5 files changed, 351 insertions, 0 deletions
diff --git a/tvix/store/src/client.rs b/tvix/store/src/client.rs
new file mode 100644
index 000000000000..3b282eacdd70
--- /dev/null
+++ b/tvix/store/src/client.rs
@@ -0,0 +1,10 @@
+use crate::proto::Directory;
+
+pub trait StoreClient {
+    fn open_blob(&self, digest: Vec<u8>) -> std::io::Result<Box<dyn std::io::BufRead>>;
+
+    // TODO: stat_blob, put_blob?
+    fn get_directory(&self, digest: Vec<u8>) -> std::io::Result<Option<Directory>>;
+
+    // TODO: put_directory
+}
diff --git a/tvix/store/src/lib.rs b/tvix/store/src/lib.rs
index 2c7f4887ce9f..a9e2382eaa1e 100644
--- a/tvix/store/src/lib.rs
+++ b/tvix/store/src/lib.rs
@@ -1,8 +1,11 @@
+pub mod client;
 pub mod proto;
 
 pub mod dummy_blob_service;
 pub mod sled_directory_service;
 pub mod sled_path_info_service;
 
+mod nar;
+
 #[cfg(test)]
 mod tests;
diff --git a/tvix/store/src/nar.rs b/tvix/store/src/nar.rs
new file mode 100644
index 000000000000..efcaf652e138
--- /dev/null
+++ b/tvix/store/src/nar.rs
@@ -0,0 +1,70 @@
+//! This provides some common "client-side" libraries to interact with a tvix-
+//! store, in this case to render NAR.
+use crate::{
+    client::StoreClient,
+    proto::{self, NamedNode},
+};
+use anyhow::Result;
+use nix_compat::nar;
+
+/// Consumes a [proto::node::Node] pointing to the root of a (store) path,
+/// and writes the contents in NAR serialization to the passed
+/// [std::io::Write].
+///
+/// It uses a [StoreClient] to do the necessary lookups as it traverses the
+/// structure.
+pub fn write_nar<W: std::io::Write, SC: StoreClient>(
+    w: &mut W,
+    proto_root_node: proto::node::Node,
+    store_client: &mut SC,
+) -> Result<()> {
+    // Initialize NAR writer
+    let nar_root_node = nar::writer::open(w)?;
+
+    walk_node(nar_root_node, proto_root_node, store_client)
+}
+
+/// Process an intermediate node in the structure.
+/// This consumes the node.
+fn walk_node<SC: StoreClient>(
+    nar_node: nar::writer::Node,
+    proto_node: proto::node::Node,
+    store_client: &mut SC,
+) -> Result<()> {
+    match proto_node {
+        proto::node::Node::Symlink(proto_symlink_node) => {
+            nar_node.symlink(&proto_symlink_node.target)?;
+        }
+        proto::node::Node::File(proto_file_node) => {
+            nar_node.file(
+                proto_file_node.executable,
+                proto_file_node.size.into(),
+                &mut store_client.open_blob(proto_file_node.digest)?,
+            )?;
+        }
+        proto::node::Node::Directory(proto_directory_node) => {
+            // look up that node from the store client
+            let proto_directory = store_client.get_directory(proto_directory_node.digest)?;
+
+            // if it's None, that's an error!
+            if proto_directory.is_none() {
+                // TODO: proper error handling
+                panic!("not found!")
+            }
+
+            // start a directory node
+            let mut nar_node_directory = nar_node.directory()?;
+
+            // for each node in the directory, create a new entry with its name,
+            // and then invoke walk_node on that entry.
+            for proto_node in proto_directory.unwrap().nodes() {
+                let child_node = nar_node_directory.entry(proto_node.get_name())?;
+                walk_node(child_node, proto_node, store_client)?;
+            }
+
+            // close the directory
+            nar_node_directory.close()?;
+        }
+    }
+    Ok(())
+}
diff --git a/tvix/store/src/tests/mod.rs b/tvix/store/src/tests/mod.rs
index 4022c329798b..57ae1df9f6ff 100644
--- a/tvix/store/src/tests/mod.rs
+++ b/tvix/store/src/tests/mod.rs
@@ -1,5 +1,6 @@
 mod directory;
 mod directory_nodes_iterator;
 mod directory_service;
+mod nar;
 mod path_info_service;
 mod pathinfo;
diff --git a/tvix/store/src/tests/nar.rs b/tvix/store/src/tests/nar.rs
new file mode 100644
index 000000000000..5a865f52b492
--- /dev/null
+++ b/tvix/store/src/tests/nar.rs
@@ -0,0 +1,267 @@
+use data_encoding::BASE64;
+
+use crate::client::StoreClient;
+use crate::nar::write_nar;
+use crate::proto;
+use crate::proto::DirectoryNode;
+use crate::proto::FileNode;
+use crate::proto::SymlinkNode;
+use lazy_static::lazy_static;
+
+const HELLOWORLD_BLOB_CONTENTS: &[u8] = b"Hello World!";
+const EMPTY_BLOB_CONTENTS: &[u8] = b"";
+
+lazy_static! {
+    static ref HELLOWORLD_BLOB_DIGEST: Vec<u8> =
+        blake3::hash(HELLOWORLD_BLOB_CONTENTS).as_bytes().to_vec();
+    static ref EMPTY_BLOB_DIGEST: Vec<u8> = blake3::hash(EMPTY_BLOB_CONTENTS).as_bytes().to_vec();
+    static ref DIRECTORY_WITH_KEEP: proto::Directory = proto::Directory {
+        directories: vec![],
+        files: vec![FileNode {
+            name: ".keep".to_string(),
+            digest: EMPTY_BLOB_DIGEST.to_vec(),
+            size: 0,
+            executable: false,
+        }],
+        symlinks: vec![],
+    };
+    static ref DIRECTORY_COMPLICATED: proto::Directory = proto::Directory {
+        directories: vec![DirectoryNode {
+            name: "keep".to_string(),
+            digest: DIRECTORY_WITH_KEEP.digest(),
+            size: DIRECTORY_WITH_KEEP.size(),
+        }],
+        files: vec![FileNode {
+            name: ".keep".to_string(),
+            digest: EMPTY_BLOB_DIGEST.to_vec(),
+            size: 0,
+            executable: false,
+        }],
+        symlinks: vec![SymlinkNode {
+            name: "aa".to_string(),
+            target: "/nix/store/somewhereelse".to_string(),
+        }],
+    };
+}
+
+/// A Store client that fails if you ask it for a blob or a directory
+#[derive(Default)]
+struct FailingStoreClient {}
+
+impl StoreClient for FailingStoreClient {
+    fn open_blob(&self, digest: Vec<u8>) -> std::io::Result<Box<dyn std::io::BufRead>> {
+        panic!(
+            "open_blob should never be called, but was called with {}",
+            BASE64.encode(&digest),
+        );
+    }
+
+    fn get_directory(&self, digest: Vec<u8>) -> std::io::Result<Option<proto::Directory>> {
+        panic!(
+            "get_directory should never be called, but was called with {}",
+            BASE64.encode(&digest),
+        );
+    }
+}
+
+/// Only allow a request for a blob with [HELLOWORLD_BLOB_DIGEST]
+/// panic on everything else.
+#[derive(Default)]
+struct HelloWorldBlobStoreClient {}
+
+impl StoreClient for HelloWorldBlobStoreClient {
+    fn open_blob(&self, digest: Vec<u8>) -> std::io::Result<Box<dyn std::io::BufRead>> {
+        if digest != HELLOWORLD_BLOB_DIGEST.to_vec() {
+            panic!("open_blob called with {}", BASE64.encode(&digest));
+        }
+
+        let b: Box<&[u8]> = Box::new(&HELLOWORLD_BLOB_CONTENTS);
+
+        Ok(b)
+    }
+
+    fn get_directory(&self, digest: Vec<u8>) -> std::io::Result<Option<proto::Directory>> {
+        panic!(
+            "get_directory should never be called, but was called with {}",
+            BASE64.encode(&digest),
+        );
+    }
+}
+
+/// Allow blob requests for [HELLOWORLD_BLOB_DIGEST] and EMPTY_BLOB_DIGEST, and
+/// allow DIRECTORY_WITH_KEEP and DIRECTORY_COMPLICATED.
+#[derive(Default)]
+struct SomeDirectoryStoreClient {}
+
+impl StoreClient for SomeDirectoryStoreClient {
+    fn open_blob(&self, digest: Vec<u8>) -> std::io::Result<Box<dyn std::io::BufRead>> {
+        if digest == HELLOWORLD_BLOB_DIGEST.to_vec() {
+            let b: Box<&[u8]> = Box::new(&HELLOWORLD_BLOB_CONTENTS);
+            return Ok(b);
+        }
+        if digest == EMPTY_BLOB_DIGEST.to_vec() {
+            let b: Box<&[u8]> = Box::new(&EMPTY_BLOB_CONTENTS);
+            return Ok(b);
+        }
+        panic!("open_blob called with {}", BASE64.encode(&digest));
+    }
+
+    fn get_directory(&self, digest: Vec<u8>) -> std::io::Result<Option<proto::Directory>> {
+        if digest == DIRECTORY_WITH_KEEP.digest() {
+            return Ok(Some(DIRECTORY_WITH_KEEP.clone()));
+        }
+        if digest == DIRECTORY_COMPLICATED.digest() {
+            return Ok(Some(DIRECTORY_COMPLICATED.clone()));
+        }
+        panic!("get_directory called with {}", BASE64.encode(&digest));
+    }
+}
+
+#[tokio::test]
+async fn single_symlink() -> anyhow::Result<()> {
+    let mut buf: Vec<u8> = vec![];
+    let mut store_client = FailingStoreClient::default();
+
+    write_nar(
+        &mut buf,
+        crate::proto::node::Node::Symlink(SymlinkNode {
+            name: "doesntmatter".to_string(),
+            target: "/nix/store/somewhereelse".to_string(),
+        }),
+        &mut store_client,
+    )
+    .expect("must succeed");
+
+    assert_eq!(
+        buf,
+        vec![
+            13, 0, 0, 0, 0, 0, 0, 0, 110, 105, 120, 45, 97, 114, 99, 104, 105, 118, 101, 45, 49, 0,
+            0, 0, // "nix-archive-1"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 116, 121, 112, 101, 0, 0, 0, 0, // "type"
+            7, 0, 0, 0, 0, 0, 0, 0, 115, 121, 109, 108, 105, 110, 107, 0, // "symlink"
+            6, 0, 0, 0, 0, 0, 0, 0, 116, 97, 114, 103, 101, 116, 0, 0, // target
+            24, 0, 0, 0, 0, 0, 0, 0, 47, 110, 105, 120, 47, 115, 116, 111, 114, 101, 47, 115, 111,
+            109, 101, 119, 104, 101, 114, 101, 101, 108, 115,
+            101, // "/nix/store/somewhereelse"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0 // ")"
+        ]
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn single_file() -> anyhow::Result<()> {
+    let mut buf: Vec<u8> = vec![];
+    let mut store_client = HelloWorldBlobStoreClient::default();
+
+    write_nar(
+        &mut buf,
+        crate::proto::node::Node::File(FileNode {
+            name: "doesntmatter".to_string(),
+            digest: HELLOWORLD_BLOB_DIGEST.to_vec(),
+            size: HELLOWORLD_BLOB_CONTENTS.len() as u32,
+            executable: false,
+        }),
+        &mut store_client,
+    )
+    .expect("must succeed");
+
+    assert_eq!(
+        buf,
+        vec![
+            13, 0, 0, 0, 0, 0, 0, 0, 110, 105, 120, 45, 97, 114, 99, 104, 105, 118, 101, 45, 49, 0,
+            0, 0, // "nix-archive-1"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 116, 121, 112, 101, 0, 0, 0, 0, // "type"
+            7, 0, 0, 0, 0, 0, 0, 0, 114, 101, 103, 117, 108, 97, 114, 0, // "regular"
+            8, 0, 0, 0, 0, 0, 0, 0, 99, 111, 110, 116, 101, 110, 116, 115, // "contents"
+            12, 0, 0, 0, 0, 0, 0, 0, 72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33, 0, 0,
+            0, 0, // "Hello World!"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0 // ")"
+        ]
+    );
+
+    Ok(())
+}
+
+#[tokio::test]
+async fn test_complicated() -> anyhow::Result<()> {
+    let mut buf: Vec<u8> = vec![];
+    let mut store_client = SomeDirectoryStoreClient::default();
+
+    write_nar(
+        &mut buf,
+        crate::proto::node::Node::Directory(DirectoryNode {
+            name: "doesntmatter".to_string(),
+            digest: DIRECTORY_COMPLICATED.digest(),
+            size: DIRECTORY_COMPLICATED.size() as u32,
+        }),
+        &mut store_client,
+    )
+    .expect("must succeed");
+
+    assert_eq!(
+        buf,
+        vec![
+            13, 0, 0, 0, 0, 0, 0, 0, 110, 105, 120, 45, 97, 114, 99, 104, 105, 118, 101, 45, 49, 0,
+            0, 0, // "nix-archive-1"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 116, 121, 112, 101, 0, 0, 0, 0, // "type"
+            9, 0, 0, 0, 0, 0, 0, 0, 100, 105, 114, 101, 99, 116, 111, 114, 121, 0, 0, 0, 0, 0, 0,
+            0, // "directory"
+            5, 0, 0, 0, 0, 0, 0, 0, 101, 110, 116, 114, 121, 0, 0, 0, // "entry"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 110, 97, 109, 101, 0, 0, 0, 0, // "name"
+            5, 0, 0, 0, 0, 0, 0, 0, 46, 107, 101, 101, 112, 0, 0, 0, // ".keep"
+            4, 0, 0, 0, 0, 0, 0, 0, 110, 111, 100, 101, 0, 0, 0, 0, // "node"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 116, 121, 112, 101, 0, 0, 0, 0, // "type"
+            7, 0, 0, 0, 0, 0, 0, 0, 114, 101, 103, 117, 108, 97, 114, 0, // "regular"
+            8, 0, 0, 0, 0, 0, 0, 0, 99, 111, 110, 116, 101, 110, 116, 115, 0, 0, 0, 0, 0, 0, 0,
+            0, // "contents"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0, // ")"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0, // ")"
+            5, 0, 0, 0, 0, 0, 0, 0, 101, 110, 116, 114, 121, 0, 0, 0, // "entry"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 110, 97, 109, 101, 0, 0, 0, 0, // "name"
+            2, 0, 0, 0, 0, 0, 0, 0, 97, 97, 0, 0, 0, 0, 0, 0, // "aa"
+            4, 0, 0, 0, 0, 0, 0, 0, 110, 111, 100, 101, 0, 0, 0, 0, // "node"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 116, 121, 112, 101, 0, 0, 0, 0, // "type"
+            7, 0, 0, 0, 0, 0, 0, 0, 115, 121, 109, 108, 105, 110, 107, 0, // "symlink"
+            6, 0, 0, 0, 0, 0, 0, 0, 116, 97, 114, 103, 101, 116, 0, 0, // "target"
+            24, 0, 0, 0, 0, 0, 0, 0, 47, 110, 105, 120, 47, 115, 116, 111, 114, 101, 47, 115, 111,
+            109, 101, 119, 104, 101, 114, 101, 101, 108, 115,
+            101, //  "/nix/store/somewhereelse"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0, // ")"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0, // ")"
+            5, 0, 0, 0, 0, 0, 0, 0, 101, 110, 116, 114, 121, 0, 0, 0, // "entry"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 110, 97, 109, 101, 0, 0, 0, 0, // "name"
+            4, 0, 0, 0, 0, 0, 0, 0, 107, 101, 101, 112, 0, 0, 0, 0, // "keep"
+            4, 0, 0, 0, 0, 0, 0, 0, 110, 111, 100, 101, 0, 0, 0, 0, // "node"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 116, 121, 112, 101, 0, 0, 0, 0, // "type"
+            9, 0, 0, 0, 0, 0, 0, 0, 100, 105, 114, 101, 99, 116, 111, 114, 121, 0, 0, 0, 0, 0, 0,
+            0, // "directory"
+            5, 0, 0, 0, 0, 0, 0, 0, 101, 110, 116, 114, 121, 0, 0, 0, // "entry"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 110, 97, 109, 101, 0, 0, 0, 0, // "name"
+            5, 0, 0, 0, 0, 0, 0, 0, 46, 107, 101, 101, 112, 0, 0, 0, // ".keep"
+            4, 0, 0, 0, 0, 0, 0, 0, 110, 111, 100, 101, 0, 0, 0, 0, // "node"
+            1, 0, 0, 0, 0, 0, 0, 0, 40, 0, 0, 0, 0, 0, 0, 0, // "("
+            4, 0, 0, 0, 0, 0, 0, 0, 116, 121, 112, 101, 0, 0, 0, 0, // "type"
+            7, 0, 0, 0, 0, 0, 0, 0, 114, 101, 103, 117, 108, 97, 114, 0, // "regular"
+            8, 0, 0, 0, 0, 0, 0, 0, 99, 111, 110, 116, 101, 110, 116, 115, 0, 0, 0, 0, 0, 0, 0,
+            0, // "contents"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0, // ")"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0, // ")"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0, // ")"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0, // ")"
+            1, 0, 0, 0, 0, 0, 0, 0, 41, 0, 0, 0, 0, 0, 0, 0 // ")"
+        ]
+    );
+
+    Ok(())
+}