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>

6 files changed, 103 insertions, 57 deletions

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()),
         };