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xarmian
3a00783557
Reading both handlers as state machines: the event channel and the gap channel are two arms of one select, so with both ready Go picks at random. Announcing first and then draining events the subscriber queued BEFORE the hole is the wrong order twice over — the client is told its position is untrustworthy and then immediately handed IDs that re-establish one, below the hole; and on an ID-space change those queued events belong to the space that was just abandoned. The gap is now latched and the announcement waits for an empty channel. Nothing is discarded to achieve it, and that restraint is the load-bearing part on the watch stream: a queued one-shot PUSH cannot be recovered by any reconcile, so dropping it to make the cursor tidy would destroy the only copy. Draining cannot starve the announcement either — one event per iteration, re-checked at the top, so it lands on the first iteration with nothing queued, immediately when the channel was already empty. Pinned by asserting the ORDER of the frames with twenty events queued ahead of the gap. Without the barrier that fails on the first or third event, roughly half the time per run.