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>

2 files changed, 35 insertions, 92 deletions

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)]