chore: comment the why of thread::spawn

angelsharkd/src/routes/extensions/simple_search/mod.rs

@@ -61,8 +61,18 @@ async fn search(
 /// Immediately returns. Spawns an asynchronous task to complete the haystack
 /// refresh in the background.
 async fn refresh(haystack: Haystack) -> Result<impl Reply, Infallible> {
+    // Note: why use `thread::spawn()`? Previously I had used `tokio::spawn()`
+    // here. The difference is that `tokio::spawn()` spawns the task in a thread
+    // from Tokio's dedicated threadpool. This means that this thread is no
+    // longer available to answer requests from the HTTP server. This
+    // effectively meant that during a refresh, large blocking portions of code
+    // were preventing people from running searches, and they would receive 504
+    // Gateway Timeouts from the reverse proxy. Using a plain `thread::spawn()`,
+    // we ensure that a thread outside of Tokio's or Rayon's threadpools.
+    // Therefore, searches can still happen during even the most intensive of
+    // refreshes.
+
     // Run refresh as a background task and immediately return.
-    // TODO: notes on why this is the way it is
     thread::spawn(move || {
         if let Err(e) = haystack.refresh() {
             error!("{}", e.to_string());