fix(scheduler): preserve cron timezone on task reschedule

This problem arisen from the fact that we rely
on the timezone of the time passed to cron trigger
for correct calculation of next activation.
We might want to improve that by switching to set
timezone as a part of cron specification, but this
is also confusing when we take into consideration
our separate timezone flag (#3818).

In this case, it wasn't done for the time of initial
activation kept always in UTC+0 timezone. This resulted
in switching from correct timezone to UTC+0 timezone
on task reschedule.

Refs #3818
Fixes #4743

Author: Michal-Leszczynski

pkg/scheduler/scheduler.go

@@ -1,4 +1,4 @@
-// Copyright (C) 2017 ScyllaDB
+// Copyright (C) 2026 ScyllaDB
 
 package scheduler
 
@@ -140,6 +140,7 @@ func (s *Scheduler[K]) reschedule(ctx *RunContext[K], initialActivation time.Tim
 	now := s.now()
 	if d.Location != nil {
 		now = now.In(d.Location)
+		initialActivation = initialActivation.In(d.Location)
 	}
 	next := d.Trigger.Next(now)
 

pkg/scheduler/scheduler_test.go

@@ -1,4 +1,4 @@
-// Copyright (C) 2017 ScyllaDB
+// Copyright (C) 2026 ScyllaDB
 
 package scheduler
 
@@ -855,4 +855,58 @@ func TestReschedule(t *testing.T) {
 			}
 		}
 	})
+
+	t.Run("preserve cron timezone on task reschedule", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(t.Context())
+		defer cancel()
+		// Use different timezones in time generator and trigger
+		zone0 := time.FixedZone("UTC", 0)
+		zone5 := time.FixedZone("UTC+5", 5*60*60)
+		// Trigger at midnight UTC+5
+		d := Details{
+			Trigger:  schedules.MustCron("0 0 * * *", time.Time{}),
+			Location: zone5,
+		}
+		// Returns noon in UTC+0
+		timeGen := func() time.Time {
+			return time.Date(2024, 1, 1, 12, 0, 0, 0, zone0)
+		}
+		// Since trigger timezone takes precedence over time generator timezone,
+		// expect next activation in 7 hours.
+		expectedNext := timeGen().Add(7 * time.Hour)
+
+		f := newFakeRunner()
+		s := NewScheduler[testKey](timeGen, f.Run, ll)
+		k := randomKey()
+
+		s.Schedule(ctx, k, d)
+		// Verify initial activation
+		a := s.Activations(k)
+		if len(a) != 1 {
+			t.Fatalf("expected 1 activation in queue, got %d", len(a))
+		}
+		if !expectedNext.Equal(a[0].Time) {
+			t.Fatalf("expected initial activation to be %v, got %v", expectedNext, a[0].Time)
+		}
+		// Start task in the background so that it's rescheduled when it finishes
+		time.AfterFunc(StartOffset, func() {
+			s.Trigger(ctx, k)
+		})
+		// Verify that task finished before timeout or closing scheduler
+		select {
+		case <-startAndWait(ctx, s):
+			t.Fatal("expected a run, scheduler exit")
+		case <-time.After(Timeout):
+			t.Fatal("expected a run, timeout")
+		case <-f.WaitKeys(k):
+		}
+		// Verify rescheduled activation
+		a = s.Activations(k)
+		if len(a) != 1 {
+			t.Fatalf("expected 1 activation in queue, got %d", len(a))
+		}
+		if !expectedNext.Equal(a[0].Time) {
+			t.Fatalf("expected rescheduled activation to be %v, got %v", expectedNext, a[0].Time)
+		}
+	})
 }