jobs: don't reset next_run when resuming active schedules

Consider an active schedule that was created with a specific first_run
time. The first_run would populate the next_run_time on the schedule.
The resumption of this schedule before it executed would re-evaluate the
next runtime based off the schedule's recurrence.

This commit changes the scheduling system to only recompute the next run
time on paused schedules.

Release justification: bug fix
Release note (bug fix): Fix a bug where resuming an active schedule
would always reset its next run time. This was sometimes undesirable
with schedules that had a first_run option specified.

pkg/jobs/schedule_control_test.go

@@ -27,6 +27,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/sql/sessiondata"
 	"github.com/cockroachdb/cockroach/pkg/testutils/sqlutils"
 	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
+	"github.com/cockroachdb/cockroach/pkg/util/timeutil"
 	"github.com/stretchr/testify/require"
 )
 
@@ -75,6 +76,21 @@ func TestScheduleControl(t *testing.T) {
 		require.True(t, th.loadSchedule(t, scheduleID).IsPaused())
 	})
 
+	t.Run("pause-active-schedule", func(t *testing.T) {
+		schedule := th.newScheduledJob(t, "test schedule", "select 42")
+		require.NoError(t, schedule.SetSchedule("@weekly"))
+		firstRunTime := timeutil.Now().Add(10 * time.Second)
+		schedule.SetNextRun(firstRunTime)
+		require.NoError(t, schedule.Create(ctx, th.cfg.InternalExecutor, nil))
+		scheduleID := schedule.ScheduleID()
+		require.Equal(t, schedule.NextRun(), firstRunTime)
+		th.sqlDB.Exec(t, "RESUME SCHEDULE $1", scheduleID)
+
+		afterSchedule := th.loadSchedule(t, scheduleID)
+		require.False(t, afterSchedule.IsPaused())
+		require.Equal(t, afterSchedule.NextRun(), firstRunTime)
+	})
+
 	t.Run("cannot-resume-one-off-schedule", func(t *testing.T) {
 		schedule := th.newScheduledJob(t, "test schedule", "select 42")
 		require.NoError(t, schedule.Create(ctx, th.cfg.InternalExecutor, nil))

pkg/sql/control_schedules.go

@@ -138,9 +138,13 @@ func (n *controlSchedulesNode) startExec(params runParams) error {
 			schedule.Pause()
 			err = updateSchedule(params, schedule)
 		case tree.ResumeSchedule:
-			err = schedule.ScheduleNextRun()
-			if err == nil {
-				err = updateSchedule(params, schedule)
+			// Only schedule the next run time on PAUSED schedules, since ACTIVE
+			// schedules may have a custom next run time set by first_run.
+			if schedule.IsPaused() {
+				err = schedule.ScheduleNextRun()
+				if err == nil {
+					err = updateSchedule(params, schedule)
+				}
 			}
 		case tree.DropSchedule:
 			var ex jobs.ScheduledJobExecutor