refactor(tvix/nix-compat): move build_store_path out of derivation

This doesn't have anything to do with ATerms, we just happen to be using
the aterm representation of a Derivation as contents.

Moving this into store_path/utils.rs makes these things much cleaner -
Have a build_store_path_from_references function, and a
build_store_path_from_fingerprint helper function that makes use of it.

build_store_path_from_references is invoked from the derivation module
which can be used to calculate the derivation path.

In the derivation module, we also invoke
build_store_path_from_fingerprint during the output path calculation.

Change-Id: Ia8d61a5e8e5d3f396f93593676ed3f5d1a3f1d66
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8367
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/nix-compat/src/store_path/utils.rs

@@ -0,0 +1,118 @@
+use crate::nixbase32;
+use crate::store_path::StorePath;
+use crate::texthash::text_hash_string;
+use sha2::{Digest, Sha256};
+
+use super::Error;
+
+/// compress_hash takes an arbitrarily long sequence of bytes (usually
+/// a hash digest), and returns a sequence of bytes of length
+/// output_size.
+///
+/// It's calculated by rotating through the bytes in the output buffer
+/// (zero- initialized), and XOR'ing with each byte of the passed
+/// input. It consumes 1 byte at a time, and XOR's it with the current
+/// value in the output buffer.
+///
+/// This mimics equivalent functionality in C++ Nix.
+pub fn compress_hash(input: &[u8], output_size: usize) -> Vec<u8> {
+    let mut output: Vec<u8> = vec![0; output_size];
+
+    for (ii, ch) in input.iter().enumerate() {
+        output[ii % output_size] ^= ch;
+    }
+
+    output
+}
+
+/// This builds a store path, by calculating the text_hash_string of either a
+/// derivation or a literal text file that may contain references.
+pub fn build_store_path_from_references<
+    S: AsRef<str>,
+    I: IntoIterator<Item = S>,
+    C: AsRef<[u8]>,
+>(
+    name: &str,
+    content: C,
+    references: I,
+) -> Result<StorePath, Error> {
+    let text_hash_str = text_hash_string(name, content, references);
+    build_store_path_from_fingerprint(name, &text_hash_str)
+}
+
+/// This builds a store path from a fingerprint.
+/// Usually, that function is used from [build_store_path_from_references] and
+/// passed a "text hash string" (starting with "text:" as fingerprint),
+/// but other fingerprints starting with "output:" are also used in Derivation
+/// output path calculation.
+///
+/// The fingerprint is hashed with sha256, its digest is compressed to 20 bytes,
+/// and nixbase32-encoded (32 characters).
+pub fn build_store_path_from_fingerprint(
+    name: &str,
+    fingerprint: &str,
+) -> Result<StorePath, Error> {
+    let digest = {
+        let hasher = Sha256::new_with_prefix(fingerprint);
+        hasher.finalize()
+    };
+    let compressed = compress_hash(&digest, 20);
+    StorePath::from_string(format!("{}-{}", nixbase32::encode(&compressed), name).as_str())
+}
+
+/// Nix placeholders (i.e. values returned by `builtins.placeholder`)
+/// are used to populate outputs with paths that must be
+/// string-replaced with the actual placeholders later, at runtime.
+///
+/// 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()
+    };
+
+    format!("/{}", nixbase32::encode(&digest))
+}
+
+#[cfg(test)]
+mod test {
+    use crate::store_path::build_store_path_from_references;
+
+    #[test]
+    fn build_store_path_with_zero_references() {
+        // This hash should match `builtins.toFile`, e.g.:
+        //
+        // nix-repl> builtins.toFile "foo" "bar"
+        // "/nix/store/vxjiwkjkn7x4079qvh1jkl5pn05j2aw0-foo"
+
+        let store_path = build_store_path_from_references("foo", "bar", Vec::<String>::new())
+            .expect("build_store_path() should succeed");
+
+        assert_eq!(
+            store_path.to_absolute_path().as_str(),
+            "/nix/store/vxjiwkjkn7x4079qvh1jkl5pn05j2aw0-foo"
+        );
+    }
+
+    #[test]
+    fn build_store_path_with_non_zero_references() {
+        // This hash should match:
+        //
+        // nix-repl> builtins.toFile "baz" "${builtins.toFile "foo" "bar"}"
+        // "/nix/store/5xd714cbfnkz02h2vbsj4fm03x3f15nf-baz"
+
+        let inner = build_store_path_from_references("foo", "bar", Vec::<String>::new())
+            .expect("path_with_references() should succeed");
+        let inner_path = inner.to_absolute_path();
+
+        let outer = build_store_path_from_references("baz", &inner_path, vec![inner_path.as_str()])
+            .expect("path_with_references() should succeed");
+
+        assert_eq!(
+            outer.to_absolute_path().as_str(),
+            "/nix/store/5xd714cbfnkz02h2vbsj4fm03x3f15nf-baz"
+        );
+    }
+}