decode_len(input.len());
+    let mut output: Vec<u8> = vec![0x00; output_len];
+
+    // loop over all characters in reverse, and keep the iteration count in n.
+    for (n, c) in input.iter().rev().enumerate() {
+        match decode_char(c) {
+            None => return Err(Nixbase32DecodeError::CharacterNotInAlphabet(*c)),
+            Some(c_decoded) => {
+                let b = n * 5;
+                let i = b / 8;
+                let j = b % 8;
+
+                let val = (c_decoded as u16).rotate_left(j as u32);
+                output[i] |= (val & 0x00ff) as u8;
+                let carry = ((val & 0xff00) >> 8) as u8;
+
+                // if we're at the end of dst…
+                if i == output_len - 1 {
+                    // but have a nonzero carry, the encoding is invalid.
+                    if carry != 0 {
+                        return Err(Nixbase32DecodeError::NonzeroCarry());
+                    }
+                } else {
+                    output[i + 1] |= carry;
+                }
+            }
+        }
+    }
+
+    Ok(output)
+}
+
+/// Returns the decoded length of an input of length len.
+pub fn decode_len(len: usize) -> usize {
+    return (len * 5) / 8;
+}
+
+/// Returns the encoded length of an input of length len
+pub fn encode_len(len: usize) -> usize {
+    if len == 0 {
+        return 0;
+    }
+    return (len * 8 - 1) / 5 + 1;
+}
+
+#[cfg(test)]
+mod tests {
+    use test_case::test_case;
+
+    #[test_case("", vec![] ; "empty bytes")]
+    #[test_case("0z", vec![0x1f]; "one byte")]
+    #[test_case("00bgd045z0d4icpbc2yyz4gx48ak44la", vec![
+                 0x8a, 0x12, 0x32, 0x15, 0x22, 0xfd, 0x91, 0xef, 0xbd, 0x60, 0xeb, 0xb2, 0x48, 0x1a,
+                 0xf8, 0x85, 0x80, 0xf6, 0x16, 0x00]; "store path")]
+    #[test_case("0c5b8vw40dy178xlpddw65q9gf1h2186jcc3p4swinwggbllv8mk", vec![
+        0xb3, 0xa2, 0x4d, 0xe9, 0x7a, 0x8f, 0xdb, 0xc8, 0x35, 0xb9, 0x83, 0x31, 0x69, 0x50, 0x10, 0x30,
+        0xb8, 0x97, 0x70, 0x31, 0xbc, 0xb5, 0x4b, 0x3b, 0x3a, 0xc1, 0x37, 0x40, 0xf8, 0x46, 0xab, 0x30,
+    ]; "sha256")]
+    fn encode(enc: &str, dec: Vec<u8>) {
+        assert_eq!(enc, super::encode(&dec));
+    }
+
+    #[test_case("", Some(vec![]) ; "empty bytes")]
+    #[test_case("0z", Some(vec![0x1f]); "one byte")]
+    #[test_case("00bgd045z0d4icpbc2yyz4gx48ak44la", Some(vec![
+                 0x8a, 0x12, 0x32, 0x15, 0x22, 0xfd, 0x91, 0xef, 0xbd, 0x60, 0xeb, 0xb2, 0x48, 0x1a,
+                 0xf8, 0x85, 0x80, 0xf6, 0x16, 0x00]); "store path")]
+    #[test_case("0c5b8vw40dy178xlpddw65q9gf1h2186jcc3p4swinwggbllv8mk", Some(vec![
+        0xb3, 0xa2, 0x4d, 0xe9, 0x7a, 0x8f, 0xdb, 0xc8, 0x35, 0xb9, 0x83, 0x31, 0x69, 0x50, 0x10, 0x30,
+        0xb8, 0x97, 0x70, 0x31, 0xbc, 0xb5, 0x4b, 0x3b, 0x3a, 0xc1, 0x37, 0x40, 0xf8, 0x46, 0xab, 0x30,
+    ]); "sha256")]
+    // this is invalid encoding, because it encodes 10 1-bytes, so the carry
+    // would be 2 1-bytes
+    #[test_case("zz", None; "invalid encoding-1")]
+    // this is an even more specific example - it'd decode as 00000000 11
+    #[test_case("c0", None; "invalid encoding-2")]
+
+    fn decode(enc: &str, dec: Option<Vec<u8>>) {
+        match dec {
+            Some(dec) => {
+                // The decode needs to match what's passed in dec
+                assert_eq!(dec, super::decode(enc.as_bytes()).unwrap());
+            }
+            None => {
+                // the decode needs to be an error
+                assert_eq!(true, super::decode(enc.as_bytes()).is_err());
+            }
+        }
+    }
+
+    #[test]
+    fn encode_len() {
+        assert_eq!(super::encode_len(20), 32)
+    }
+
+    #[test]
+    fn decode_len() {
+        assert_eq!(super::decode_len(32), 20)
+    }
+}
diff --git a/tvix/nix-compat/src/store_path.rs b/tvix/nix-compat/src/store_path.rs
new file mode 100644
index 000000000000..5032a73fb19b
--- /dev/null
+++ b/tvix/nix-compat/src/store_path.rs
@@ -0,0 +1,198 @@
+use crate::nixbase32::{self, Nixbase32DecodeError};
+use std::fmt;
+use thiserror::Error;
+
+pub const DIGEST_SIZE: usize = 20;
+// lazy_static doesn't allow us to call NIXBASE32.encode_len(), so we ran it
+// manually and have an assert in the tests.
+pub const ENCODED_DIGEST_SIZE: usize = 32;
+
+// The store dir prefix, without trailing slash.
+// That's usually where the Nix store is mounted at.
+pub const STORE_DIR: &str = "/nix/store";
+pub const STORE_DIR_WITH_SLASH: &str = "/nix/store/";
+
+/// Errors that can occur during the validation of name characters.
+#[derive(Debug, PartialEq, Eq, Error)]
+pub enum ParseStorePathError {
+    #[error("Dash is missing between hash and name")]
+    MissingDash(),
+    #[error("Hash encoding is invalid: {0}")]
+    InvalidHashEncoding(Nixbase32DecodeError),
+    #[error("Invalid name: {0}")]
+    InvalidName(String),
+    #[error("Tried to parse an absolute path which was missing the store dir prefix.")]
+    MissingStoreDir(),
+}
+
+/// Represents a path in the Nix store (a direct child of [STORE_DIR]).
+///
+/// It starts with a digest (20 bytes), [struct@NIXBASE32]-encoded, followed by
+/// a `-`, and ends with a `name`, which is a string, consisting only of ASCCI
+/// alphanumeric characters, or one of the following characters: `-`, `_`, `.`,
+/// `+`, `?`, `=`.
+///
+/// The name is usually used to describe the pname and version of a package.
+/// Derivations paths can also be represented as store paths, they end
+/// with .drv.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct StorePath {
+    pub digest: [u8; DIGEST_SIZE],
+    pub name: String,
+}
+
+impl StorePath {
+    pub fn from_string(s: &str) -> Result<StorePath, ParseStorePathError> {
+        // the whole string needs to be at least:
+        //
+        // - 32 characters (encoded hash)
+        // - 1 dash
+        // - 1 character for the name
+        if s.len() < ENCODED_DIGEST_SIZE + 2 {
+            return Err(ParseStorePathError::InvalidName("".to_string()));
+        }
+
+        let digest = match nixbase32::decode(s[..ENCODED_DIGEST_SIZE].as_bytes()) {
+            Ok(decoded) => decoded,
+            Err(decoder_error) => {
+                return Err(ParseStorePathError::InvalidHashEncoding(decoder_error))
+            }
+        };
+
+        if s.as_bytes()[ENCODED_DIGEST_SIZE] != b'-' {
+            return Err(ParseStorePathError::MissingDash());
+        }
+
+        StorePath::validate_name(&s[ENCODED_DIGEST_SIZE + 2..])?;
+
+        Ok(StorePath {
+            name: s[ENCODED_DIGEST_SIZE + 1..].to_string(),
+            digest: digest.try_into().expect("size is known"),
+        })
+    }
+
+    /// Construct a [StorePath] from an absolute store path string.
+    /// That is a string starting with the store prefix (/nix/store)
+    pub fn from_absolute_path(s: &str) -> Result<StorePath, ParseStorePathError> {
+        match s.strip_prefix(STORE_DIR_WITH_SLASH) {
+            Some(s_stripped) => Self::from_string(s_stripped),
+            None => Err(ParseStorePathError::MissingStoreDir()),
+        }
+    }
+
+    /// Converts the [StorePath] to an absolute store path string.
+    /// That is a string starting with the store prefix (/nix/store)
+    pub fn to_absolute_path(&self) -> String {
+        format!("{}/{}", STORE_DIR, self)
+    }
+
+    /// Checks a given &str to match the restrictions for store path names.
+    pub fn validate_name(s: &str) -> Result<(), ParseStorePathError> {
+        for c in s.chars() {
+            if c.is_ascii_alphanumeric()
+                || c == '-'
+                || c == '_'
+                || c == '.'
+                || c == '+'
+                || c == '?'
+                || c == '='
+            {
+                continue;
+            }
+
+            return Err(ParseStorePathError::InvalidName(s.to_string()));
+        }
+
+        Ok(())
+    }
+}
+
+impl fmt::Display for StorePath {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}-{}", nixbase32::encode(&self.digest), self.name)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::nixbase32;
+    use crate::store_path::{DIGEST_SIZE, ENCODED_DIGEST_SIZE};
+
+    use super::{ParseStorePathError, StorePath};
+
+    #[test]
+    fn encoded_digest_size() {
+        assert_eq!(ENCODED_DIGEST_SIZE, nixbase32::encode_len(DIGEST_SIZE));
+    }
+
+    #[test]
+    fn happy_path() {
+        let example_nix_path_str =
+            "00bgd045z0d4icpbc2yyz4gx48ak44la-net-tools-1.60_p20170221182432";
+        let nixpath =
+            StorePath::from_string(&example_nix_path_str).expect("Error parsing example string");
+
+        let expected_digest: [u8; DIGEST_SIZE] = [
+            0x8a, 0x12, 0x32, 0x15, 0x22, 0xfd, 0x91, 0xef, 0xbd, 0x60, 0xeb, 0xb2, 0x48, 0x1a,
+            0xf8, 0x85, 0x80, 0xf6, 0x16, 0x00,
+        ];
+
+        assert_eq!("net-tools-1.60_p20170221182432", nixpath.name);
+        assert_eq!(nixpath.digest, expected_digest);
+
+        assert_eq!(example_nix_path_str, nixpath.to_string())
+    }
+
+    #[test]
+    fn invalid_hash_length() {
+        StorePath::from_string("00bgd045z0d4icpbc2yy-net-tools-1.60_p20170221182432")
+            .expect_err("No error raised.");
+    }
+
+    #[test]
+    fn invalid_encoding_hash() {
+        StorePath::from_string("00bgd045z0d4icpbc2yyz4gx48aku4la-net-tools-1.60_p20170221182432")
+            .expect_err("No error raised.");
+    }
+
+    #[test]
+    fn more_than_just_the_bare_nix_store_path() {
+        StorePath::from_string(
+            "00bgd045z0d4icpbc2yyz4gx48aku4la-net-tools-1.60_p20170221182432/bin/arp",
+        )
+        .expect_err("No error raised.");
+    }
+
+    #[test]
+    fn no_dash_between_hash_and_name() {
+        StorePath::from_string("00bgd045z0d4icpbc2yyz4gx48ak44lanet-tools-1.60_p20170221182432")
+            .expect_err("No error raised.");
+    }
+
+    #[test]
+    fn absolute_path() {
+        let example_nix_path_str =
+            "00bgd045z0d4icpbc2yyz4gx48ak44la-net-tools-1.60_p20170221182432";
+        let nixpath_expected = StorePath::from_string(&example_nix_path_str).expect("must parse");
+
+        let nixpath_actual = StorePath::from_absolute_path(
+            "/nix/store/00bgd045z0d4icpbc2yyz4gx48ak44la-net-tools-1.60_p20170221182432",
+        )
+        .expect("must parse");
+
+        assert_eq!(nixpath_expected, nixpath_actual);
+
+        assert_eq!(
+            "/nix/store/00bgd045z0d4icpbc2yyz4gx48ak44la-net-tools-1.60_p20170221182432",
+            nixpath_actual.to_absolute_path(),
+        );
+    }
+
+    #[test]
+    fn absolute_path_missing_prefix() {
+        assert_eq!(
+            ParseStorePathError::MissingStoreDir(),
+            StorePath::from_absolute_path("foobar-123").expect_err("must fail")
+        );
+    }
+}