feat: use hashicorp/cronexpr for cron

This replaces our cron calculation with those provided by a 3rd party library.

Unfortunatly we still need to do our own parsing however, as we support additional syntax, and other parts of the code expect to be able to parse the Pattern using participle.

closess #1744

backend/controller/cronjobs/cronjobs.go

@@ -307,7 +307,7 @@ func (s *Service) watchForUpdates(ctx context.Context) {
 		now := s.clock.Now()
 		next := now.Add(time.Hour) // should never be reached, expect a different signal long beforehand
 		for _, j := range state.jobs {
-			if possibleNext, err := s.nextAttemptForJob(j, state, false); err == nil && possibleNext.Before(next) {
+			if possibleNext, err := s.nextAttemptForJob(j, state, now,false); err == nil && possibleNext.Before(next) {
 				next = possibleNext
 			}
 		}
@@ -329,8 +329,11 @@ func (s *Service) watchForUpdates(ctx context.Context) {
 			return
 		case <-s.clock.After(next.Sub(now)):
 			// Try starting jobs in db
+			// note that we use next here are the current time
+			// as if there is a pause of over a second we could miss jobs if we use the current time
+			// this is very unlikely to happen, but if it did it would be hard to diagnose
 			jobsToAttempt := slices.Filter(state.jobs, func(j model.CronJob) bool {
-				if n, err := s.nextAttemptForJob(j, state, true); err == nil {
+				if n, err := s.nextAttemptForJob(j, state, next,true); err == nil {
 					return !n.After(s.clock.Now().UTC())
 				}
 				return false
@@ -386,7 +389,8 @@ func (s *Service) watchForUpdates(ctx context.Context) {
 	}
 }
 
-func (s *Service) nextAttemptForJob(job model.CronJob, state *state, allowsNow bool) (time.Time, error) {
+func (s *Service) nextAttemptForJob(job model.CronJob, state *state, currentTime time.Time, allowsNow bool) (time.Time, error) {
+	currentTime = currentTime.UTC()
 	if !s.isResponsibleForJob(job, state) {
 		return s.clock.Now(), fmt.Errorf("controller is not responsible for job")
 	}
@@ -401,7 +405,7 @@ func (s *Service) nextAttemptForJob(job model.CronJob, state *state, allowsNow b
 		if err != nil {
 			return s.clock.Now(), fmt.Errorf("failed to parse cron schedule %q", job.Schedule)
 		}
-		next, err := cron.NextAfter(pattern, s.clock.Now().UTC(), allowsNow)
+		next, err := cron.NextAfter(pattern, currentTime, allowsNow)
 		if err == nil {
 			return next, nil
 		}

examples/go/echo/go.mod

@@ -19,6 +19,7 @@ require (
 	github.com/go-logr/logr v1.4.2 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/godbus/dbus/v5 v5.1.0 // indirect
+	github.com/hashicorp/cronexpr v1.1.2 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20221227161230-091c0ba34f0a // indirect
 	github.com/jackc/pgx/v5 v5.6.0 // indirect

go.mod

@@ -30,6 +30,7 @@ require (
 	github.com/docker/go-connections v0.5.0
 	github.com/go-logr/logr v1.4.2
 	github.com/google/uuid v1.6.0
+	github.com/hashicorp/cronexpr v1.1.2
 	github.com/jackc/pgerrcode v0.0.0-20240316143900-6e2875d9b438
 	github.com/jackc/pgx/v5 v5.6.0
 	github.com/jellydator/ttlcache/v3 v3.2.0