kv.listenQueue should warn if called multiple times

[roziscoding]

Although the docs don’t clearly state it (at least I couldn’t find it), the runtime doesn’t appear to support multiple listeners for the queue—or at least it doesn’t call all of them. This means it’s not safe to call listenQueue more than once, because you don’t know which listener will be used.

This observation came from the following code:

kv.listenQueue(async (event) => {
    if (!ProcessNotificationEvent.safeParse(event).success) return;

    await sendNotification(event.data);
});

kv.listenQueue(async (event) => {
    if (!DeleteSubscriptionEvent.safeParse(event).success) return;

    await deleteSubscription(event.data);
});

I usually have ~25 notification events enqueued at the same time, but not all of them were actually run—there were no error logs, and no undelivered messages. I then realized I was probably only supposed to call listenQueue once. So I moved the handlers to functions and called them inside a single listener:

kv.listenQueue(async (event) => {
    await processNotification(event);
    await deleteSubscription(event);
});

After that, all notifications were correctly delivered.

I don’t know if adding a warning or even throwing an error is feasible, but I believe this behavior could at least be documented more clearly.