fix nextRun with singleton mode reporting incorrect time (#705)

* fix nextRun with singleton mode reporting incorrect time

* only remove past if >1, sort next scheduled

* update test, remove no longer needed lastScheduledRun

job.go

@@ -24,8 +24,11 @@ type internalJob struct {
 	name   string
 	tags   []string
 	jobSchedule
-	lastScheduledRun   time.Time
-	nextScheduled      time.Time
+
+	// as some jobs may queue up, it's possible to
+	// have multiple nextScheduled times
+	nextScheduled []time.Time
+
 	lastRun            time.Time
 	function           any
 	parameters         []any
@@ -894,7 +897,12 @@ func (j job) NextRun() (time.Time, error) {
 	if ij == nil || ij.id == uuid.Nil {
 		return time.Time{}, ErrJobNotFound
 	}
-	return ij.nextScheduled, nil
+	if len(ij.nextScheduled) == 0 {
+		return time.Time{}, nil
+	}
+	// the first element is the next scheduled run with subsequent
+	// runs following after in the slice
+	return ij.nextScheduled[0], nil
 }
 
 func (j job) Tags() []string {

job_test.go

@@ -492,3 +492,59 @@ func TestWithEventListeners(t *testing.T) {
 		})
 	}
 }
+
+func TestJob_NextRun(t *testing.T) {
+	tests := []struct {
+		name string
+		f    func()
+	}{
+		{
+			"simple",
+			func() {},
+		},
+		{
+			"sleep 3 seconds",
+			func() {
+				time.Sleep(300 * time.Millisecond)
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			testTime := time.Now()
+
+			s := newTestScheduler(t)
+
+			// run a job every 10 milliseconds that starts 10 milliseconds after the current time
+			j, err := s.NewJob(
+				DurationJob(
+					100*time.Millisecond,
+				),
+				NewTask(
+					func() {},
+				),
+				WithStartAt(WithStartDateTime(testTime.Add(100*time.Millisecond))),
+				WithSingletonMode(LimitModeReschedule),
+			)
+			require.NoError(t, err)
+
+			s.Start()
+			nextRun, err := j.NextRun()
+			require.NoError(t, err)
+
+			assert.Equal(t, testTime.Add(100*time.Millisecond), nextRun)
+
+			time.Sleep(150 * time.Millisecond)
+
+			nextRun, err = j.NextRun()
+			assert.NoError(t, err)
+
+			assert.Equal(t, testTime.Add(200*time.Millisecond), nextRun)
+			assert.Equal(t, 200*time.Millisecond, nextRun.Sub(testTime))
+
+			err = s.Shutdown()
+			require.NoError(t, err)
+		})
+	}
+}

scheduler.go

@@ -298,9 +298,18 @@ func (s *scheduler) selectExecJobsOutForRescheduling(id uuid.UUID) {
 		// so we don't need to reschedule it.
 		return
 	}
-	j.lastScheduledRun = j.nextScheduled
-
-	next := j.next(j.lastScheduledRun)
+	var scheduleFrom time.Time
+	if len(j.nextScheduled) > 0 {
+		// always grab the last element in the slice as that is the furthest
+		// out in the future and the time from which we want to calculate
+		// the subsequent next run time.
+		slices.SortStableFunc(j.nextScheduled, func(a, b time.Time) int {
+			return a.Compare(b)
+		})
+		scheduleFrom = j.nextScheduled[len(j.nextScheduled)-1]
+	}
+
+	next := j.next(scheduleFrom)
 	if next.IsZero() {
 		// the job's next function will return zero for OneTime jobs.
 		// since they are one time only, they do not need rescheduling.
@@ -316,7 +325,7 @@ func (s *scheduler) selectExecJobsOutForRescheduling(id uuid.UUID) {
 			next = j.next(next)
 		}
 	}
-	j.nextScheduled = next
+	j.nextScheduled = append(j.nextScheduled, next)
 	j.timer = s.clock.AfterFunc(next.Sub(s.now()), func() {
 		// set the actual timer on the job here and listen for
 		// shut down events so that the job doesn't attempt to
@@ -340,6 +349,19 @@ func (s *scheduler) selectExecJobsOutCompleted(id uuid.UUID) {
 		return
 	}
 
+	// if the job has more than one nextScheduled time,
+	// we need to remove any that are in the past.
+	if len(j.nextScheduled) > 1 {
+		var newNextScheduled []time.Time
+		for _, t := range j.nextScheduled {
+			if t.Before(s.now()) {
+				continue
+			}
+			newNextScheduled = append(newNextScheduled, t)
+		}
+		j.nextScheduled = newNextScheduled
+	}
+
 	// if the job has a limited number of runs set, we need to
 	// check how many runs have occurred and stop running this
 	// job if it has reached the limit.
@@ -400,7 +422,7 @@ func (s *scheduler) selectNewJob(in newJobIn) {
 				}
 			})
 		}
-		j.nextScheduled = next
+		j.nextScheduled = append(j.nextScheduled, next)
 	}
 
 	s.jobs[j.id] = j
@@ -451,7 +473,7 @@ func (s *scheduler) selectStart() {
 				}
 			})
 		}
-		j.nextScheduled = next
+		j.nextScheduled = append(j.nextScheduled, next)
 		s.jobs[id] = j
 	}
 	select {

scheduler_test.go

@@ -1622,7 +1622,7 @@ func TestScheduler_RunJobNow(t *testing.T) {
 				WithSingletonMode(LimitModeReschedule),
 			},
 			func() time.Duration {
-				return 20 * time.Second
+				return 10 * time.Second
 			},
 			1,
 		},