fix: Fix bug that causes cronpatterns with no next execution-time to …

…fail. (#435)

Introduce an `Instant.NEVER` that represents future execution-time that
will never run, and use this for cron-patterns with no next
execution-time.

## Fixes
* #361 


## Reminders
- [ ] Added/ran automated tests
- [x] Update README and/or examples
- [x] Ran `mvn spotless:apply`

db-scheduler/src/main/java/com/github/kagkarlsson/scheduler/task/schedule/CronSchedule.java

@@ -24,7 +24,6 @@
 import java.time.Instant;
 import java.time.ZoneId;
 import java.time.ZonedDateTime;
-import java.time.temporal.ChronoUnit;
 import java.util.Optional;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -69,19 +68,23 @@ public CronSchedule(String pattern, ZoneId zoneId, CronStyle cronStyle) {
   @Override
   public Instant getNextExecutionTime(ExecutionComplete executionComplete) {
     lazyInitExecutionTime(); // for deserialized objects
-    ZonedDateTime lastDone =
-        ZonedDateTime.ofInstant(
-            executionComplete.getTimeDone(),
-            zoneId); // frame the 'last done' time in the context of the time zone for this schedule
+
+    // frame the 'last done' time in the context of the time zone for this schedule
     // so that expressions like "0 05 13,20 * * ?" (New York) can operate in the
     // context of the desired time zone
+    ZonedDateTime lastDone = ZonedDateTime.ofInstant(executionComplete.getTimeDone(), zoneId);
 
     Optional<ZonedDateTime> nextTime = cronExecutionTime.nextExecution(lastDone);
     if (!nextTime.isPresent()) {
       LOG.error(
-          "Cron-pattern did not return any further execution-times. This behavior is currently not supported by the scheduler. Setting next execution-time to far-future.");
-      return Instant.now().plus(1000, ChronoUnit.YEARS);
+          "Cron-pattern did not return any further execution-times. This behavior is currently "
+              + "not supported by the scheduler. Setting next execution-time to far-future, "
+              + "aka \"NEVER\" ({})",
+          NEVER);
+
+      return Schedule.NEVER;
     }
+
     return nextTime.get().toInstant();
   }
 

db-scheduler/src/main/java/com/github/kagkarlsson/scheduler/task/schedule/Schedule.java

@@ -18,6 +18,8 @@
 
 public interface Schedule {
 
+  Instant NEVER = Instant.parse("2500-01-01T12:00:00.00Z");
+
   Instant getNextExecutionTime(ExecutionComplete executionComplete);
 
   /** Used to get the first execution-time for a schedule. Simulates an ExecutionComplete event. */

db-scheduler/src/test/java/com/github/kagkarlsson/scheduler/concurrent/MssqlClusterTest.java

@@ -66,7 +66,7 @@ public void test_concurrency_optimistic_locking() throws InterruptedException {
     // Observed failed heartbeats due to deadlock.
     // FIXLATER: add retry of update heartbeats
     Utils.retryOnFailed(
-        1,
+        3,
         () -> {
           DEBUG_LOG.info("Starting test_concurrency_optimistic_locking");
           ClusterTests.testConcurrencyForPollingStrategy(

db-scheduler/src/test/java/com/github/kagkarlsson/scheduler/task/CronTest.java

@@ -121,6 +121,13 @@ public void should_mark_schedule_as_disabled() {
     assertFalse(Schedules.cron("0 * * * * ?").isDisabled());
   }
 
+  @Test
+  public void should_not_fail_on_cronschedule_without_next_execution_time() {
+    CronSchedule cron =
+        new CronSchedule("0 0 0 29 2 MON#1", ZoneId.systemDefault(), CronStyle.SPRING53);
+    Assertions.assertEquals(Schedule.NEVER, cron.getNextExecutionTime(complete(Instant.now())));
+  }
+
   private void assertNextExecutionTime(
       ZonedDateTime timeDone, String cronPattern, ZonedDateTime expectedTime) {
     assertNextExecutionTime(timeDone, expectedTime, new CronSchedule(cronPattern));