[BUG]Invalid cron date schedule creates infinite loop in flytescheduler (#3312)

Signed-off-by: Alex Wu <c.alexwu@gmail.com>

Author: popojk

flytekit/core/schedule.py

@@ -142,12 +142,12 @@ def _validate_expression(cron_expression: str):
     def _validate_schedule(schedule: str):
         if schedule.lower() not in CronSchedule._VALID_CRON_ALIASES:
             try:
-                croniter.croniter(schedule)
-            except Exception:
-                raise ValueError(
-                    "Schedule is invalid. It must be set to either a cron alias or valid cron expression."
-                    f" Provided schedule: {schedule}"
-                )
+                # Validate the cron expression
+                cron = croniter.croniter(schedule)
+                # Try to get the next occurrence to validate the schedule
+                cron.get_next(datetime.datetime)
+            except Exception as e:
+                raise ValueError(f"Schedule is invalid. Provided schedule: {schedule} Error: {str(e)}")
 
     @staticmethod
     def _validate_offset(offset: str):

tests/flytekit/unit/core/test_schedule.py

@@ -153,3 +153,35 @@ def quadruple(a: int) -> int:
     assert lp.schedule == _schedule_models.Schedule(
         "kickoff_input", rate=_schedule_models.Schedule.FixedRate(12, _schedule_models.Schedule.FixedRateUnit.HOUR)
     )
+
+
+@pytest.mark.parametrize(
+    "invalid_schedule",
+    [
+        "0 0 31 2 *",    # February 31st (does not exist)
+        "0 0 30 2 *",    # February 30th (does not exist)
+        "0 0 31 4 *",    # April 31st (does not exist)
+        "0 0 31 6 *",    # June 31st (does not exist)
+    ],
+)
+def test_cron_invalid_date_combinations(invalid_schedule):
+    """Test that CronSchedule rejects invalid date combinations like 31st of February."""
+    with pytest.raises(ValueError, match="Schedule is invalid."):
+        CronSchedule(schedule=invalid_schedule)
+
+
+@pytest.mark.parametrize(
+    "valid_schedule",
+    [
+        "0 0 28 2 *",    # February 28th (always valid)
+        "0 0 29 2 *",    # February 29th (valid in leap years - handled by croniter)
+        "0 0 30 4 *",    # April 30th (valid)
+        "0 0 31 1 *",    # January 31st (valid)
+        "0 0 31 3 *",    # March 31st (valid)
+    ],
+)
+def test_cron_valid_date_combinations(valid_schedule):
+    """Test that CronSchedule accepts valid date combinations."""
+    # These should not raise any exceptions
+    obj = CronSchedule(schedule=valid_schedule)
+    assert obj.cron_schedule.schedule == valid_schedule