2 files changed, 106 insertions, 2 deletions
diff --git a/tvix/store/src/proto/mod.rs b/tvix/store/src/proto/mod.rs
index b6d0f35bfc56..fb8e172a6c37 100644
--- a/tvix/store/src/proto/mod.rs
+++ b/tvix/store/src/proto/mod.rs
@@ -3,8 +3,9 @@ use bytes::Bytes;
 use data_encoding::BASE64;
 // https://github.com/hyperium/tonic/issues/1056
 use nix_compat::{
+    narinfo::Flags,
     nixhash::{CAHash, NixHash},
-    store_path,
+    store_path::{self, StorePathRef},
 };
 use thiserror::Error;
 use tvix_castore::proto::{self as castorepb, NamedNode, ValidateNodeError};
@@ -174,6 +175,67 @@ impl PathInfo {
         // return the root nix path
         Ok(root_nix_path)
     }
+
+    /// With self and a given StorePathRef, this reconstructs a
+    /// [nix_compat::narinfo::NarInfo<'_>].
+    /// It can be used to validate Signatures, or get back a (sparse) NarInfo
+    /// struct to prepare writing it out.
+    ///
+    /// This doesn't allocate any new data.
+    ///
+    /// Keep in mind this is not able to reconstruct all data present in the
+    /// NarInfo<'_>, as some of it is not stored at all:
+    /// - the `system`, `file_hash` and `file_size` fields are set to `None`.
+    /// - the URL is set to an empty string.
+    /// - Compression is set to "none"
+    ///
+    /// If you want to render it out to a string and be able to parse it back
+    /// in, at least URL *must* be set again.
+    pub fn as_narinfo<'a>(
+        &'a self,
+        store_path: store_path::StorePathRef<'a>,
+    ) -> Option<nix_compat::narinfo::NarInfo<'_>> {
+        let narinfo = &self.narinfo.as_ref()?;
+
+        Some(nix_compat::narinfo::NarInfo {
+            flags: Flags::empty(),
+            store_path,
+            nar_hash: narinfo.nar_sha256.to_vec().try_into().unwrap(),
+            nar_size: narinfo.nar_size,
+            references: narinfo
+                .reference_names
+                .iter()
+                .map(|ref_name| {
+                    // This shouldn't pass validation
+                    StorePathRef::from_bytes(ref_name.as_bytes()).expect("invalid reference")
+                })
+                .collect(),
+            signatures: narinfo
+                .signatures
+                .iter()
+                .map(|sig| {
+                    nix_compat::narinfo::Signature::new(
+                        &sig.name,
+                        // This shouldn't pass validation
+                        sig.data[..].try_into().expect("invalid signature len"),
+                    )
+                })
+                .collect(),
+            ca: narinfo
+                .ca
+                .as_ref()
+                .map(|ca| ca.try_into().expect("invalid ca")),
+            system: None,
+            deriver: narinfo.deriver.as_ref().map(|deriver| {
+                StorePathRef::from_name_and_digest(&deriver.name, &deriver.digest)
+                    .expect("invalid deriver")
+            }),
+            url: "",
+            compression: Some("none"),
+            file_hash: None,
+            file_size: None,
+        })
+    }
 }
 
 /// Errors that can occur when converting from a [&nar_info::Ca] to a (stricter)
diff --git a/tvix/store/src/proto/tests/pathinfo.rs b/tvix/store/src/proto/tests/pathinfo.rs
index 31f4790b73ee..7c1b69c2a376 100644
--- a/tvix/store/src/proto/tests/pathinfo.rs
+++ b/tvix/store/src/proto/tests/pathinfo.rs
@@ -3,7 +3,7 @@ use crate::tests::fixtures::*;
 use bytes::Bytes;
 use data_encoding::BASE64;
 use nix_compat::nixbase32;
-use nix_compat::store_path::{self, StorePath};
+use nix_compat::store_path::{self, StorePath, StorePathRef};
 use std::str::FromStr;
 use test_case::test_case;
 use tvix_castore::proto as castorepb;
@@ -404,3 +404,45 @@ CA: fixed:sha256:086vqwk2wl8zfs47sq2xpjc9k066ilmb8z6dn0q6ymwjzlm196cd"#
         (&narinfo_parsed).into()
     );
 }
+
+/// Exercise .as_narinfo() on a PathInfo and ensure important fields are preserved..
+#[test]
+fn as_narinfo() {
+    let narinfo_parsed = nix_compat::narinfo::NarInfo::parse(
+        r#"StorePath: /nix/store/pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz
+URL: nar/1zjrhzhaizsrlsvdkqfl073vivmxcqnzkff4s50i0cdf541ary1r.nar.xz
+Compression: xz
+FileHash: sha256:1zjrhzhaizsrlsvdkqfl073vivmxcqnzkff4s50i0cdf541ary1r
+FileSize: 1033524
+NarHash: sha256:1lvqpbk2k1sb39z8jfxixf7p7v8sj4z6mmpa44nnmff3w1y6h8lh
+NarSize: 1033416
+References: 
+Deriver: dyivpmlaq2km6c11i0s6bi6mbsx0ylqf-hello-2.12.1.tar.gz.drv
+Sig: cache.nixos.org-1:ywnIG629nQZQhEr6/HLDrLT/mUEp5J1LC6NmWSlJRWL/nM7oGItJQUYWGLvYGhSQvHrhIuvMpjNmBNh/WWqCDg==
+CA: fixed:sha256:086vqwk2wl8zfs47sq2xpjc9k066ilmb8z6dn0q6ymwjzlm196cd"#
+    ).expect("must parse");
+
+    let path_info: PathInfo = (&narinfo_parsed).into();
+
+    let mut narinfo_returned = path_info
+        .as_narinfo(
+            StorePathRef::from_bytes(b"pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz")
+                .expect("invalid storepath"),
+        )
+        .expect("must be some");
+    narinfo_returned.url = "some.nar";
+
+    assert_eq!(
+        r#"StorePath: /nix/store/pa10z4ngm0g83kx9mssrqzz30s84vq7k-hello-2.12.1.tar.gz
+URL: some.nar
+Compression: none
+NarHash: sha256:1lvqpbk2k1sb39z8jfxixf7p7v8sj4z6mmpa44nnmff3w1y6h8lh
+NarSize: 1033416
+References: 
+Deriver: dyivpmlaq2km6c11i0s6bi6mbsx0ylqf-hello-2.12.1.tar.gz.drv
+Sig: cache.nixos.org-1:ywnIG629nQZQhEr6/HLDrLT/mUEp5J1LC6NmWSlJRWL/nM7oGItJQUYWGLvYGhSQvHrhIuvMpjNmBNh/WWqCDg==
+CA: fixed:sha256:086vqwk2wl8zfs47sq2xpjc9k066ilmb8z6dn0q6ymwjzlm196cd
+"#,
+        narinfo_returned.to_string(),
+    );
+}