Generate native identifiers in creation order

rust/src/runtime/queries.rs

@@ -234,7 +234,7 @@ where
                 "query_snapshot_exceeds_budget",
             ));
         }
-        let task_id = ulid::Ulid::new().to_string();
+        let task_id = super::store::id();
         let key = key(&task_id)?;
         let admitted = authority::now();
         let (deadline, retained_until) = config.timing(admitted)?;

rust/src/runtime/store.rs

@@ -27,8 +27,28 @@ pub(super) struct Store<DB: Backend> {
     pub(super) history_budget: super::history_budget::Config,
     pub(super) search_attribute_limit: i64,
 }
+/// A ULID that sorts after every earlier one from this process, as Laravel's
+/// (Symfony's) generator does. Rows created in the same millisecond, such as
+/// the activity tasks of one completion, keep their creation order in
+/// `ORDER BY available_at,id`.
 pub(super) fn id() -> String {
-    Ulid::new().to_string()
+    static GENERATOR: std::sync::Mutex<ulid::Generator> =
+        std::sync::Mutex::new(ulid::Generator::new());
+    let mut generator = GENERATOR
+        .lock()
+        .unwrap_or_else(std::sync::PoisonError::into_inner);
+    // Overflow needs 2^80 identifiers in one millisecond.
+    generator
+        .generate()
+        .unwrap_or_else(|_| Ulid::new())
+        .to_string()
+}
+
+#[cfg(test)]
+#[test]
+fn identifiers_keep_creation_order_within_a_millisecond() {
+    let ids: Vec<String> = (0..10_000).map(|_| id()).collect();
+    assert!(ids.windows(2).all(|pair| pair[0] < pair[1]));
 }
 pub(super) fn now() -> DateTime<Utc> {
     DateTime::from_timestamp_millis(Utc::now().timestamp_millis()).expect("current timestamp")