feat(nix-compat/{aterm,derivation}): init parser r/6831

This provides a nom-based parser for Nix derivations in ATerm format,
which can be reached via `Derivation::from_aterm_bytes`.

Some of the lower-level ATerm primitives are moved into a (new) aterm
module, and some more higher-level ones that construct derivation-
specific types.

Also, move the escape_bytes function into there, this is a generic ATerm
thing.

Change-Id: I2b03b8a1461c7ea2fcb8640c2fc3d1fa3ea719fb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9730
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

11 files changed, 612 insertions, 3 deletions

diff --git a/tvix/nix-compat/src/derivation/escape.rs b/tvix/nix-compat/src/aterm/escape.rs
index 06b550bbf0..06b550bbf0 100644
--- a/tvix/nix-compat/src/derivation/escape.rs
+++ b/tvix/nix-compat/src/aterm/escape.rs
diff --git a/tvix/nix-compat/src/aterm/mod.rs b/tvix/nix-compat/src/aterm/mod.rs
new file mode 100644
index 0000000000..8806b6caf2
--- /dev/null
+++ b/tvix/nix-compat/src/aterm/mod.rs
@@ -0,0 +1,7 @@
+mod escape;
+mod parser;
+
+pub(crate) use escape::escape_bytes;
+pub(crate) use parser::parse_bstr_field;
+pub(crate) use parser::parse_str_list;
+pub(crate) use parser::parse_string_field;
diff --git a/tvix/nix-compat/src/aterm/parser.rs b/tvix/nix-compat/src/aterm/parser.rs
new file mode 100644
index 0000000000..883eeb60b9
--- /dev/null
+++ b/tvix/nix-compat/src/aterm/parser.rs
@@ -0,0 +1,123 @@
+//! 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 mut input: Vec<u8> = Vec::new();
+        input.push(b'"');
+        input.push(0xc5);
+        input.push(0xc4);
+        input.push(0xd6);
+        input.push(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);
+    }
+}
diff --git a/tvix/nix-compat/src/derivation/mod.rs b/tvix/nix-compat/src/derivation/mod.rs
index a06f8eb160..d7f894c89a 100644
--- a/tvix/nix-compat/src/derivation/mod.rs
+++ b/tvix/nix-compat/src/derivation/mod.rs
@@ -8,8 +8,9 @@ use std::collections::{BTreeMap, BTreeSet};
 use std::io;
 
 mod errors;
-mod escape;
 mod output;
+mod parse_error;
+mod parser;
 mod validate;
 mod write;
 
@@ -89,6 +90,12 @@ impl Derivation {
         buffer
     }
 
+    /// Parse an Derivation in ATerm serialization, and validate it passes our
+    /// set of validations.
+    pub fn from_aterm_bytes(b: &[u8]) -> Result<Derivation, parser::Error<&[u8]>> {
+        parser::parse(b)
+    }
+
     /// Returns the drv path of a [Derivation] struct.
     ///
     /// The drv path is calculated by invoking [build_text_path], using
diff --git a/tvix/nix-compat/src/derivation/parse_error.rs b/tvix/nix-compat/src/derivation/parse_error.rs
new file mode 100644
index 0000000000..a064d4faba
--- /dev/null
+++ b/tvix/nix-compat/src/derivation/parse_error.rs
@@ -0,0 +1,73 @@
+//! This contains error and result types that can happen while parsing
+//! Derivations from ATerm.
+use nom::IResult;
+
+use crate::nixhash;
+
+pub type NomResult<I, O> = IResult<I, O, NomError<I>>;
+
+#[derive(Debug, PartialEq)]
+pub enum ErrorKind {
+    // duplicate key in map
+    DuplicateMapKey(String),
+
+    // Digest parsing error
+    NixHashError(nixhash::Error),
+
+    // error kind wrapped from native nom errors
+    Nom(nom::error::ErrorKind),
+}
+
+/// Our own error type to pass along parser-related errors.
+#[derive(Debug, PartialEq)]
+pub struct NomError<I> {
+    /// position of the error in the input data
+    pub input: I,
+    /// error code
+    pub code: ErrorKind,
+}
+
+impl<I, E> nom::error::FromExternalError<I, E> for NomError<I> {
+    fn from_external_error(input: I, kind: nom::error::ErrorKind, _e: E) -> Self {
+        Self {
+            input,
+            code: ErrorKind::Nom(kind),
+        }
+    }
+}
+
+impl<I> nom::error::ParseError<I> for NomError<I> {
+    fn from_error_kind(input: I, kind: nom::error::ErrorKind) -> Self {
+        Self {
+            input,
+            code: ErrorKind::Nom(kind),
+        }
+    }
+
+    // FUTUREWORK: implement, so we have support for backtracking through the
+    // parse tree?
+    fn append(_input: I, _kind: nom::error::ErrorKind, other: Self) -> Self {
+        other
+    }
+}
+
+/// This wraps a [nom::error::Error] into our error.
+impl<I> From<nom::error::Error<I>> for NomError<I> {
+    fn from(value: nom::error::Error<I>) -> Self {
+        Self {
+            input: value.input,
+            code: ErrorKind::Nom(value.code),
+        }
+    }
+}
+
+/// This essentially implements
+/// From<nom::Err<nom::error::Error<I>>> for nom::Err<NomError<I>>,
+/// which we can't because nom::Err<_> is a foreign type.
+pub(crate) fn into_nomerror<I>(e: nom::Err<nom::error::Error<I>>) -> nom::Err<NomError<I>> {
+    match e {
+        nom::Err::Incomplete(n) => nom::Err::Incomplete(n),
+        nom::Err::Error(e) => nom::Err::Error(e.into()),
+        nom::Err::Failure(e) => nom::Err::Failure(e.into()),
+    }
+}
diff --git a/tvix/nix-compat/src/derivation/parser.rs b/tvix/nix-compat/src/derivation/parser.rs
new file mode 100644
index 0000000000..dfcd327e4f
--- /dev/null
+++ b/tvix/nix-compat/src/derivation/parser.rs
@@ -0,0 +1,327 @@
+//! This module constructs a [Derivation] by parsing its [ATerm][]
+//! serialization.
+//!
+//! [ATerm]: http://program-transformation.org/Tools/ATermFormat.html
+
+use bstr::BString;
+use nom::bytes::complete::tag;
+use nom::character::complete::char as nomchar;
+use nom::combinator::{all_consuming, map_res};
+use nom::multi::{separated_list0, separated_list1};
+use nom::sequence::{delimited, preceded, separated_pair, terminated, tuple};
+use std::collections::{BTreeMap, BTreeSet};
+use thiserror;
+
+use super::parse_error::{into_nomerror, ErrorKind, NomError, NomResult};
+use super::{write, Derivation, NixHashWithMode, Output};
+use crate::{aterm, nixhash};
+
+#[derive(Debug, thiserror::Error)]
+pub enum Error<I> {
+    #[error("parsing error: {0}")]
+    ParseError(NomError<I>),
+    #[error("premature EOF")]
+    Incomplete,
+    #[error("validation error: {0}")]
+    ValidationError(super::DerivationError),
+}
+
+pub(crate) fn parse(i: &[u8]) -> Result<Derivation, Error<&[u8]>> {
+    match all_consuming(parse_derivation)(i) {
+        Ok((rest, derivation)) => {
+            // this shouldn't happen, as all_consuming shouldn't return.
+            debug_assert!(rest.is_empty());
+
+            // invoke validate
+            derivation.validate(true).map_err(Error::ValidationError)?;
+
+            Ok(derivation)
+        }
+        Err(nom::Err::Incomplete(_)) => Err(Error::Incomplete),
+        Err(nom::Err::Error(e) | nom::Err::Failure(e)) => Err(Error::ParseError(e)),
+    }
+}
+
+/// Parse one output in ATerm. This is 4 string fields inside parans:
+/// output name, output path, algo (and mode), digest.
+/// Returns the output name and [Output] struct.
+fn parse_output(i: &[u8]) -> NomResult<&[u8], (String, Output)> {
+    delimited(
+        nomchar('('),
+        map_res(
+            |i| {
+                tuple((
+                    terminated(aterm::parse_string_field, nomchar(',')),
+                    terminated(aterm::parse_string_field, nomchar(',')),
+                    terminated(aterm::parse_string_field, nomchar(',')),
+                    aterm::parse_bstr_field,
+                ))(i)
+                .map_err(into_nomerror)
+            },
+            |(output_name, output_path, algo_and_mode, encoded_digest)| {
+                // convert these 4 fields into an [Output].
+                let hash_with_mode_res = {
+                    if algo_and_mode.is_empty() && encoded_digest.is_empty() {
+                        None
+                    } else {
+                        match data_encoding::HEXLOWER.decode(&encoded_digest) {
+                            Ok(digest) => Some(NixHashWithMode::from_algo_mode_hash(
+                                &algo_and_mode,
+                                &digest,
+                            )),
+                            Err(e) => Some(Err(nixhash::Error::InvalidBase64Encoding(e))),
+                        }
+                    }
+                }
+                .transpose();
+
+                match hash_with_mode_res {
+                    Ok(hash_with_mode) => Ok((
+                        output_name,
+                        Output {
+                            path: output_path,
+                            hash_with_mode,
+                        },
+                    )),
+                    Err(e) => Err(nom::Err::Failure(NomError {
+                        input: i,
+                        code: ErrorKind::NixHashError(e),
+                    })),
+                }
+            },
+        ),
+        nomchar(')'),
+    )(i)
+}
+
+/// Parse multiple outputs in ATerm. This is a list of things acccepted by
+/// parse_output, and takes care of turning the (String, Output) returned from
+/// it to a BTreeMap.
+/// We don't use parse_kv here, as it's dealing with 2-tuples, and these are
+/// 4-tuples.
+fn parse_outputs(i: &[u8]) -> NomResult<&[u8], BTreeMap<String, Output>> {
+    let res = delimited(
+        nomchar('['),
+        separated_list1(tag(","), parse_output),
+        nomchar(']'),
+    )(i);
+
+    match res {
+        Ok((rst, outputs_lst)) => {
+            let mut outputs: BTreeMap<String, Output> = BTreeMap::default();
+            for (output_name, output) in outputs_lst.into_iter() {
+                if outputs.contains_key(&output_name) {
+                    return Err(nom::Err::Failure(NomError {
+                        input: i,
+                        code: ErrorKind::DuplicateMapKey(output_name),
+                    }));
+                }
+                outputs.insert(output_name, output);
+            }
+            Ok((rst, outputs))
+        }
+        // pass regular parse errors along
+        Err(e) => Err(e),
+    }
+}
+
+fn parse_input_derivations(i: &[u8]) -> NomResult<&[u8], BTreeMap<String, Vec<String>>> {
+    parse_kv::<Vec<String>, _>(aterm::parse_str_list)(i)
+}
+
+pub fn parse_derivation(i: &[u8]) -> NomResult<&[u8], Derivation> {
+    use nom::Parser;
+    preceded(
+        tag(write::DERIVATION_PREFIX),
+        delimited(
+            // inside parens
+            nomchar('('),
+            // tuple requires all errors to be of the same type, so we need to be a
+            // bit verbose here wrapping generic IResult into [NomATermResult].
+            tuple((
+                // parse outputs
+                terminated(parse_outputs, nomchar(',')),
+                // // parse input derivations
+                terminated(parse_input_derivations, nomchar(',')),
+                // // parse input sources
+                |i| terminated(aterm::parse_str_list, nomchar(','))(i).map_err(into_nomerror),
+                // // parse system
+                |i| terminated(aterm::parse_string_field, nomchar(','))(i).map_err(into_nomerror),
+                // // parse builder
+                |i| terminated(aterm::parse_string_field, nomchar(','))(i).map_err(into_nomerror),
+                // // parse arguments
+                |i| terminated(aterm::parse_str_list, nomchar(','))(i).map_err(into_nomerror),
+                // parse environment
+                parse_kv::<BString, _>(aterm::parse_bstr_field),
+            )),
+            nomchar(')'),
+        )
+        .map(
+            |(
+                outputs,
+                input_derivations,
+                input_sources,
+                system,
+                builder,
+                arguments,
+                environment,
+            )| {
+                // All values in input_derivations need to be converted from
+                // Vec<String> to BTreeSet<String>
+                let mut input_derivations_new: BTreeMap<_, BTreeSet<_>> = BTreeMap::new();
+                for (k, v) in input_derivations.into_iter() {
+                    let values_new: BTreeSet<_> = BTreeSet::from_iter(v.into_iter());
+                    input_derivations_new.insert(k, values_new);
+                    // TODO: actually check they're not duplicate in the parser side!
+                }
+
+                // Input sources need to be converted from Vec<_> to BTreeSet<_>
+                let input_sources_new: BTreeSet<_> = BTreeSet::from_iter(input_sources);
+
+                Derivation {
+                    arguments,
+                    builder,
+                    environment,
+                    input_derivations: input_derivations_new,
+                    input_sources: input_sources_new,
+                    outputs,
+                    system,
+                }
+            },
+        ),
+    )(i)
+}
+
+/// Parse a list of key/value pairs into a BTreeMap.
+/// The parser for the values can be passed in.
+/// In terms of ATerm, this is just a 2-tuple,
+/// but we have the additional restriction that the first element needs to be
+/// unique across all tuples.
+pub(crate) fn parse_kv<'a, V, VF>(
+    vf: VF,
+) -> impl FnMut(&'a [u8]) -> NomResult<&'a [u8], BTreeMap<String, V>> + 'static
+where
+    VF: FnMut(&'a [u8]) -> nom::IResult<&'a [u8], V, nom::error::Error<&'a [u8]>> + Clone + 'static,
+{
+    move |i|
+    // inside brackets
+    delimited(
+        nomchar('['),
+        |ii| {
+            let res = separated_list0(
+                nomchar(','),
+                // inside parens
+                delimited(
+                    nomchar('('),
+                    separated_pair(
+                        aterm::parse_string_field,
+                        nomchar(','),
+                        vf.clone(),
+                    ),
+                    nomchar(')'),
+                ),
+            )(ii).map_err(into_nomerror);
+
+            match res {
+                Ok((rest, pairs)) => {
+                    let mut kvs: BTreeMap<String, V> = BTreeMap::new();
+                    for (k, v) in pairs.into_iter() {
+                        // collect the 2-tuple to a BTreeMap,
+                        // and fail if the key was already seen before.
+                        if kvs.insert(k.clone(), v).is_some() {
+                            return Err(nom::Err::Failure(NomError {
+                                input: i,
+                                code: ErrorKind::DuplicateMapKey(k),
+                            }));
+                        }
+                    }
+                    Ok((rest, kvs))
+                }
+                Err(e) => Err(e),
+            }
+        },
+        nomchar(']'),
+    )(i)
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::BTreeMap;
+
+    use crate::derivation::Output;
+    use bstr::{BString, ByteSlice};
+    use lazy_static::lazy_static;
+    use test_case::test_case;
+
+    lazy_static! {
+        static ref EXP_MULTI_OUTPUTS: BTreeMap<String, Output> = {
+            let mut b = BTreeMap::new();
+            b.insert(
+                "lib".to_string(),
+                Output {
+                    path: "/nix/store/2vixb94v0hy2xc6p7mbnxxcyc095yyia-has-multi-out-lib"
+                        .to_string(),
+                    hash_with_mode: None,
+                },
+            );
+            b.insert(
+                "out".to_string(),
+                Output {
+                    path: "/nix/store/55lwldka5nyxa08wnvlizyqw02ihy8ic-has-multi-out".to_string(),
+                    hash_with_mode: None,
+                },
+            );
+            b
+        };
+        static ref EXP_AB_MAP: BTreeMap<String, BString> = {
+            let mut b = BTreeMap::new();
+            b.insert("a".to_string(), b"1".as_bstr().to_owned());
+            b.insert("b".to_string(), b"2".as_bstr().to_owned());
+            b
+        };
+    }
+
+    #[test_case(b"[(\"a\",\"1\"),(\"b\",\"2\")]", &EXP_AB_MAP, b""; "simple")]
+    fn parse_kv(input: &'static [u8], expected: &BTreeMap<String, BString>, exp_rest: &[u8]) {
+        let (rest, parsed) = super::parse_kv::<BString, _>(crate::aterm::parse_bstr_field)(input)
+            .expect("must parse");
+        assert_eq!(exp_rest, rest, "expected remainder");
+        assert_eq!(*expected, parsed);
+    }
+
+    #[test_case(
+        br#"("out","/nix/store/5vyvcwah9l9kf07d52rcgdk70g2f4y13-foo","","")"#,
+        ("out".to_string(), Output {
+            path: "/nix/store/5vyvcwah9l9kf07d52rcgdk70g2f4y13-foo".to_string(),
+            hash_with_mode: None
+        }); "simple"
+    )]
+    #[test_case(
+        br#"("out","/nix/store/4q0pg5zpfmznxscq3avycvf9xdvx50n3-bar","r:sha256","08813cbee9903c62be4c5027726a418a300da4500b2d369d3af9286f4815ceba")"#,
+        ("out".to_string(), Output {
+            path: "/nix/store/4q0pg5zpfmznxscq3avycvf9xdvx50n3-bar".to_string(),
+            hash_with_mode: Some(crate::derivation::NixHashWithMode::Recursive(
+                crate::nixhash::from_algo_and_digest (
+                   crate::nixhash::HashAlgo::Sha256,
+                   &data_encoding::HEXLOWER.decode(b"08813cbee9903c62be4c5027726a418a300da4500b2d369d3af9286f4815ceba").unwrap()
+                ).unwrap()
+            )),
+        }); "fod"
+     )]
+    fn parse_output(input: &[u8], expected: (String, Output)) {
+        let (rest, parsed) = super::parse_output(input).expect("must parse");
+        assert!(rest.is_empty());
+        assert_eq!(expected, parsed);
+    }
+
+    #[test_case(
+        br#"[("lib","/nix/store/2vixb94v0hy2xc6p7mbnxxcyc095yyia-has-multi-out-lib","",""),("out","/nix/store/55lwldka5nyxa08wnvlizyqw02ihy8ic-has-multi-out","","")]"#,
+        &EXP_MULTI_OUTPUTS;
+        "multi-out"
+    )]
+    fn parse_outputs(input: &[u8], expected: &BTreeMap<String, Output>) {
+        let (rest, parsed) = super::parse_outputs(input).expect("must parse");
+        assert!(rest.is_empty());
+        assert_eq!(*expected, parsed);
+    }
+}
diff --git a/tvix/nix-compat/src/derivation/tests/derivation_tests/duplicate.drv b/tvix/nix-compat/src/derivation/tests/derivation_tests/duplicate.drv
new file mode 100644
index 0000000000..072561a29e
--- /dev/null
+++ b/tvix/nix-compat/src/derivation/tests/derivation_tests/duplicate.drv
@@ -0,0 +1 @@
+Derive([("out","/nix/store/5vyvcwah9l9kf07d52rcgdk70g2f4y13-foo","","")],[("/nix/store/0hm2f1psjpcwg8fijsmr4wwxrx59s092-bar.drv",["out"])],[],":",":",[],[("bar","/nix/store/4q0pg5zpfmznxscq3avycvf9xdvx50n3-bar"),("builder",":"),("name","foo"),("name","bar"),("out","/nix/store/5vyvcwah9l9kf07d52rcgdk70g2f4y13-foo"),("system",":")])
\ No newline at end of file
diff --git a/tvix/nix-compat/src/derivation/tests/mod.rs b/tvix/nix-compat/src/derivation/tests/mod.rs
index 2ae80b620d..9f0e548471 100644
--- a/tvix/nix-compat/src/derivation/tests/mod.rs
+++ b/tvix/nix-compat/src/derivation/tests/mod.rs
@@ -1,4 +1,7 @@
+use super::parse_error::ErrorKind;
 use crate::derivation::output::Output;
+use crate::derivation::parse_error::NomError;
+use crate::derivation::parser::Error::ParseError;
 use crate::derivation::Derivation;
 use crate::store_path::StorePath;
 use bstr::{BStr, BString};
@@ -70,6 +73,73 @@ fn check_to_aterm_bytes(path_to_drv_file: &str) {
     assert_eq!(expected, BStr::new(&derivation.to_aterm_bytes()));
 }
 
+/// Reads in derivations in ATerm representation, parses with that parser,
+/// then compares the structs with the ones obtained by parsing the JSON
+/// representations.
+#[test_resources("src/derivation/tests/derivation_tests/ok/*.drv")]
+fn from_aterm_bytes(path_to_drv_file: &str) {
+    // Read in ATerm representation.
+    let aterm_bytes = read_file(path_to_drv_file);
+    let parsed_drv = Derivation::from_aterm_bytes(&aterm_bytes).expect("must succeed");
+
+    // For where we're able to load JSON fixtures, parse them and compare the structs.
+    // For where we're not, compare the bytes manually.
+    if path_to_drv_file.ends_with("cp1252.drv") || path_to_drv_file.ends_with("latin1.drv") {
+        assert_eq!(
+            &[0xc5, 0xc4, 0xd6][..],
+            parsed_drv.environment.get("chars").unwrap(),
+            "expected bytes to match",
+        );
+    } else {
+        let json_bytes = read_file(&format!("{}.json", path_to_drv_file));
+        let fixture_derivation: Derivation =
+            serde_json::from_slice(&json_bytes).expect("JSON was not well-formatted");
+
+        assert_eq!(fixture_derivation, parsed_drv);
+    }
+
+    // Finally, write the ATerm serialization to another buffer, ensuring it's
+    // stable (and we compare all fields we couldn't compare in the non-utf8
+    // derivations)
+
+    assert_eq!(
+        &aterm_bytes,
+        &parsed_drv.to_aterm_bytes(),
+        "expected serialized ATerm to match initial input"
+    );
+}
+
+#[test]
+fn from_aterm_bytes_duplicate_map_key() {
+    let buf: Vec<u8> = read_file(&format!("{}/{}", RESOURCES_PATHS, "duplicate.drv")).into();
+
+    let err = Derivation::from_aterm_bytes(&buf).expect_err("must fail");
+
+    match err {
+        ParseError(NomError { input: _, code }) => {
+            assert_eq!(code, ErrorKind::DuplicateMapKey("name".to_string()));
+        }
+        _ => {
+            panic!("unexpected error");
+        }
+    }
+}
+
+/// Read in a derivation in ATerm, but add some garbage at the end.
+/// Ensure the parser detects and fails in this case.
+#[test]
+fn from_aterm_bytes_trailer() {
+    let mut buf: Vec<u8> = read_file(&format!(
+        "{}/ok/{}",
+        RESOURCES_PATHS, "0hm2f1psjpcwg8fijsmr4wwxrx59s092-bar.drv"
+    ))
+    .into();
+
+    buf.push(0x00);
+
+    Derivation::from_aterm_bytes(&buf).expect_err("must fail");
+}
+
 #[test_case("bar","0hm2f1psjpcwg8fijsmr4wwxrx59s092-bar.drv"; "fixed_sha256")]
 #[test_case("foo", "4wvvbi4jwn0prsdxb7vs673qa5h9gr7x-foo.drv"; "simple-sha256")]
 #[test_case("bar", "ss2p4wmxijn652haqyd7dckxwl4c7hxx-bar.drv"; "fixed-sha1")]
diff --git a/tvix/nix-compat/src/derivation/write.rs b/tvix/nix-compat/src/derivation/write.rs
index 0655cb8cde..7ebbbffa4b 100644
--- a/tvix/nix-compat/src/derivation/write.rs
+++ b/tvix/nix-compat/src/derivation/write.rs
@@ -3,7 +3,7 @@
 //!
 //! [ATerm]: http://program-transformation.org/Tools/ATermFormat.html
 
-use crate::derivation::escape::escape_bytes;
+use crate::aterm::escape_bytes;
 use crate::derivation::output::Output;
 use bstr::BString;
 use std::{
diff --git a/tvix/nix-compat/src/lib.rs b/tvix/nix-compat/src/lib.rs
index 37d79f38fb..6ec60f3cc8 100644
--- a/tvix/nix-compat/src/lib.rs
+++ b/tvix/nix-compat/src/lib.rs
@@ -1,3 +1,4 @@
+pub(crate) mod aterm;
 pub mod derivation;
 pub mod nar;
 pub mod nixbase32;
diff --git a/tvix/nix-compat/src/nixhash/mod.rs b/tvix/nix-compat/src/nixhash/mod.rs
index 9586473d1b..5b12f466e3 100644
--- a/tvix/nix-compat/src/nixhash/mod.rs
+++ b/tvix/nix-compat/src/nixhash/mod.rs
@@ -81,7 +81,7 @@ pub fn from_algo_and_digest(algo: HashAlgo, digest: &[u8]) -> Result<NixHash> {
 }
 
 /// Errors related to NixHash construction.
-#[derive(Debug, thiserror::Error)]
+#[derive(Debug, Eq, PartialEq, thiserror::Error)]
 pub enum Error {
     #[error("invalid hash algo: {0}")]
     InvalidAlgo(String),