Account for jitter in determining whether CronLastExec is acceptable

src/Cron.hs

@@ -150,10 +150,11 @@ genMatch p m st (CronMatchUnion aGen bGen) = merge (genMatch p m st aGen) (genMa
 
 nextCronMatch :: TZ -- ^ Timezone of the `Cron`-Entry
               -> Maybe UTCTime -- ^ Time of last execution of the job
+              -> NominalDiffTime -- ^ Scheduling precision
               -> UTCTime -- ^ Current time, used only for `CronCalendar`
               -> Cron
               -> CronNextMatch UTCTime
-nextCronMatch tz mPrev now c@Cron{..} = case notAfter of
+nextCronMatch tz mPrev prec now c@Cron{..} = case notAfter of
   MatchAsap -> MatchNone
   MatchAt ts
     | MatchAt ts' <- nextMatch
@@ -183,7 +184,7 @@ nextCronMatch tz mPrev now c@Cron{..} = case notAfter of
       Just prevT
         -> case cronRepeat of
             CronRepeatOnChange
-              | not $ matchesCron tz Nothing prevT c
+              | not $ matchesCron tz Nothing prec prevT c
                 -> let
                      cutoffTime = addUTCTime cronRateLimit prevT
                    in case execRef now False cronInitial of
@@ -240,13 +241,14 @@ nextCronMatch tz mPrev now c@Cron{..} = case notAfter of
 
 matchesCron :: TZ -- ^ Timezone of the `Cron`-Entry
             -> Maybe UTCTime -- ^ Previous execution of the job
+            -> NominalDiffTime -- ^ Scheduling precision
             -> UTCTime -- ^ "Current" time
             -> Cron
             -> Bool
 -- ^ @matchesCron tz prev prec now c@ determines whether the given `Cron`
 --   specification @c@ should match @now@, under the assumption that the next
 --   check will occur no earlier than @now + prec@.
-matchesCron tz mPrev now cron = case nextCronMatch tz mPrev now cron of
+matchesCron tz mPrev prec now cron = case nextCronMatch tz mPrev prec now cron of
   MatchAsap -> True
   MatchNone -> False
-  MatchAt ts -> ts <= now
+  MatchAt ts -> ts <= addUTCTime prec now

src/Jobs.hs

@@ -207,7 +207,7 @@ execCrontab = evalStateT go HashMap.empty
               | otherwise
               = Just (jobCtl, t)
               where
-                t = nextCronMatch appTZ (getMax <$> HashMap.lookup jobCtl lastTimes) now cron
+                t = nextCronMatch appTZ (getMax <$> HashMap.lookup jobCtl lastTimes) acc now cron
 
         waitUntil :: (Eq a, MonadResourceBase m, MonadLogger m) => TMVar a -> a -> UTCTime -> m Bool
         waitUntil crontabTV crontab nextTime = runResourceT $ do