periodic.go

func (t *Periodic) Run() {
	t.ctx, t.cancel = context.WithCancel(context.Background())
	interval := t.getInterval()
	num := int(t.period / interval) // number of revs to keep for t.period
	notify := make(chan struct{}) // interval is defined as "duration from the last successfull compaction"
	// notify := make(chan struct{}, 1) // interval is defined as "duration from the last expexcted time of compaction nethersless its result"
	retryInterval := interval / 10

	// periodically updates t.revs and notify to the other goroutine
	go func() {
		for {
			rev := t.rg.Rev()
			t.mu.Lock()
			t.revs = append(t.revs, rev)
			if len(t.revs) > num {
				t.revs = t.revs[1:] // t.revs[0] is always the rev at t.period ago
			}
			t.mu.Unlock()

			select {
			case notify <- struct{}{}:
			default:
				// compaction can take time more than interval
			}

			select {
			case <-t.ctx.Done():
				return
			case <-t.clock.After(interval):
			}
		}
	}()

	// run compaction triggered by the other goroutine thorough the notify channel
	// or internal periodic retry
	go func() {
		var lastCompactedRev int64
		for {
			select {
			case <-t.ctx.Done():
				return
			case <-notify:
				// from the other goroutine
			case <-t.clock.After(retryInterval):
				// for retry
				// when t.rev is not updated, this event will be ignored later,
				// so we don't need to think about race with <-notify.
			}

			t.mu.Lock()
			p := t.paused
			rev := t.revs[0]
			t.mu.Unlock()
			if p {
				continue
			}

			// if t.revs is not updated, we can ignore the event.
			// it's not the first time to try comapction in this interval.
			if rev == lastCompactedRev {
				continue
			}

			plog.Noticef("Starting auto-compaction at revision %d (retention: %v)", rev, t.period)
			_, err := t.c.Compact(t.ctx, &pb.CompactionRequest{Revision: rev})
			if err == nil || err == mvcc.ErrCompacted {
				plog.Noticef("Finished auto-compaction at revision %d", rev)
				lastCompactedRev = rev
			} else {
				plog.Noticef("Failed auto-compaction at revision %d (%v)", rev, err)
				plog.Noticef("Retry after %s", retryInterval)
			}
		}
	}()
}