use channels to process ready tasks (#25)

This introduces a trigger which will notify a channel when tasks are
inserted or updated in a "pending" state. Workers now listen on this
channel and if their queue is named as the one the notification is for
begin processing the next task.

Because channel notifications can fail, we retain the polling mechanism
only now it's set to a default polling interval of one minute. This can
be configured to be shorter or longer as well.

Closes #19

migrations/20241024174106_1.sql

@@ -0,0 +1,19 @@
+-- function to notify about task changes
+create or replace function underway.task_change_notify()
+returns trigger as $$
+begin
+  if (new.state = 'pending') then
+    perform pg_notify('task_change', json_build_object(
+      'task_queue_name', new.task_queue_name
+    )::text);
+  end if;
+
+  return new;
+end;
+$$ language plpgsql;
+
+-- trigger that calls the function after task changes
+create trigger task_changed
+after insert or update on underway.task
+for each row
+execute procedure underway.task_change_notify();

src/job.rs

@@ -1521,7 +1521,7 @@ mod tests {
         job.start();
 
         // Give the job a moment to process.
-        tokio::time::sleep(tokio::time::Duration::from_millis(250)).await;
+        tokio::time::sleep(tokio::time::Duration::from_millis(500)).await;
 
         assert_eq!(
             *state.data.lock().expect("Mutex should not be poisoned"),