Happy New Year, Dropped Messages — JavaScript Bug Hunt

Briefing

Inspired by the messaging-app meltdowns every New Year's Eve at midnight, when send queues overflow worldwide. Policy for the bounded outbox: when full, drop the oldest — the newest message is the one the user just hit send on.

outbox.js does the opposite, and its capacity check is off by one.

Bug report

BUG-NYE · Priority: High · Reported by: messaging core

enqueue(queue, msg, cap) rules:

• the returned queue never exceeds cap
• when full, the OLDEST message is dropped to admit the new one

Observed: at capacity the NEW message is silently discarded (users' midnight wishes vanish), and the queue actually grows to cap+1 first.

Logs

[outbox] cap=3 size=4
[outbox] dropped incoming msg id=m5 (queue kept stale m1)

The code as shipped

src/msg/outbox.js (editable)

// Bounded send queue. Returns the new queue (never mutates the input).
exports.enqueue = function (queue, msg, cap) {
  if (queue.length > cap) {
    return queue.slice();
  }
  return queue.concat([msg]);
};