Implement negative caching for narinfo

Fixes #4.

nix-actions-cache/src/binary_cache.rs

@@ -50,15 +50,23 @@ async fn get_narinfo(
     let store_path_hash = components[0].to_string();
     let key = format!("{}.narinfo", store_path_hash);
 
+    if state
+        .narinfo_nagative_cache
+        .read()
+        .await
+        .contains(&store_path_hash)
+    {
+        return pull_through(&state, &path);
+    }
+
     if let Some(url) = state.api.get_file_url(&[&key]).await? {
         return Ok(Redirect::temporary(&url));
     }
 
-    if let Some(upstream) = &state.upstream {
-        Ok(Redirect::temporary(&format!("{}/{}", upstream, path)))
-    } else {
-        Err(Error::NotFound)
-    }
+    let mut negative_cache = state.narinfo_nagative_cache.write().await;
+    negative_cache.insert(store_path_hash);
+
+    pull_through(&state, &path)
 }
 async fn put_narinfo(
     Extension(state): Extension<State>,
@@ -83,6 +91,12 @@ async fn put_narinfo(
     );
     state.api.upload_file(allocation, stream).await?;
 
+    state
+        .narinfo_nagative_cache
+        .write()
+        .await
+        .remove(&store_path_hash);
+
     Ok(())
 }
 
@@ -110,3 +124,11 @@ async fn put_nar(
 
     Ok(())
 }
+
+fn pull_through(state: &State, path: &str) -> Result<Redirect> {
+    if let Some(upstream) = &state.upstream {
+        Ok(Redirect::temporary(&format!("{}/{}", upstream, path)))
+    } else {
+        Err(Error::NotFound)
+    }
+}

nix-actions-cache/src/main.rs

@@ -30,7 +30,7 @@ use clap::Parser;
 use daemonize::Daemonize;
 use tokio::{
     runtime::Runtime,
-    sync::{oneshot, Mutex},
+    sync::{oneshot, Mutex, RwLock},
 };
 use tracing_subscriber::filter::EnvFilter;
 
@@ -89,6 +89,9 @@ struct StateInner {
 
     /// List of store paths originally present.
     original_paths: Mutex<HashSet<PathBuf>>,
+
+    /// Set of store path hashes that are not present in GHAC.
+    narinfo_nagative_cache: RwLock<HashSet<String>>,
 }
 
 fn main() {
@@ -120,6 +123,7 @@ fn main() {
         upstream: args.upstream.clone(),
         shutdown_sender: Mutex::new(Some(shutdown_sender)),
         original_paths: Mutex::new(HashSet::new()),
+        narinfo_nagative_cache: RwLock::new(HashSet::new()),
     });
 
     let app = Router::new()