1 files changed, 289 insertions, 0 deletions
diff --git a/tvix/nix-compat/src/store_path/utils.rs b/tvix/nix-compat/src/store_path/utils.rs
new file mode 100644
index 000000000000..fd3785568bac
--- /dev/null
+++ b/tvix/nix-compat/src/store_path/utils.rs
@@ -0,0 +1,289 @@
+use crate::nixbase32;
+use crate::nixhash::{HashAlgo, NixHash, NixHashWithMode};
+use crate::store_path::StorePath;
+use sha2::{Digest, Sha256};
+use thiserror::Error;
+
+use super::{NameError, STORE_DIR};
+
+/// Errors that can occur when creating a content-addressed store path.
+///
+/// This wraps the main [Error] which is just about invalid store path names.
+#[derive(Debug, PartialEq, Eq, Error)]
+pub enum BuildStorePathError {
+    #[error("{0}")]
+    InvalidName(NameError),
+    /// This error occurs when we have references outside the SHA-256 +
+    /// Recursive case. The restriction comes from upstream Nix. It may be
+    /// lifted at some point but there isn't a pressing need to anticipate that.
+    #[error("References were not supported as much as requested")]
+    InvalidReference(),
+}
+
+/// 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<const OUTPUT_SIZE: usize>(input: &[u8]) -> [u8; OUTPUT_SIZE] {
+    let mut output = [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_text_path<S: AsRef<str>, I: IntoIterator<Item = S>, C: AsRef<[u8]>>(
+    name: &str,
+    content: C,
+    references: I,
+) -> Result<StorePath, NameError> {
+    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()
+            };
+            NixHash::new(crate::nixhash::HashAlgo::Sha256, content_digest.to_vec())
+        },
+        name,
+    )
+}
+
+/// 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,
+    references: I,
+    self_reference: bool,
+) -> Result<StorePath, BuildStorePathError> {
+    match &hash_with_mode {
+        NixHashWithMode::Recursive(
+            ref hash @ NixHash {
+                algo: HashAlgo::Sha256,
+                ..
+            },
+        ) => build_store_path_from_fingerprint_parts(
+            &make_type("source", references, self_reference),
+            hash,
+            name,
+        )
+        .map_err(BuildStorePathError::InvalidName),
+        _ => {
+            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",
+                &{
+                    let content_digest = {
+                        let mut hasher = Sha256::new_with_prefix("fixed:out:");
+                        hasher.update(hash_with_mode.mode().prefix());
+                        hasher.update(hash_with_mode.digest().algo.to_string());
+                        hasher.update(":");
+                        hasher.update(
+                            &data_encoding::HEXLOWER.encode(&hash_with_mode.digest().digest),
+                        );
+                        hasher.update(":");
+                        hasher.finalize()
+                    };
+                    NixHash::new(crate::nixhash::HashAlgo::Sha256, content_digest.to_vec())
+                },
+                name,
+            )
+            .map_err(BuildStorePathError::InvalidName)
+        }
+    }
+}
+
+/// 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.
+pub fn build_output_path(
+    drv_hash: &NixHash,
+    output_name: &str,
+    output_path_name: &str,
+) -> Result<StorePath, NameError> {
+    build_store_path_from_fingerprint_parts(
+        &(String::from("output:") + output_name),
+        drv_hash,
+        output_path_name,
+    )
+}
+
+/// This builds a store path from fingerprint parts.
+/// Usually, that function is used from [build_text_path] 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).
+fn build_store_path_from_fingerprint_parts(
+    ty: &str,
+    hash: &NixHash,
+    name: &str,
+) -> Result<StorePath, NameError> {
+    let fingerprint =
+        String::from(ty) + ":" + &hash.to_nix_hash_string() + ":" + STORE_DIR + ":" + name;
+    let digest = {
+        let hasher = Sha256::new_with_prefix(fingerprint);
+        hasher.finalize()
+    };
+    let compressed = compress_hash::<20>(&digest);
+    StorePath::validate_name(name)?;
+    Ok(StorePath {
+        digest: compressed,
+        name: name.to_string(),
+    })
+}
+
+/// This contains the Nix logic to create "text hash strings", which are used
+/// in `builtins.toFile`, as well as in Derivation Path calculation.
+///
+/// A text hash is calculated by concatenating the following fields, separated by a `:`:
+///
+///  - text
+///  - references, individually joined by `:`
+///  - 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>>(
+    ty: &str,
+    references: I,
+    self_ref: bool,
+) -> String {
+    let mut s = String::from(ty);
+
+    for reference in references {
+        s.push(':');
+        s.push_str(reference.as_ref());
+    }
+
+    if self_ref {
+        s.push_str(":self");
+    }
+
+    s
+}
+
+/// 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 super::*;
+    use crate::nixhash::{NixHash, NixHashWithMode};
+
+    #[test]
+    fn build_text_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_text_path("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_text_path_with_non_zero_references() {
+        // This hash should match:
+        //
+        // nix-repl> builtins.toFile "baz" "${builtins.toFile "foo" "bar"}"
+        // "/nix/store/5xd714cbfnkz02h2vbsj4fm03x3f15nf-baz"
+
+        let inner = build_text_path("foo", "bar", Vec::<String>::new())
+            .expect("path_with_references() should succeed");
+        let inner_path = inner.to_absolute_path();
+
+        let outer = build_text_path("baz", &inner_path, vec![inner_path.as_str()])
+            .expect("path_with_references() should succeed");
+
+        assert_eq!(
+            outer.to_absolute_path().as_str(),
+            "/nix/store/5xd714cbfnkz02h2vbsj4fm03x3f15nf-baz"
+        );
+    }
+
+    #[test]
+    fn build_sha1_path() {
+        let outer = build_regular_ca_path(
+            "bar",
+            &NixHashWithMode::Recursive(NixHash {
+                algo: HashAlgo::Sha1,
+                digest: data_encoding::HEXLOWER
+                    .decode(b"0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33")
+                    .expect("hex should decode"),
+            }),
+            Vec::<String>::new(),
+            false,
+        )
+        .expect("path_with_references() should succeed");
+
+        assert_eq!(
+            outer.to_absolute_path().as_str(),
+            "/nix/store/mp57d33657rf34lzvlbpfa1gjfv5gmpg-bar"
+        );
+    }
+
+    #[test]
+    fn build_store_path_with_non_zero_references() {
+        // This hash should match:
+        //
+        // nix-repl> builtins.toFile "baz" "${builtins.toFile "foo" "bar"}"
+        // "/nix/store/5xd714cbfnkz02h2vbsj4fm03x3f15nf-baz"
+        //
+        // $ 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(
+            "baz",
+            &NixHashWithMode::Recursive(NixHash {
+                algo: HashAlgo::Sha256,
+                digest: nixbase32::decode(b"1xqkzcb3909fp07qngljr4wcdnrh1gdam1m2n29i6hhrxlmkgkv1")
+                    .expect("hex should decode"),
+            }),
+            vec!["/nix/store/dxwkwjzdaq7ka55pkk252gh32bgpmql4-foo"],
+            false,
+        )
+        .expect("path_with_references() should succeed");
+
+        assert_eq!(
+            outer.to_absolute_path().as_str(),
+            "/nix/store/s89y431zzhmdn3k8r96rvakryddkpv2v-baz"
+        );
+    }
+}