Keep the recall future Send: the keep predicate must be Sync

crates/tinymemory-tools/src/recall/gather.rs

@@ -236,7 +236,7 @@ async fn with_listed_beliefs(
     engine: &dyn MemoryEngine,
     section: &ScopeSection,
     want: usize,
-    keep: &dyn Fn(&Hit) -> bool,
+    keep: &(dyn Fn(&Hit) -> bool + Sync),
 ) -> tinymemory_api::Result<(Vec<Hit>, Vec<Hit>)> {
     if !reads_learnings(section) {
         return Ok((
@@ -324,7 +324,7 @@ async fn fetch(
     query: &str,
     limit: usize,
     beliefs: usize,
-    keep: &dyn Fn(&Hit) -> bool,
+    keep: &(dyn Fn(&Hit) -> bool + Sync),
 ) -> tinymemory_api::Result<(Vec<Hit>, Vec<Hit>)> {
     let Some(mode) = preferred_mode(engine) else {
         return Ok((latest(engine, filter, limit, keep).await?, Vec::new()));
@@ -347,7 +347,7 @@ async fn latest(
     engine: &dyn MemoryEngine,
     filter: &MetaFilter,
     limit: usize,
-    keep: &dyn Fn(&Hit) -> bool,
+    keep: &(dyn Fn(&Hit) -> bool + Sync),
 ) -> tinymemory_api::Result<Vec<Hit>> {
     let mut all: Vec<Hit> = Vec::new();
     let mut cursor: Option<String> = None;

crates/tinymemory-tools/src/recall/mod_tests.rs

@@ -615,3 +615,19 @@ async fn a_belief_the_filter_rules_out_is_left_out() {
     .unwrap();
     assert!(!pack.markdown.contains("pnpm"), "{}", pack.markdown);
 }
+
+/// Hosts run recall on multi-threaded runtimes (a spawned pre-turn, an
+/// `async_trait` method), which need its future to be `Send`. Every
+/// reference it holds across an `.await` must therefore be `Sync`,
+/// including the `keep` predicate the gathering passes down.
+#[test]
+fn a_recall_future_can_cross_threads() {
+    fn send<T: Send>(_: &T) {}
+    let engine = ReferenceEngine::new();
+    let request = HolisticRecall::new(
+        Some("refunds".into()),
+        vec![ScopeSection::fetch("Docs", docs(), 5)],
+    );
+    let future = holistic_recall(&engine, &request);
+    send(&future);
+}