refactor(tvix/nix-compat): rename NixHashWithMode -> CAHash r/6871

This specific struct is only used to represent content-addressed paths
(in case a Derivation has a fixed-output hash, for example).
Rename `Output`'s `hash_with_mode` to `ca_hash`.

We now also include `CAHash::Text`, and update the `validate` function
of the `Output` struct to reject text hashes there.

This allows cleaning up the various output path calculation functions
inside nix-compat/src/store_path/utils.rs, as they can now match on
the type.

`make_type` is renamed to `make_references_string`,
`build_regular_ca_path` is renamed to `build_ca_path`, and
`build_text_path` has a disclaimer added, because you might not actually
want to use it.

Change-Id: I674d065f2ed5c804012ddfed56e161ac49d23931
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9814
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

10 files changed, 222 insertions, 209 deletions

diff --git a/tvix/cli/src/derivation.rs b/tvix/cli/src/derivation.rs
index 76cc3f60f77e..15df6ed76102 100644
--- a/tvix/cli/src/derivation.rs
+++ b/tvix/cli/src/derivation.rs
@@ -115,7 +115,7 @@ fn populate_output_configuration(
 ) -> Result<(), ErrorKind> {
     // We only do something when `digest` and `algo` are `Some(_)``, and
     // there's an `out` output.
-    if let (Some(hash), Some(algo), hash_mode) = (hash, hash_algo, hash_mode) {
+    if let (Some(nixhash_str), Some(algo), hash_mode) = (hash, hash_algo, hash_mode) {
         match drv.outputs.get_mut("out") {
             None => return Err(Error::ConflictingOutputTypes.into()),
             Some(out) => {
@@ -126,12 +126,12 @@ fn populate_output_configuration(
                     Some(algo.as_ref())
                 };
 
-                let output_hash = nixhash::from_str(&hash, a).map_err(Error::InvalidOutputHash)?;
+                let hash = nixhash::from_str(&nixhash_str, a).map_err(Error::InvalidOutputHash)?;
 
                 // construct the NixHashWithMode.
-                out.hash_with_mode = match hash_mode.as_deref() {
-                    None | Some("flat") => Some(nixhash::NixHashWithMode::Flat(output_hash)),
-                    Some("recursive") => Some(nixhash::NixHashWithMode::Recursive(output_hash)),
+                out.ca_hash = match hash_mode.as_deref() {
+                    None | Some("flat") => Some(nixhash::CAHash::Flat(hash)),
+                    Some("recursive") => Some(nixhash::CAHash::Nar(hash)),
                     Some(other) => {
                         return Err(Error::InvalidOutputHashMode(other.to_string()).into())
                     }
diff --git a/tvix/nix-compat/src/derivation/errors.rs b/tvix/nix-compat/src/derivation/errors.rs
index 8e9e6a121096..305957b10f6d 100644
--- a/tvix/nix-compat/src/derivation/errors.rs
+++ b/tvix/nix-compat/src/derivation/errors.rs
@@ -1,6 +1,8 @@
-use crate::{nixbase32::Nixbase32DecodeError, store_path};
+use crate::store_path;
 use thiserror::Error;
 
+use super::CAHash;
+
 /// Errors that can occur during the validation of Derivation structs.
 #[derive(Debug, Error, PartialEq)]
 pub enum DerivationError {
@@ -50,10 +52,6 @@ pub enum DerivationError {
 pub enum OutputError {
     #[error("Invalid output path {0}: {1}")]
     InvalidOutputPath(String, store_path::Error),
-    #[error("Invalid hash encoding: {0}")]
-    InvalidHashEncoding(String, Nixbase32DecodeError),
-    #[error("Invalid hash algo: {0}")]
-    InvalidHashAlgo(String),
-    #[error("Invalid Digest size {0} for algo {1}")]
-    InvalidDigestSizeForAlgo(usize, String),
+    #[error("Invalid CAHash: {:?}", .0)]
+    InvalidCAHash(CAHash),
 }
diff --git a/tvix/nix-compat/src/derivation/mod.rs b/tvix/nix-compat/src/derivation/mod.rs
index d7f894c89a8f..a765e343096e 100644
--- a/tvix/nix-compat/src/derivation/mod.rs
+++ b/tvix/nix-compat/src/derivation/mod.rs
@@ -1,6 +1,4 @@
-use crate::store_path::{
-    self, build_output_path, build_regular_ca_path, build_text_path, StorePath,
-};
+use crate::store_path::{self, build_ca_path, build_output_path, build_text_path, StorePath};
 use bstr::BString;
 use serde::{Deserialize, Serialize};
 use sha2::{Digest, Sha256};
@@ -18,7 +16,7 @@ mod write;
 mod tests;
 
 // Public API of the crate.
-pub use crate::nixhash::{NixHash, NixHashWithMode};
+pub use crate::nixhash::{CAHash, NixHash};
 pub use errors::{DerivationError, OutputError};
 pub use output::Output;
 
@@ -122,16 +120,22 @@ impl Derivation {
 
     /// Returns the FOD digest, if the derivation is fixed-output, or None if
     /// it's not.
+    /// TODO: this is kinda the string from [build_ca_path] with a
+    /// [CAHash::Flat], what's fed to `build_store_path_from_fingerprint_parts`
+    /// (except the out_output.path being an empty string)
     fn fod_digest(&self) -> Option<[u8; 32]> {
         if self.outputs.len() != 1 {
             return None;
         }
 
         let out_output = self.outputs.get("out")?;
+        let ca_hash = &out_output.ca_hash.as_ref()?;
+
         Some(
             Sha256::new_with_prefix(format!(
-                "fixed:out:{}:{}",
-                out_output.hash_with_mode.clone()?.to_nix_hash_string(),
+                "fixed:out:{}{}:{}",
+                ca_kind_prefix(ca_hash),
+                ca_hash.digest().to_nix_hash_string(),
                 out_output.path
             ))
             .finalize()
@@ -229,10 +233,10 @@ impl Derivation {
 
             // For fixed output derivation we use the per-output info, otherwise we use the
             // derivation hash.
-            let abs_store_path = if let Some(ref hwm) = output.hash_with_mode {
-                build_regular_ca_path(&path_name, hwm, Vec::<String>::new(), false).map_err(
-                    |e| DerivationError::InvalidOutputDerivationPath(output_name.to_string(), e),
-                )?
+            let abs_store_path = if let Some(ref hwm) = output.ca_hash {
+                build_ca_path(&path_name, hwm, Vec::<String>::new(), false).map_err(|e| {
+                    DerivationError::InvalidOutputDerivationPath(output_name.to_string(), e)
+                })?
             } else {
                 build_output_path(derivation_or_fod_hash, output_name, &path_name).map_err(|e| {
                     DerivationError::InvalidOutputDerivationPath(
@@ -265,3 +269,15 @@ fn output_path_name(derivation_name: &str, output_name: &str) -> String {
     }
     output_path_name
 }
+
+/// For a [CAHash], return the "prefix" used for NAR purposes.
+/// For [CAHash::Flat], this is an empty string, for [CAHash::Nar], it's "r:".
+/// Panics for other [CAHash] kinds, as they're not valid in a derivation
+/// context.
+fn ca_kind_prefix(ca_hash: &CAHash) -> &'static str {
+    match ca_hash {
+        CAHash::Flat(_) => "",
+        CAHash::Nar(_) => "r:",
+        _ => panic!("invalid ca hash in derivation context: {:?}", ca_hash),
+    }
+}
diff --git a/tvix/nix-compat/src/derivation/output.rs b/tvix/nix-compat/src/derivation/output.rs
index 78a83b03be45..c13f94859dc1 100644
--- a/tvix/nix-compat/src/derivation/output.rs
+++ b/tvix/nix-compat/src/derivation/output.rs
@@ -1,5 +1,5 @@
 use crate::derivation::OutputError;
-use crate::nixhash::{HashAlgo, NixHashWithMode};
+use crate::nixhash::CAHash;
 use crate::store_path::StorePath;
 use serde::{Deserialize, Serialize};
 use serde_json::Map;
@@ -9,7 +9,7 @@ pub struct Output {
     pub path: String,
 
     #[serde(flatten)]
-    pub hash_with_mode: Option<NixHashWithMode>,
+    pub ca_hash: Option<CAHash>, // we can only represent a subset here.
 }
 
 impl<'de> Deserialize<'de> for Output {
@@ -30,26 +30,26 @@ impl<'de> Deserialize<'de> for Output {
                     &"a string",
                 ))?
                 .to_owned(),
-            hash_with_mode: NixHashWithMode::from_map::<D>(&fields)?,
+            ca_hash: CAHash::from_map::<D>(&fields)?,
         })
     }
 }
 
 impl Output {
     pub fn is_fixed(&self) -> bool {
-        self.hash_with_mode.is_some()
+        self.ca_hash.is_some()
     }
 
     pub fn validate(&self, validate_output_paths: bool) -> Result<(), OutputError> {
-        if let Some(hash) = &self.hash_with_mode {
-            match hash {
-                NixHashWithMode::Flat(h) | NixHashWithMode::Recursive(h) => {
-                    if h.algo() != HashAlgo::Sha1 || h.algo() != HashAlgo::Sha256 {
-                        return Err(OutputError::InvalidHashAlgo(h.algo().to_string()));
-                    }
+        if let Some(fixed_output_hash) = &self.ca_hash {
+            match fixed_output_hash {
+                CAHash::Flat(_) | CAHash::Nar(_) => {
+                    // all hashes allowed for Flat, and Nar.
                 }
+                _ => return Err(OutputError::InvalidCAHash(fixed_output_hash.clone())),
             }
         }
+
         if validate_output_paths {
             if let Err(e) = StorePath::from_absolute_path(self.path.as_bytes()) {
                 return Err(OutputError::InvalidOutputPath(self.path.to_string(), e));
diff --git a/tvix/nix-compat/src/derivation/parser.rs b/tvix/nix-compat/src/derivation/parser.rs
index 48f2b92d9168..b04187c433dc 100644
--- a/tvix/nix-compat/src/derivation/parser.rs
+++ b/tvix/nix-compat/src/derivation/parser.rs
@@ -12,8 +12,8 @@ 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::derivation::parse_error::{into_nomerror, ErrorKind, NomError, NomResult};
+use crate::derivation::{write, CAHash, Derivation, Output};
 use crate::{aterm, nixhash};
 
 #[derive(Debug, thiserror::Error)]
@@ -42,6 +42,24 @@ pub(crate) fn parse(i: &[u8]) -> Result<Derivation, Error<&[u8]>> {
     }
 }
 
+/// Consume a string containing the algo, and optionally a `r:`
+/// prefix, and a digest (bytes), return a [CAHash::Nar] or [CAHash::Flat].
+fn from_algo_and_mode_and_digest<B: AsRef<[u8]>>(
+    algo_and_mode: &str,
+    digest: B,
+) -> crate::nixhash::Result<CAHash> {
+    Ok(match algo_and_mode.strip_prefix("r:") {
+        Some(algo) => nixhash::CAHash::Nar(nixhash::from_algo_and_digest(
+            algo.try_into()?,
+            digest.as_ref(),
+        )?),
+        None => nixhash::CAHash::Flat(nixhash::from_algo_and_digest(
+            algo_and_mode.try_into()?,
+            digest.as_ref(),
+        )?),
+    })
+}
+
 /// 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.
@@ -60,27 +78,26 @@ fn parse_output(i: &[u8]) -> NomResult<&[u8], (String, Output)> {
             },
             |(output_name, output_path, algo_and_mode, encoded_digest)| {
                 // convert these 4 fields into an [Output].
-                let hash_with_mode_res = {
+                let ca_hash_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,
-                            )),
+                            Ok(digest) => {
+                                Some(from_algo_and_mode_and_digest(&algo_and_mode, digest))
+                            }
                             Err(e) => Some(Err(nixhash::Error::InvalidBase64Encoding(e))),
                         }
                     }
                 }
                 .transpose();
 
-                match hash_with_mode_res {
+                match ca_hash_res {
                     Ok(hash_with_mode) => Ok((
                         output_name,
                         Output {
                             path: output_path,
-                            hash_with_mode,
+                            ca_hash: hash_with_mode,
                         },
                     )),
                     Err(e) => Err(nom::Err::Failure(NomError {
@@ -279,12 +296,20 @@ where
 mod tests {
     use std::collections::{BTreeMap, BTreeSet};
 
-    use crate::derivation::{parse_error::ErrorKind, Output};
+    use crate::derivation::{
+        parse_error::ErrorKind, parser::from_algo_and_mode_and_digest, CAHash, NixHash, Output,
+    };
     use bstr::{BString, ByteSlice};
     use lazy_static::lazy_static;
     use test_case::test_case;
+    const DIGEST_SHA256: [u8; 32] = [
+        0xa5, 0xce, 0x9c, 0x15, 0x5e, 0xd0, 0x93, 0x97, 0x61, 0x46, 0x46, 0xc9, 0x71, 0x7f, 0xc7,
+        0xcd, 0x94, 0xb1, 0x02, 0x3d, 0x7b, 0x76, 0xb6, 0x18, 0xd4, 0x09, 0xe4, 0xfe, 0xfd, 0x6e,
+        0x9d, 0x39,
+    ];
 
     lazy_static! {
+        pub static ref NIXHASH_SHA256: NixHash = NixHash::Sha256(DIGEST_SHA256);
         static ref EXP_MULTI_OUTPUTS: BTreeMap<String, Output> = {
             let mut b = BTreeMap::new();
             b.insert(
@@ -292,14 +317,14 @@ mod tests {
                 Output {
                     path: "/nix/store/2vixb94v0hy2xc6p7mbnxxcyc095yyia-has-multi-out-lib"
                         .to_string(),
-                    hash_with_mode: None,
+                    ca_hash: None,
                 },
             );
             b.insert(
                 "out".to_string(),
                 Output {
                     path: "/nix/store/55lwldka5nyxa08wnvlizyqw02ihy8ic-has-multi-out".to_string(),
-                    hash_with_mode: None,
+                    ca_hash: None,
                 },
             );
             b
@@ -441,19 +466,15 @@ mod tests {
         br#"("out","/nix/store/5vyvcwah9l9kf07d52rcgdk70g2f4y13-foo","","")"#,
         ("out".to_string(), Output {
             path: "/nix/store/5vyvcwah9l9kf07d52rcgdk70g2f4y13-foo".to_string(),
-            hash_with_mode: None
+            ca_hash: 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()
-            )),
+            ca_hash: Some(from_algo_and_mode_and_digest("r:sha256",
+                   &data_encoding::HEXLOWER.decode(b"08813cbee9903c62be4c5027726a418a300da4500b2d369d3af9286f4815ceba").unwrap()            ).unwrap()),
         }); "fod"
      )]
     fn parse_output(input: &[u8], expected: (String, Output)) {
@@ -472,4 +493,19 @@ mod tests {
         assert!(rest.is_empty());
         assert_eq!(*expected, parsed);
     }
+
+    #[test_case("sha256", &DIGEST_SHA256, CAHash::Flat(NIXHASH_SHA256.clone()); "sha256 flat")]
+    #[test_case("r:sha256", &DIGEST_SHA256, CAHash::Nar(NIXHASH_SHA256.clone()); "sha256 recursive")]
+    fn test_from_algo_and_mode_and_digest(algo_and_mode: &str, digest: &[u8], expected: CAHash) {
+        assert_eq!(
+            expected,
+            from_algo_and_mode_and_digest(algo_and_mode, digest).unwrap()
+        );
+    }
+
+    #[test]
+    fn from_algo_and_mode_and_digest_failure() {
+        assert!(from_algo_and_mode_and_digest("r:sha256", &[]).is_err());
+        assert!(from_algo_and_mode_and_digest("ha256", &DIGEST_SHA256).is_err());
+    }
 }
diff --git a/tvix/nix-compat/src/derivation/tests/mod.rs b/tvix/nix-compat/src/derivation/tests/mod.rs
index 36b44e047f9a..b969625d9760 100644
--- a/tvix/nix-compat/src/derivation/tests/mod.rs
+++ b/tvix/nix-compat/src/derivation/tests/mod.rs
@@ -315,7 +315,7 @@ fn output_path_construction() {
         "out".to_string(),
         Output {
             path: "".to_string(), // will be calculated
-            hash_with_mode: Some(crate::nixhash::NixHashWithMode::Recursive(
+            ca_hash: Some(crate::nixhash::CAHash::Nar(
                 crate::nixhash::from_algo_and_digest(
                     crate::nixhash::HashAlgo::Sha256,
                     &data_encoding::HEXLOWER
@@ -376,7 +376,7 @@ fn output_path_construction() {
         "out".to_string(),
         Output {
             path: "".to_string(), // will be calculated
-            hash_with_mode: None,
+            ca_hash: None,
         },
     );
 
diff --git a/tvix/nix-compat/src/derivation/write.rs b/tvix/nix-compat/src/derivation/write.rs
index 7ebbbffa4b55..087227c99998 100644
--- a/tvix/nix-compat/src/derivation/write.rs
+++ b/tvix/nix-compat/src/derivation/write.rs
@@ -4,7 +4,7 @@
 //! [ATerm]: http://program-transformation.org/Tools/ATermFormat.html
 
 use crate::aterm::escape_bytes;
-use crate::derivation::output::Output;
+use crate::derivation::{ca_kind_prefix, output::Output};
 use bstr::BString;
 use std::{
     collections::{BTreeMap, BTreeSet},
@@ -79,14 +79,10 @@ pub fn write_outputs(
 
         let mut elements: Vec<&str> = vec![output_name, &output.path];
 
-        let (mode_and_algo, digest) = match &output.hash_with_mode {
-            Some(crate::nixhash::NixHashWithMode::Flat(h)) => (
-                h.algo().to_string(),
-                data_encoding::HEXLOWER.encode(h.digest_as_bytes()),
-            ),
-            Some(crate::nixhash::NixHashWithMode::Recursive(h)) => (
-                format!("r:{}", h.algo()),
-                data_encoding::HEXLOWER.encode(h.digest_as_bytes()),
+        let (mode_and_algo, digest) = match &output.ca_hash {
+            Some(ca_hash) => (
+                format!("{}{}", ca_kind_prefix(ca_hash), ca_hash.digest().algo()),
+                data_encoding::HEXLOWER.encode(ca_hash.digest().digest_as_bytes()),
             ),
             None => ("".to_string(), "".to_string()),
         };
diff --git a/tvix/nix-compat/src/nixhash/with_mode.rs b/tvix/nix-compat/src/nixhash/ca_hash.rs
index 6e6d2e76b4a4..5d9ae3f3a861 100644
--- a/tvix/nix-compat/src/nixhash/with_mode.rs
+++ b/tvix/nix-compat/src/nixhash/ca_hash.rs
@@ -4,74 +4,41 @@ use serde::de::Unexpected;
 use serde::ser::SerializeMap;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use serde_json::{Map, Value};
+use std::borrow::Cow;
 
 use super::algos::SUPPORTED_ALGOS;
 use super::from_algo_and_digest;
 
-pub enum NixHashMode {
-    Flat,
-    Recursive,
-}
-
-impl NixHashMode {
-    pub fn prefix(self) -> &'static str {
-        match self {
-            Self::Flat => "",
-            Self::Recursive => "r:",
-        }
-    }
-}
-
-/// A Nix Hash can either be flat or recursive.
+/// A Nix CAHash describes a content-addressed hash of a path.
+/// Semantically, it can be split into the following components:
+///
+///  - "content address prefix". Currently, "fixed" and "text" are supported.
+///  - "hash mode". Currently, "flat" and "recursive" are supported.
+///  - "hash type". The underlying hash function used.
+///    Currently, sha1, md5, sha256, sha512.
+///  - "digest". The digest itself.
 #[derive(Clone, Debug, Eq, PartialEq)]
-pub enum NixHashWithMode {
-    Flat(NixHash),
-    Recursive(NixHash),
+pub enum CAHash {
+    Flat(NixHash),       // "fixed flat"
+    Nar(NixHash),        // "fixed recursive"
+    Text(Box<[u8; 32]>), // "text", only supports sha256
 }
 
-impl NixHashWithMode {
-    /// Construct a [NixHashWithMode] from a string containing the algo, and
-    /// optionally a `r:` prefix, and a digest (bytes).
-    pub fn from_algo_mode_hash(algo_and_mode: &str, digest: &[u8]) -> super::Result<Self> {
-        Ok(match algo_and_mode.strip_prefix("r:") {
-            Some(algo) => nixhash::NixHashWithMode::Recursive(nixhash::from_algo_and_digest(
-                algo.try_into()?,
-                digest,
-            )?),
-            None => nixhash::NixHashWithMode::Flat(nixhash::from_algo_and_digest(
-                algo_and_mode.try_into()?,
-                digest,
-            )?),
-        })
-    }
-
-    pub fn mode(&self) -> NixHashMode {
-        match self {
-            Self::Flat(_) => NixHashMode::Flat,
-            Self::Recursive(_) => NixHashMode::Recursive,
-        }
-    }
-
-    pub fn digest(&self) -> &NixHash {
+impl CAHash {
+    pub fn digest(&self) -> Cow<NixHash> {
         match self {
-            Self::Flat(ref h) => h,
-            Self::Recursive(ref h) => h,
+            CAHash::Nar(ref digest) => Cow::Borrowed(digest),
+            CAHash::Text(ref digest) => Cow::Owned(NixHash::Sha256(*digest.clone())),
+            CAHash::Flat(ref digest) => Cow::Borrowed(digest),
         }
     }
 
-    /// Formats a [NixHashWithMode] in the Nix default hash format,
-    /// which is the algo, followed by a colon, then the lower hex encoded digest.
-    /// In case the hash itself is recursive, a `r:` is added as prefix
-    pub fn to_nix_hash_string(&self) -> String {
-        String::from(self.mode().prefix()) + &self.digest().to_nix_hash_string()
-    }
-
     /// This takes a serde_json::Map and turns it into this structure. This is necessary to do such
     /// shenigans because we have external consumers, like the Derivation parser, who would like to
     /// know whether we have a invalid or a missing NixHashWithMode structure in another structure,
     /// e.g. Output.
     /// This means we have this combinatorial situation:
-    /// - no hash, no hashAlgo: no NixHashWithMode so we return Ok(None).
+    /// - no hash, no hashAlgo: no [CAHash] so we return Ok(None).
     /// - present hash, missing hashAlgo: invalid, we will return missing_field
     /// - missing hash, present hashAlgo: same
     /// - present hash, present hashAlgo: either we return ourselves or a type/value validation
@@ -79,7 +46,7 @@ impl NixHashWithMode {
     /// This function is for internal consumption regarding those needs until we have a better
     /// solution. Now this is said, let's explain how this works.
     ///
-    /// We want to map the serde data model into a NixHashWithMode.
+    /// We want to map the serde data model into a [CAHash].
     ///
     /// The serde data model has a `hash` field (containing a digest in nixbase32),
     /// and a `hashAlgo` field, containing the stringified hash algo.
@@ -118,7 +85,7 @@ impl NixHashWithMode {
         if let Some(v) = map.get("hashAlgo") {
             if let Some(s) = v.as_str() {
                 match s.strip_prefix("r:") {
-                    Some(rest) => Ok(Some(Self::Recursive(
+                    Some(rest) => Ok(Some(Self::Nar(
                         from_algo_and_digest(
                             HashAlgo::try_from(rest).map_err(|e| {
                                 serde::de::Error::invalid_value(
@@ -167,28 +134,35 @@ impl NixHashWithMode {
     }
 }
 
-impl Serialize for NixHashWithMode {
-    /// map a NixHashWithMode into the serde data model.
+impl Serialize for CAHash {
+    /// map a CAHash into the serde data model.
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
         S: Serializer,
     {
         let mut map = serializer.serialize_map(Some(2))?;
         match self {
-            NixHashWithMode::Flat(h) => {
+            CAHash::Flat(h) => {
                 map.serialize_entry("hash", &nixbase32::encode(h.digest_as_bytes()))?;
                 map.serialize_entry("hashAlgo", &h.algo())?;
             }
-            NixHashWithMode::Recursive(h) => {
+            CAHash::Nar(h) => {
                 map.serialize_entry("hash", &nixbase32::encode(h.digest_as_bytes()))?;
                 map.serialize_entry("hashAlgo", &format!("r:{}", &h.algo()))?;
             }
+            // It is not legal for derivations to use this (which is where
+            // we're currently exercising [Serialize] mostly,
+            // but it's still good to be able to serialize other CA hashes too.
+            CAHash::Text(h) => {
+                map.serialize_entry("hash", &nixbase32::encode(h.as_ref()))?;
+                map.serialize_entry("hashAlgo", "text")?;
+            }
         };
         map.end()
     }
 }
 
-impl<'de> Deserialize<'de> for NixHashWithMode {
+impl<'de> Deserialize<'de> for CAHash {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
         D: Deserializer<'de>,
@@ -201,34 +175,3 @@ impl<'de> Deserialize<'de> for NixHashWithMode {
         }
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use crate::nixhash::{NixHash, NixHashWithMode};
-    use lazy_static::lazy_static;
-    use test_case::test_case;
-
-    const DIGEST_SHA256: [u8; 32] = [
-        0xa5, 0xce, 0x9c, 0x15, 0x5e, 0xd0, 0x93, 0x97, 0x61, 0x46, 0x46, 0xc9, 0x71, 0x7f, 0xc7,
-        0xcd, 0x94, 0xb1, 0x02, 0x3d, 0x7b, 0x76, 0xb6, 0x18, 0xd4, 0x09, 0xe4, 0xfe, 0xfd, 0x6e,
-        0x9d, 0x39,
-    ];
-    lazy_static! {
-        pub static ref NIXHASH_SHA256: NixHash = NixHash::Sha256(DIGEST_SHA256);
-    }
-
-    #[test_case("sha256", &DIGEST_SHA256, NixHashWithMode::Flat(NIXHASH_SHA256.clone()); "sha256 flat")]
-    #[test_case("r:sha256", &DIGEST_SHA256, NixHashWithMode::Recursive(NIXHASH_SHA256.clone()); "sha256 recursive")]
-    fn from_from_algo_mode_hash(algo_and_mode: &str, digest: &[u8], expected: NixHashWithMode) {
-        assert_eq!(
-            expected,
-            NixHashWithMode::from_algo_mode_hash(algo_and_mode, digest).unwrap()
-        );
-    }
-
-    #[test]
-    fn from_algo_mode_failure() {
-        assert!(NixHashWithMode::from_algo_mode_hash("r:sha256", &[]).is_err());
-        assert!(NixHashWithMode::from_algo_mode_hash("ha256", &DIGEST_SHA256).is_err());
-    }
-}
diff --git a/tvix/nix-compat/src/nixhash/mod.rs b/tvix/nix-compat/src/nixhash/mod.rs
index 5b12f466e3fb..1983d16feee9 100644
--- a/tvix/nix-compat/src/nixhash/mod.rs
+++ b/tvix/nix-compat/src/nixhash/mod.rs
@@ -3,10 +3,10 @@ use data_encoding::{BASE64, BASE64_NOPAD, HEXLOWER};
 use thiserror;
 
 mod algos;
-mod with_mode;
+mod ca_hash;
 
 pub use algos::HashAlgo;
-pub use with_mode::NixHashWithMode;
+pub use ca_hash::CAHash;
 
 /// NixHash represents hashes known by Nix.
 #[derive(Clone, Debug, Eq, PartialEq)]
diff --git a/tvix/nix-compat/src/store_path/utils.rs b/tvix/nix-compat/src/store_path/utils.rs
index b774fbdb662d..b625abdfd0b9 100644
--- a/tvix/nix-compat/src/store_path/utils.rs
+++ b/tvix/nix-compat/src/store_path/utils.rs
@@ -1,7 +1,6 @@
-use super::{Error, STORE_DIR};
 use crate::nixbase32;
-use crate::nixhash::{NixHash, NixHashWithMode};
-use crate::store_path::StorePath;
+use crate::nixhash::{CAHash, NixHash};
+use crate::store_path::{Error, StorePath, STORE_DIR};
 use sha2::{Digest, Sha256};
 use thiserror;
 
@@ -41,43 +40,51 @@ pub fn compress_hash<const OUTPUT_SIZE: usize>(input: &[u8]) -> [u8; OUTPUT_SIZE
 
 /// This builds a store path, by calculating the text_hash_string of either a
 /// derivation or a literal text file that may contain references.
+/// If you don't want to have to pass the entire contents, you might want to use
+/// [build_ca_path] instead.
 pub fn build_text_path<S: AsRef<str>, I: IntoIterator<Item = S>, C: AsRef<[u8]>>(
     name: &str,
     content: C,
     references: I,
-) -> Result<StorePath, Error> {
-    build_store_path_from_fingerprint_parts(
-        &make_type("text", references, false),
-        // the nix_hash_string representation of the sha256 digest of some contents
-        &{
-            let content_digest = {
-                let hasher = Sha256::new_with_prefix(content);
-                hasher.finalize()
-            };
+) -> Result<StorePath, BuildStorePathError> {
+    // produce the sha256 digest of the contents
+    let content_digest = Sha256::new_with_prefix(content).finalize().into();
 
-            NixHash::Sha256(content_digest.into())
-        },
+    build_ca_path(
         name,
+        &CAHash::Text(Box::new(content_digest)),
+        references,
+        false,
     )
 }
 
-/// This builds a more "regular" content-addressed store path
-pub fn build_regular_ca_path<S: AsRef<str>, I: IntoIterator<Item = S>>(
-    name: &str,
-    hash_with_mode: &NixHashWithMode,
+/// This builds a store path from a [CAHash] and a list of references.
+pub fn build_ca_path<B: AsRef<[u8]>, S: AsRef<str>, I: IntoIterator<Item = S>>(
+    name: B,
+    ca_hash: &CAHash,
     references: I,
     self_reference: bool,
 ) -> Result<StorePath, BuildStorePathError> {
-    match &hash_with_mode {
-        NixHashWithMode::Recursive(ref hash @ NixHash::Sha256(_)) => {
+    match &ca_hash {
+        CAHash::Text(ref digest) => {
+            if self_reference {
+                return Err(BuildStorePathError::InvalidReference());
+            }
             build_store_path_from_fingerprint_parts(
-                &make_type("source", references, self_reference),
-                hash,
+                &make_references_string("text", references, false),
+                &NixHash::Sha256(*digest.to_owned()),
                 name,
             )
             .map_err(BuildStorePathError::InvalidStorePath)
         }
-        _ => {
+        CAHash::Nar(ref hash @ NixHash::Sha256(_)) => build_store_path_from_fingerprint_parts(
+            &make_references_string("source", references, self_reference),
+            hash,
+            name,
+        )
+        .map_err(BuildStorePathError::InvalidStorePath),
+        // for all other CAHash::Nar, another custom scheme is used.
+        CAHash::Nar(ref hash) => {
             if references.into_iter().next().is_some() {
                 return Err(BuildStorePathError::InvalidReference());
             }
@@ -87,14 +94,38 @@ pub fn build_regular_ca_path<S: AsRef<str>, I: IntoIterator<Item = S>>(
             build_store_path_from_fingerprint_parts(
                 "output:out",
                 &{
-                    let content_digest = {
-                        let mut hasher = Sha256::new_with_prefix("fixed:out:");
-                        hasher.update(hash_with_mode.to_nix_hash_string());
-                        hasher.update(":");
-                        hasher.finalize()
-                    };
-
-                    NixHash::Sha256(content_digest.into())
+                    NixHash::Sha256(
+                        Sha256::new_with_prefix(format!(
+                            "fixed:out:r:{}:",
+                            hash.to_nix_hash_string()
+                        ))
+                        .finalize()
+                        .into(),
+                    )
+                },
+                name,
+            )
+            .map_err(BuildStorePathError::InvalidStorePath)
+        }
+        // CaHash::Flat is using something very similar, except the `r:` prefix.
+        CAHash::Flat(ref hash) => {
+            if references.into_iter().next().is_some() {
+                return Err(BuildStorePathError::InvalidReference());
+            }
+            if self_reference {
+                return Err(BuildStorePathError::InvalidReference());
+            }
+            build_store_path_from_fingerprint_parts(
+                "output:out",
+                &{
+                    NixHash::Sha256(
+                        Sha256::new_with_prefix(format!(
+                            "fixed:out:{}:",
+                            hash.to_nix_hash_string()
+                        ))
+                        .finalize()
+                        .into(),
+                    )
                 },
                 name,
             )
@@ -105,13 +136,12 @@ pub fn build_regular_ca_path<S: AsRef<str>, I: IntoIterator<Item = S>>(
 
 /// For given NAR sha256 digest and name, return the new [StorePath] this would have.
 pub fn build_nar_based_store_path(nar_sha256_digest: &[u8; 32], name: &str) -> StorePath {
-    let nar_hash_with_mode =
-        NixHashWithMode::Recursive(NixHash::Sha256(nar_sha256_digest.to_owned()));
+    let nar_hash_with_mode = CAHash::Nar(NixHash::Sha256(nar_sha256_digest.to_owned()));
 
-    build_regular_ca_path(name, &nar_hash_with_mode, Vec::<String>::new(), false).unwrap()
+    build_ca_path(name, &nar_hash_with_mode, Vec::<String>::new(), false).unwrap()
 }
 
-/// This builds an input-addressed store path
+/// This builds an input-addressed store path.
 ///
 /// Input-addresed store paths are always derivation outputs, the "input" in question is the
 /// derivation and its closure.
@@ -135,22 +165,20 @@ pub fn build_output_path(
 ///
 /// The fingerprint is hashed with sha256, its digest is compressed to 20 bytes,
 /// and nixbase32-encoded (32 characters).
-fn build_store_path_from_fingerprint_parts(
+fn build_store_path_from_fingerprint_parts<B: AsRef<[u8]>>(
     ty: &str,
     hash: &NixHash,
-    name: &str,
+    name: B,
 ) -> Result<StorePath, Error> {
+    let name = super::validate_name(name.as_ref())?;
     let fingerprint =
-        String::from(ty) + ":" + &hash.to_nix_hash_string() + ":" + STORE_DIR + ":" + name;
-    let digest = {
-        let hasher = Sha256::new_with_prefix(fingerprint);
-        hasher.finalize()
-    };
+        String::from(ty) + ":" + &hash.to_nix_hash_string() + ":" + STORE_DIR + ":" + &name;
+    let digest = Sha256::new_with_prefix(fingerprint).finalize();
     let compressed = compress_hash::<20>(&digest);
-    super::validate_name(name.as_bytes())?;
+
     Ok(StorePath {
         digest: compressed,
-        name: name.to_string(),
+        name,
     })
 }
 
@@ -164,7 +192,7 @@ fn build_store_path_from_fingerprint_parts(
 ///  - the nix_hash_string representation of the sha256 digest of some contents
 ///  - the value of `storeDir`
 ///  - the name
-fn make_type<S: AsRef<str>, I: IntoIterator<Item = S>>(
+fn make_references_string<S: AsRef<str>, I: IntoIterator<Item = S>>(
     ty: &str,
     references: I,
     self_ref: bool,
@@ -190,11 +218,7 @@ fn make_type<S: AsRef<str>, I: IntoIterator<Item = S>>(
 /// The actual placeholder is basically just a SHA256 hash encoded in
 /// cppnix format.
 pub fn hash_placeholder(name: &str) -> String {
-    let digest = {
-        let mut hasher = Sha256::new();
-        hasher.update(format!("nix-output:{}", name));
-        hasher.finalize()
-    };
+    let digest = Sha256::new_with_prefix(format!("nix-output:{}", name)).finalize();
 
     format!("/{}", nixbase32::encode(&digest))
 }
@@ -202,7 +226,7 @@ pub fn hash_placeholder(name: &str) -> String {
 #[cfg(test)]
 mod test {
     use super::*;
-    use crate::nixhash::{NixHash, NixHashWithMode};
+    use crate::nixhash::{CAHash, NixHash};
 
     #[test]
     fn build_text_path_with_zero_references() {
@@ -242,9 +266,9 @@ mod test {
 
     #[test]
     fn build_sha1_path() {
-        let outer = build_regular_ca_path(
+        let outer = build_ca_path(
             "bar",
-            &NixHashWithMode::Recursive(NixHash::Sha1(
+            &CAHash::Nar(NixHash::Sha1(
                 data_encoding::HEXLOWER
                     .decode(b"0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33")
                     .expect("hex should decode")
@@ -271,9 +295,9 @@ mod test {
         //
         // $ nix store make-content-addressed /nix/store/5xd714cbfnkz02h2vbsj4fm03x3f15nf-baz
         // rewrote '/nix/store/5xd714cbfnkz02h2vbsj4fm03x3f15nf-baz' to '/nix/store/s89y431zzhmdn3k8r96rvakryddkpv2v-baz'
-        let outer = build_regular_ca_path(
+        let outer = build_ca_path(
             "baz",
-            &NixHashWithMode::Recursive(NixHash::Sha256(
+            &CAHash::Nar(NixHash::Sha256(
                 nixbase32::decode(b"1xqkzcb3909fp07qngljr4wcdnrh1gdam1m2n29i6hhrxlmkgkv1")
                     .expect("hex should decode")
                     .try_into()