some progress at an attempt

src/bindings/mod.rs

@@ -211,6 +211,13 @@ mod ffi {
         /// Obtains a handle to the Nix store.
         fn open_nix_store() -> Result<UniquePtr<CNixStore>>;
 
+        /// Creates a NAR dump from a path.
+        fn nar_from_path(
+            self: Pin<&mut CNixStore>,
+            base_name: Vec<u8>,
+            sender: Box<AsyncWriteSender>,
+        ) -> Result<()>;
+
         // =========
         // CPathInfo
         // =========

src/bindings/nix.cpp

@@ -108,6 +108,17 @@ std::unique_ptr<std::vector<std::string>> CNixStore::compute_fs_closure(RBasePat
 	return std::make_unique<std::vector<std::string>>(result);
 }
 
+void CNixStore::nar_from_path(RVec<unsigned char> base_name, RBox<AsyncWriteSender> sender) {
+	RustSink sink(std::move(sender));
+
+	std::string_view sv((const char *)base_name.data(), base_name.size());
+	nix::StorePath store_path(sv);
+
+	// exceptions will be thrown into Rust
+	this->store->narFromPath(store_path, sink);
+	sink.eof();
+}
+
 std::unique_ptr<CNixStore> open_nix_store() {
 	return std::make_unique<CNixStore>();
 }

src/bindings/nix.hpp

@@ -79,6 +79,7 @@ public:
 		bool flip_direction,
 		bool include_outputs,
 		bool include_derivers);
+	void nar_from_path(RVec<unsigned char> base_name, RBox<AsyncWriteSender> sender);
 };
 
 std::unique_ptr<CNixStore> open_nix_store();

src/store.rs

@@ -4,7 +4,10 @@ use anyhow::{Context, Result};
 use nix_compat::store_path::StorePath;
 use tokio::task;
 
-use crate::{bindings, path_info::PathInfo};
+use crate::{
+    bindings::{self, AsyncWriteAdapter},
+    path_info::PathInfo,
+};
 
 pub struct Store {
     inner: Arc<bindings::FfiNixStore>,
@@ -75,4 +78,19 @@ impl Store {
         .await
         .unwrap()
     }
+
+    pub fn make_nar(&self, path: StorePath<String>) -> AsyncWriteAdapter {
+        let inner = self.inner.clone();
+        let (adapter, mut sender) = AsyncWriteAdapter::new();
+
+        task::spawn_blocking(move || {
+            if let Err(e) = inner
+                .store()
+                .nar_from_path(path.to_string().as_bytes().to_vec(), sender.clone())
+            {
+                let _ = sender.rust_error(e);
+            }
+        });
+        adapter
+    }
 }

src/uploader.rs

@@ -139,16 +139,6 @@ impl<'a> Uploader<'a> {
         Ok(())
     }
 
-    async fn make_nar(&self) -> Result<Vec<u8>> {
-        Ok(Command::new("nix")
-            .arg("nar")
-            .arg("dump-path")
-            .arg(self.path.absolute_path())
-            .output()
-            .await?
-            .stdout)
-    }
-
     fn narinfo_from_nar(&self, nar: &[u8]) -> Result<NarInfo> {
         let mut hasher = Sha256::new();
         hasher.update(nar);