snapper: fix delayed snapshots caused by system suspend/resume

See explainer comment in periodic.go for details.

fixes https://github.com/zrepl/zrepl/issues/611

daemon/snapper/cron.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/zrepl/zrepl/config"
 	"github.com/zrepl/zrepl/daemon/hooks"
+	"github.com/zrepl/zrepl/util/suspendresumesafetimer"
 	"github.com/zrepl/zrepl/zfs"
 )
 
@@ -42,67 +43,45 @@ type Cron struct {
 
 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():
+		ctxDone := suspendresumesafetimer.SleepUntil(ctx, s.wakeupTime)
+		if ctxDone != nil {
 			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")
-					}
-				}
-			}()
 		}
+
+		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")
+				}
+			}
+		}()
 	}
 
 }

daemon/snapper/periodic.go

@@ -15,6 +15,7 @@ import (
 	"github.com/zrepl/zrepl/daemon/hooks"
 	"github.com/zrepl/zrepl/daemon/logging"
 	"github.com/zrepl/zrepl/util/envconst"
+	"github.com/zrepl/zrepl/util/suspendresumesafetimer"
 	"github.com/zrepl/zrepl/zfs"
 )
 
@@ -171,16 +172,13 @@ func periodicStateSyncUp(a periodicArgs, u updater) state {
 	u(func(s *Periodic) {
 		s.sleepUntil = syncPoint
 	})
-	t := time.NewTimer(time.Until(syncPoint))
-	defer t.Stop()
-	select {
-	case <-t.C:
-		return u(func(s *Periodic) {
-			s.state = Planning
-		}).sf()
-	case <-a.ctx.Done():
+	ctxDone := suspendresumesafetimer.SleepUntil(a.ctx, syncPoint)
+	if ctxDone != nil {
 		return onMainCtxDone(a.ctx, u)
 	}
+	return u(func(s *Periodic) {
+		s.state = Planning
+	}).sf()
 }
 
 func periodicStatePlan(a periodicArgs, u updater) state {
@@ -241,17 +239,13 @@ func periodicStateWait(a periodicArgs, u updater) state {
 		logFunc("enter wait-state after error")
 	})
 
-	t := time.NewTimer(time.Until(sleepUntil))
-	defer t.Stop()
-
-	select {
-	case <-t.C:
-		return u(func(snapper *Periodic) {
-			snapper.state = Planning
-		}).sf()
-	case <-a.ctx.Done():
+	ctxDone := suspendresumesafetimer.SleepUntil(a.ctx, sleepUntil)
+	if ctxDone != nil {
 		return onMainCtxDone(a.ctx, u)
 	}
+	return u(func(snapper *Periodic) {
+		snapper.state = Planning
+	}).sf()
 }
 
 func listFSes(ctx context.Context, mf zfs.DatasetFilter) (fss []*zfs.DatasetPath, err error) {

util/suspendresumesafetimer/suspendresumesafetimer.go

@@ -0,0 +1,96 @@
+package suspendresumesafetimer
+
+import (
+	"context"
+	"time"
+
+	"github.com/zrepl/zrepl/util/envconst"
+)
+
+// The returned error is guaranteed to be the ctx.Err()
+func SleepUntil(ctx context.Context, sleepUntil time.Time) error {
+
+	// We use .Round(0) to strip the monotonic clock reading from the time.Time
+	// returned by time.Now(). That will make the before/after check in the ticker
+	// for-loop compare wall-clock times instead of monotonic time.
+	// Comparing wall clock time is necessary because monotonic time does not progress
+	// while the system is suspended.
+	//
+	// Background
+	//
+	// A time.Time carries a wallclock timestamp and optionally a monotonic clock timestamp.
+	// time.Now() returns a time.Time that carries both.
+	// time.Time.Add() applies the same delta to both timestamps in the time.Time.
+	// x.Sub(y) will return the *monotonic* delta if both x and y carry a monotonic timestamp.
+	// time.Until(x) == x.Sub(now) where `now` will have a monotonic timestamp.
+	// So, time.Until(x) with an `x` that has monotonic timestamp will return monotonic delta.
+	//
+	// Why Do We Care?
+	//
+	// On systems that suspend/resume, wall clock time progresses during suspend but
+	// monotonic time does not.
+	//
+	// So, suppose the following sequence of events:
+	//   x <== time.Now()
+	//   System suspends for 1 hour
+	//   delta <== time.Now().Sub(x)
+	// `delta` will be near 0 because time.Until() subtracts the monotonic
+	// timestamps, and monotonic time didn't progress during suspend.
+	//
+	// Now strip the timestamp using .Round(0)
+	//   x <== time.Now().Round(0)
+	//   System suspends for 1 hour
+	//   delta <== time.Now().Sub(x)
+	// `delta` will be 1 hour because time.Sub() subtracted wallclock timestamps
+	// because x didn't have a monotonic timestamp because we stripped it using .Round(0).
+	//
+	//
+	sleepUntil = sleepUntil.Round(0)
+
+	// Set up a timer so that, if the system doesn't suspend/resume,
+	// we get a precise wake-up time from the native Go timer.
+	monotonicClockTimer := time.NewTimer(time.Until(sleepUntil))
+	defer func() {
+		if !monotonicClockTimer.Stop() {
+			// non-blocking read since we can come here when
+			// we've already drained the channel through
+			//		case <-monotonicClockTimer.C
+			// in the `for` loop below.
+			select {
+			case <-monotonicClockTimer.C:
+			default:
+			}
+		}
+	}()
+
+	// Set up a ticker so that we're guaranteed to wake up periodically.
+	// We'll then get the current wall-clock time and check ourselves
+	// whether we're past the requested expiration time.
+	// Pick a 10 second check interval by default since it's rare that
+	// suspend/resume is done more frequently.
+	ticker := time.NewTicker(envconst.Duration("ZREPL_WALLCLOCKTIMER_MAX_DELAY", 10*time.Second))
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-monotonicClockTimer.C:
+			return nil
+		case <-ticker.C:
+			now := time.Now()
+			if now.Before(sleepUntil) {
+				// Continue waiting.
+
+				// Reset the monotonic timer to reset drift.
+				if !monotonicClockTimer.Stop() {
+					<-monotonicClockTimer.C
+				}
+				monotonicClockTimer.Reset(time.Until(sleepUntil))
+
+				continue
+			}
+			return nil
+		case <-ctx.Done():
+			return ctx.Err()
+		}
+	}
+}