feat(tvix/store): add write_nar function r/5794

This adds a function that 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].

We need this in various places:

 - tvix-store's calculate_nar() RPC method needs to render a NAR stream
   to get the nar hash, which is necessary to give things imported in
   the store a "NAR-based" store path.

 - communication with (remote) Nix (via daemon protocol) needs a NAR
   representation.

 - Things like nar-bridge, exposing a NAR/NARInfo HTTP interface need a
   NAR representation.

Change-Id: I7fb2e0bf01814a1c09094c0e35394d9d6b3e43b6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7956
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 10 insertions, 0 deletions

diff --git a/tvix/store/src/client.rs b/tvix/store/src/client.rs
new file mode 100644
index 0000000000..3b282eacdd
--- /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
+}