}
		r.spillErr = err

		// Reset CPU samples cursor.
		r.cpuSamples = r.gen.cpuSamples

		// Reset frontier.
		for m, batches := range r.gen.batches {
			bc := &batchCursor{m: m}
			ok, err := bc.nextEvent(batches, r.gen.freq)
			if err != nil {
				return Event{}, err
			}
			if !ok {
				// Turns out there aren't actually any events in these batches.
				continue
			}

type batchCursor struct {
	m       ThreadID
	lastTs  Time
	idx     int       // next index into []batch
	dataOff int       // next index into batch.data
	ev      baseEvent // last read event
}

func (b *batchCursor) nextEvent(batches []batch, freq frequency) (ok bool, err error) {
	// Batches should generally always have at least one event,
	// but let's be defensive about that and accept empty batches.