refactor snapper & support cron-based snapshotting

fixes https://github.com/zrepl/zrepl/issues/554
refs https://github.com/zrepl/zrepl/discussions/547#discussioncomment-1936126

daemon/snapper/cron.go

@@ -0,0 +1,172 @@
+package snapper
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/pkg/errors"
+
+	"github.com/zrepl/zrepl/config"
+	"github.com/zrepl/zrepl/daemon/hooks"
+	"github.com/zrepl/zrepl/zfs"
+)
+
+func cronFromConfig(fsf zfs.DatasetFilter, in config.SnapshottingCron) (*Cron, error) {
+
+	hooksList, err := hooks.ListFromConfig(&in.Hooks)
+	if err != nil {
+		return nil, errors.Wrap(err, "hook config error")
+	}
+	planArgs := planArgs{
+		prefix: in.Prefix,
+		hooks:  hooksList,
+	}
+	return &Cron{config: in, fsf: fsf, planArgs: planArgs}, nil
+}
+
+type Cron struct {
+	config   config.SnapshottingCron
+	fsf      zfs.DatasetFilter
+	planArgs planArgs
+
+	mtx sync.RWMutex
+
+	running                 bool
+	wakeupTime              time.Time // zero value means uninit
+	lastError               error
+	lastPlan                *plan
+	wakeupWhileRunningCount int
+}
+
+func (s *Cron) Run(ctx context.Context, snapshotsTaken chan<- struct{}) {
+
+	t := time.NewTimer(0)
+	defer func() {
+		if !t.Stop() {
+			select {
+			case <-t.C:
+			default:
+			}
+		}
+	}()
+	for {
+		now := time.Now()
+		s.mtx.Lock()
+		s.wakeupTime = s.config.Cron.Schedule.Next(now)
+		s.mtx.Unlock()
+
+		// Re-arm the timer.
+		// Need to Stop before Reset, see docs.
+		if !t.Stop() {
+			// Use non-blocking read from timer channel
+			// because, except for the first loop iteration,
+			// the channel is already drained
+			select {
+			case <-t.C:
+			default:
+			}
+		}
+		t.Reset(s.wakeupTime.Sub(now))
+
+		select {
+		case <-ctx.Done():
+			return
+		case <-t.C:
+			getLogger(ctx).Debug("cron timer fired")
+			s.mtx.Lock()
+			if s.running {
+				getLogger(ctx).Warn("snapshotting triggered according to cron rules but previous snapshotting is not done; not taking a snapshot this time")
+				s.wakeupWhileRunningCount++
+				s.mtx.Unlock()
+				continue
+			}
+			s.lastError = nil
+			s.lastPlan = nil
+			s.wakeupWhileRunningCount = 0
+			s.running = true
+			s.mtx.Unlock()
+			go func() {
+				err := s.do(ctx)
+				s.mtx.Lock()
+				s.lastError = err
+				s.running = false
+				s.mtx.Unlock()
+
+				select {
+				case snapshotsTaken <- struct{}{}:
+				default:
+					if snapshotsTaken != nil {
+						getLogger(ctx).Warn("callback channel is full, discarding snapshot update event")
+					}
+				}
+			}()
+		}
+	}
+
+}
+
+func (s *Cron) do(ctx context.Context) error {
+	fss, err := zfs.ZFSListMapping(ctx, s.fsf)
+	if err != nil {
+		return errors.Wrap(err, "cannot list filesystems")
+	}
+	p := makePlan(s.planArgs, fss)
+
+	s.mtx.Lock()
+	s.lastPlan = p
+	s.lastError = nil
+	s.mtx.Unlock()
+
+	ok := p.execute(ctx, false)
+	if !ok {
+		return errors.New("one or more snapshots could not be created, check logs for details")
+	} else {
+		return nil
+	}
+}
+
+type CronState string
+
+const (
+	CronStateRunning CronState = "running"
+	CronStateWaiting CronState = "waiting"
+)
+
+type CronReport struct {
+	State      CronState
+	WakeupTime time.Time
+	Errors     []string
+	Progress   []*ReportFilesystem
+}
+
+func (s *Cron) Report() Report {
+
+	s.mtx.Lock()
+	defer s.mtx.Unlock()
+
+	r := CronReport{}
+
+	r.WakeupTime = s.wakeupTime
+
+	if s.running {
+		r.State = CronStateRunning
+	} else {
+		r.State = CronStateWaiting
+	}
+
+	if s.lastError != nil {
+		r.Errors = append(r.Errors, s.lastError.Error())
+	}
+	if s.wakeupWhileRunningCount > 0 {
+		r.Errors = append(r.Errors, fmt.Sprintf("cron frequency is too high; snapshots were not taken %d times", s.wakeupWhileRunningCount))
+	}
+
+	r.Progress = nil
+	if s.lastPlan != nil {
+		r.Progress = s.lastPlan.report()
+	}
+
+	return Report{Type: TypeCron, Cron: &r}
+}