coder(dispatcher): react to new tasks via LISTEN/NOTIFY, poll as fallback

AFTER INSERT trigger on tasks fires pg_notify('tasks_new'); the dispatcher listens via porsager sql.listen and triggers an immediate poll, with the setInterval poll kept at 2s as a missed-notification safety net. Per-session guard unchanged (no double-dispatch).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/schema.sql

@@ -71,3 +71,22 @@ ALTER TABLE available_agents ADD COLUMN IF NOT EXISTS transport TEXT DEFAULT 'pt
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS mode_id TEXT;
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS thinking_option_id TEXT;
 ALTER TABLE tasks ADD COLUMN IF NOT EXISTS feature_values JSONB;
+
+-- LISTEN/NOTIFY fast path: every tasks INSERT (from any call site — routes,
+-- new_task tool, arena, MCP server) fires pg_notify('tasks_new') in the same
+-- transaction, so the dispatcher reacts immediately instead of waiting for the
+-- fallback poll. Postgres holds the notification until COMMIT, so the listener
+-- always sees the committed row. A trigger covers all insert paths with no
+-- app-code drift. Idempotent: re-applied on every startup.
+CREATE OR REPLACE FUNCTION notify_tasks_new() RETURNS trigger AS $$
+BEGIN
+  PERFORM pg_notify('tasks_new', '');
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS tasks_notify_new ON tasks;
+CREATE TRIGGER tasks_notify_new
+  AFTER INSERT ON tasks
+  FOR EACH ROW
+  EXECUTE FUNCTION notify_tasks_new();

apps/coder/src/services/dispatcher.ts

@@ -24,16 +24,29 @@ interface Deps {
   config: Config;
 }
 
-const POLL_INTERVAL_MS = 5_000;
+// LISTEN/NOTIFY ('tasks_new') is the fast path — the dispatcher reacts to new
+// tasks immediately. The poll is only a safety net for notifications missed
+// during a listen-connection drop (porsager auto-reconnects), so it can stay slow.
+const POLL_INTERVAL_MS = 2_000;
 const COMPLETION_POLL_MS = 2_000;
 
 export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<void> } {
   const { sql, inference, broker, log, config } = deps;
   let timer: ReturnType<typeof setInterval> | null = null;
+  let listener: { unlisten: () => Promise<void> } | null = null;
   let running = false;
   let stopping = false;
   let inflightPromise: Promise<void> | null = null;
 
+  // Shared entry point for both the poll timer and the NOTIFY listener. poll()'s
+  // `running`/`stopping` guard makes this safe to call concurrently — a notify
+  // arriving mid-task returns immediately and never double-dispatches.
+  function triggerPoll(reason: string): void {
+    poll().catch((err) => {
+      log.error({ err, reason }, 'dispatcher: poll error');
+    });
+  }
+
   async function poll(): Promise<void> {
     if (running || stopping) return;
 
@@ -463,12 +476,28 @@ export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<v
 
   return {
     start() {
-      log.info('dispatcher: starting poll loop');
-      timer = setInterval(() => {
-        poll().catch((err) => {
-          log.error({ err }, 'dispatcher: poll error');
+      log.info('dispatcher: starting poll loop + tasks_new listener');
+
+      // Fallback poll — catches notifications missed while the listen connection
+      // was down. The fast path is the NOTIFY listener below.
+      timer = setInterval(() => triggerPoll('interval'), POLL_INTERVAL_MS);
+
+      // Fast path: react immediately to new tasks. porsager reserves a dedicated
+      // connection and auto-resubscribes on reconnect; the onlisten callback
+      // fires on each (re)subscribe, so we kick a catch-up poll there too to
+      // sweep up anything inserted during a disconnect.
+      sql
+        .listen(
+          'tasks_new',
+          () => triggerPoll('notify'),
+          () => triggerPoll('listen-subscribed'),
+        )
+        .then((meta) => {
+          listener = meta;
+        })
+        .catch((err) => {
+          log.error({ err }, 'dispatcher: failed to LISTEN tasks_new — relying on poll fallback');
         });
-      }, POLL_INTERVAL_MS);
     },
 
     async stop() {
@@ -477,6 +506,12 @@ export function createDispatcher(deps: Deps): { start(): void; stop(): Promise<v
         clearInterval(timer);
         timer = null;
       }
+      if (listener) {
+        await listener.unlisten().catch((err) => {
+          log.error({ err }, 'dispatcher: unlisten error');
+        });
+        listener = null;
+      }
       if (inflightPromise) {
         log.info('dispatcher: waiting for in-flight task');
         await inflightPromise;