Stopped executor service will not cause an error in polling strategy.

Signed-off-by: Tomas Langer <tomas.langer@oracle.com>

config/config/src/main/java/io/helidon/config/ScheduledPollingStrategy.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020 Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2020 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,11 +19,14 @@ package io.helidon.config;
 import java.time.Duration;
 import java.time.Instant;
 import java.util.concurrent.Executors;
+import java.util.concurrent.RejectedExecutionException;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.BiFunction;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 
 import io.helidon.config.spi.ChangeEventType;
 import io.helidon.config.spi.PollingStrategy;
@@ -32,6 +35,8 @@ import io.helidon.config.spi.PollingStrategy;
  * A strategy which allows the user to schedule periodically fired polling event.
  */
 public final class ScheduledPollingStrategy implements PollingStrategy {
+    private static final Logger LOGGER = Logger.getLogger(ScheduledPollingStrategy.class.getName());
+
     /*
      * This class will trigger checks in a periodic manner.
      * The actual check if the source has changed is done elsewhere, this is just responsible for telling us
@@ -108,9 +113,19 @@ public final class ScheduledPollingStrategy implements PollingStrategy {
     }
 
     private void scheduleNext() {
-        scheduledFuture = executor.schedule(this::fireEvent,
-                                            recurringPolicy.interval().toMillis(),
-                                            TimeUnit.MILLISECONDS);
+        try {
+            scheduledFuture = executor.schedule(this::fireEvent,
+                                                recurringPolicy.interval().toMillis(),
+                                                TimeUnit.MILLISECONDS);
+        } catch (RejectedExecutionException e) {
+            if (executor.isShutdown()) {
+                // intentional shutdown of an executor service
+                LOGGER.log(Level.FINEST, "Executor service is shut down, polling is terminated for " + this, e);
+            } else {
+                // exceptional condition
+                LOGGER.log(Level.SEVERE, "Failed to schedule next polling for " + this + ", polling will stop", e);
+            }
+        }
     }
 
     private synchronized void fireEvent() {