A message held during startup is no longer overtaken by a later one

Everything in the mesh is addressed, and a message sent to an address that is still waking up is held rather than dropped. The moment the target is ready, the held messages run — and they run in the order they were sent. That ordering is not a detail: it is why submitting two code cells back to back works, why an edit followed by a read sees the edit, and why any handler may rely on what the previous message did.

There was a gap of a few milliseconds in which it did not hold.

What went wrong

Becoming ready is two steps — stop holding and put the held messages back at the front of the queue — and a message that was already being processed sat between them. It had been taken off the queue before the target became ready, and by the time it asked "am I being held?" the answer was no. So it ran immediately, ahead of messages that had been waiting longer, and the queue was put back in order behind it.

The effect a person would notice is a later message winning: a cell that uses a value running before the cell that defines it, a view rendering against state an earlier write had not applied yet. It needed the target to be busy at the exact instant it became ready, which is why it showed up under load and never on a quiet machine — measured at about one occurrence in a hundred on our own build runners, with a different-looking symptom each time.

An earlier fix had closed the half of this that was visible in the queues. This is the other half: the message that was in no queue at all when the switch flipped.

What changes

The two steps are now one: the held messages are put back before the target stops holding, so there is no instant in which a message can be told "nothing is waiting" while something is. And a message that is already running when the switch flips now notices that older work has appeared ahead of it and rejoins the queue in its own place instead of overtaking.

Nothing to configure, and nothing to change in any code that sends messages — the ordering that was always promised now actually holds, including at that boundary.