refactor(nix-compat/store_path): take [u8;32] for outer fingerprint r/7691

The outer fingerprint used for store path calculation is always a sha256
digest. This includes both input and output-addressed store paths.

We used a NixHash here, which can also represent other hash types, and
that had a bunch of annoyances:

 - Whenever we had the bytes, we had to wrap them in a NixHash::Sha256().
 - Things like AtermWriteable had to be implemented on NixHash,
   even though we then had an assertion it was only called in the
   NixHash::Sha256 case.

Change-Id: Ic895503d9b071800d2e52ae057666f44bd0ab9d6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11142
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: John Ericson <git@johnericson.me>
Reviewed-by: picnoir picnoir <picnoir@alternativebit.fr>

1 files changed, 5 insertions, 9 deletions

diff --git a/tvix/nix-compat/src/derivation/write.rs b/tvix/nix-compat/src/derivation/write.rs
index f20bf4e121..735b781574 100644
--- a/tvix/nix-compat/src/derivation/write.rs
+++ b/tvix/nix-compat/src/derivation/write.rs
@@ -8,7 +8,8 @@ use crate::derivation::{ca_kind_prefix, output::Output};
 use crate::nixbase32;
 use crate::store_path::{StorePath, StorePathRef, STORE_DIR_WITH_SLASH};
 use bstr::BString;
-use std::fmt::Display;
+use data_encoding::HEXLOWER;
+
 use std::{
     collections::{BTreeMap, BTreeSet},
     io,
@@ -16,8 +17,6 @@ use std::{
     io::Write,
 };
 
-use super::NixHash;
-
 pub const DERIVATION_PREFIX: &str = "Derive";
 pub const PAREN_OPEN: char = '(';
 pub const PAREN_CLOSE: char = ')';
@@ -31,7 +30,7 @@ pub const QUOTE: char = '"';
 /// Note that we mostly use explicit `write_*` calls
 /// instead since the serialization of the items depends on
 /// the context a lot.
-pub(crate) trait AtermWriteable: Display {
+pub(crate) trait AtermWriteable {
     fn aterm_write(&self, writer: &mut impl Write) -> std::io::Result<()>;
 
     fn aterm_bytes(&self) -> Vec<u8> {
@@ -67,12 +66,9 @@ impl AtermWriteable for String {
     }
 }
 
-impl AtermWriteable for NixHash {
+impl AtermWriteable for [u8; 32] {
     fn aterm_write(&self, writer: &mut impl Write) -> std::io::Result<()> {
-        // When we serialize the placeholder hashes,
-        // they need to be SHA256.
-        debug_assert!(matches!(self, NixHash::Sha256(_)));
-        write_field(writer, self.to_plain_hex_string(), false)
+        write_field(writer, HEXLOWER.encode(self), false)
     }
 }