Disable time cache on interval == 0

Author: matriv

server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -550,22 +550,39 @@ static class CachedTimeThread extends Thread {
             this.absoluteMillis = System.currentTimeMillis();
             this.counter = new TimeCounter();
             setDaemon(true);
+            if (interval <= 0) {
+                this.running = false;
+            }
         }
 
         /**
          * Return the current time used for relative calculations. This is
          * {@link System#nanoTime()} truncated to milliseconds.
+         * <p>
+         * If {@link ThreadPool#ESTIMATED_TIME_INTERVAL_SETTING} is set to 0
+         * then the cache is disabled and the method calls {@link System#nanoTime()}
+         * whenever called. Typically used for testing.
          */
         long relativeTimeInMillis() {
-            return relativeMillis;
+            if (running) {
+                return relativeMillis;
+            }
+            return TimeValue.nsecToMSec(System.nanoTime());
         }
 
         /**
          * Return the current epoch time, used to find absolute time. This is
          * a cached version of {@link System#currentTimeMillis()}.
+         * <p>
+         * If {@link ThreadPool#ESTIMATED_TIME_INTERVAL_SETTING} is set to 0
+         * then the cache is disabled and the method calls {@link System#currentTimeMillis()}
+         * whenever called. Typically used for testing.
          */
         long absoluteTimeInMillis() {
-            return absoluteMillis;
+            if (running) {
+                return absoluteMillis;
+            }
+            return System.currentTimeMillis();
         }
 
         @Override

server/src/test/java/org/elasticsearch/indices/settings/UpdateSettingsIT.java

@@ -134,7 +134,7 @@ public List<Setting<?>> getSettings() {
     @Override
     protected Settings nodeSettings(int nodeOrdinal) {
         return Settings.builder().put(super.nodeSettings(nodeOrdinal))
-            .put("thread_pool.estimated_time_interval", TimeValue.timeValueMillis(1))
+            .put("thread_pool.estimated_time_interval", 0)
             .build();
     }