lock: fix timer expiry monitoring during standby

Monotonic timers are paused during standby. Thus these timers won't fire
after waking up. Fall back to periodic polling to detect too large clock
jumps. See golang/go#35012 for a discussion of
go timers during standby.

cmd/restic/lock.go

@@ -119,7 +119,18 @@ func refreshLocks(ctx context.Context, lock *restic.Lock, lockInfo *lockContext,
 }
 
 func monitorLockRefresh(ctx context.Context, lock *restic.Lock, lockInfo *lockContext, refreshed <-chan struct{}) {
-	timer := time.NewTimer(refreshabilityTimeout)
+	// time.Now() might use a monotonic timer which is paused during standby
+	// convert to unix time to ensure we compare real time values
+	lastRefresh := time.Now().Unix()
+	pollDuration := 1 * time.Second
+	if refreshInterval < pollDuration {
+		// require for TestLockFailedRefresh
+		pollDuration = refreshInterval / 5
+	}
+	// timers are paused during standby, which is a problem as the refresh timeout
+	// _must_ expire if the host was too long in standby. Thus fall back to periodic checks
+	// https://github.com/golang/go/issues/35012
+	timer := time.NewTimer(pollDuration)
 	defer func() {
 		timer.Stop()
 		lockInfo.cancel()
@@ -132,9 +143,14 @@ func monitorLockRefresh(ctx context.Context, lock *restic.Lock, lockInfo *lockCo
 			debug.Log("terminate expiry monitoring")
 			return
 		case <-refreshed:
-			// reset timer once the lock was refreshed successfully
-			timer.Reset(refreshabilityTimeout)
+			lastRefresh = time.Now().Unix()
 		case <-timer.C:
+			if float64(time.Now().Unix()-lastRefresh) < refreshInterval.Seconds() {
+				// restart timer
+				timer.Reset(pollDuration)
+				continue
+			}
+
 			Warnf("Fatal: failed to refresh lock in time\n")
 			return
 		}