1 files changed, 118 insertions, 0 deletions
diff --git a/tvix/nix-compat/src/aterm/parser.rs b/tvix/nix-compat/src/aterm/parser.rs
new file mode 100644
index 000000000000..72648d5ef590
--- /dev/null
+++ b/tvix/nix-compat/src/aterm/parser.rs
@@ -0,0 +1,118 @@
+//! This module implements parsing code for some basic building blocks
+//! of the [ATerm][] format, which is used by C++ Nix to serialize Derivations.
+//!
+//! [ATerm]: http://program-transformation.org/Tools/ATermFormat.html
+use bstr::BString;
+use nom::branch::alt;
+use nom::bytes::complete::{escaped_transform, is_not, tag};
+use nom::character::complete::char as nomchar;
+use nom::combinator::{map, value};
+use nom::multi::separated_list0;
+use nom::sequence::delimited;
+use nom::IResult;
+
+/// Parse a bstr and undo any escaping.
+fn parse_escaped_bstr(i: &[u8]) -> IResult<&[u8], BString> {
+    escaped_transform(
+        is_not("\"\\"),
+        '\\',
+        alt((
+            value("\\".as_bytes(), nomchar('\\')),
+            value("\n".as_bytes(), nomchar('n')),
+            value("\t".as_bytes(), nomchar('t')),
+            value("\r".as_bytes(), nomchar('r')),
+            value("\"".as_bytes(), nomchar('\"')),
+        )),
+    )(i)
+    .map(|(i, v)| (i, BString::new(v)))
+}
+
+/// Parse a field in double quotes, undo any escaping, and return the unquoted
+/// and decoded `Vec<u8>`.
+pub(crate) fn parse_bstr_field(i: &[u8]) -> IResult<&[u8], BString> {
+    // inside double quotes…
+    delimited(
+        nomchar('\"'),
+        // There is
+        alt((
+            // …either is a bstr after unescaping
+            parse_escaped_bstr,
+            // …or an empty string.
+            map(tag(b""), |_| BString::default()),
+        )),
+        nomchar('\"'),
+    )(i)
+}
+
+/// Parse a field in double quotes, undo any escaping, and return the unquoted
+/// and decoded string, if it's a valid string. Or fail parsing if the bytes are
+/// no valid UTF-8.
+pub(crate) fn parse_string_field(i: &[u8]) -> IResult<&[u8], String> {
+    // inside double quotes…
+    delimited(
+        nomchar('\"'),
+        // There is
+        alt((
+            // either is a String after unescaping
+            nom::combinator::map_opt(parse_escaped_bstr, |escaped_bstr| {
+                String::from_utf8(escaped_bstr.into()).ok()
+            }),
+            // or an empty string.
+            map(tag(b""), |_| String::new()),
+        )),
+        nomchar('\"'),
+    )(i)
+}
+
+/// Parse a list of of string fields (enclosed in brackets)
+pub(crate) fn parse_str_list(i: &[u8]) -> IResult<&[u8], Vec<String>> {
+    // inside brackets
+    delimited(
+        nomchar('['),
+        separated_list0(nomchar(','), parse_string_field),
+        nomchar(']'),
+    )(i)
+}
+
+#[cfg(test)]
+mod tests {
+    use test_case::test_case;
+
+    #[test_case(br#""""#, b"", b""; "empty")]
+    #[test_case(br#""Hello World""#, b"Hello World", b""; "hello world")]
+    #[test_case(br#""\"""#, br#"""#, b""; "doublequote")]
+    #[test_case(br#"":""#, b":", b""; "colon")]
+    #[test_case(br#""\""Rest"#, br#"""#, b"Rest"; "doublequote rest")]
+    fn parse_bstr_field(input: &[u8], expected: &[u8], exp_rest: &[u8]) {
+        let (rest, parsed) = super::parse_bstr_field(input).expect("must parse");
+        assert_eq!(exp_rest, rest, "expected remainder");
+        assert_eq!(expected, parsed);
+    }
+
+    #[test_case(br#""""#, "", b""; "empty")]
+    #[test_case(br#""Hello World""#, "Hello World", b""; "hello world")]
+    #[test_case(br#""\"""#, r#"""#, b""; "doublequote")]
+    #[test_case(br#"":""#, ":", b""; "colon")]
+    #[test_case(br#""\""Rest"#, r#"""#, b"Rest"; "doublequote rest")]
+    fn parse_string_field(input: &[u8], expected: &str, exp_rest: &[u8]) {
+        let (rest, parsed) = super::parse_string_field(input).expect("must parse");
+        assert_eq!(exp_rest, rest, "expected remainder");
+        assert_eq!(expected, &parsed);
+    }
+
+    #[test]
+    fn parse_string_field_invalid_encoding_fail() {
+        let input: Vec<_> = vec![b'"', 0xc5, 0xc4, 0xd6, b'"'];
+
+        super::parse_string_field(&input).expect_err("must fail");
+    }
+
+    #[test_case(br#"["foo"]"#, vec!["foo".to_string()], b""; "single foo")]
+    #[test_case(b"[]", vec![], b""; "empty list")]
+    #[test_case(b"[]blub", vec![], b"blub"; "empty list with rest")]
+    fn parse_list(input: &[u8], expected: Vec<String>, exp_rest: &[u8]) {
+        let (rest, parsed) = super::parse_str_list(input).expect("must parse");
+        assert_eq!(exp_rest, rest, "expected remainder");
+        assert_eq!(expected, parsed);
+    }
+}