[GitHub] jihoonson commented on a change in pull request #6033: Synchronize scheduled poll() calls in SQLMetadataRuleManager to prevent flakiness in SqlMetadataRuleManagerTest

[GitBox <gi...@apache.org>]

jihoonson commented on a change in pull request #6033: Synchronize scheduled poll() calls in SQLMetadataRuleManager to prevent flakiness in SqlMetadataRuleManagerTest
URL: https://github.com/apache/incubator-druid/pull/6033#discussion_r204518835
 
 

 ##########
 File path: server/src/main/java/io/druid/metadata/SQLMetadataRuleManager.java
 ##########
 @@ -142,13 +139,19 @@ public Void withHandle(Handle handle) throws Exception
   private final AuditManager auditManager;
 
   private final Object lock = new Object();
-
-  private volatile boolean started = false;
-
-  private volatile ListeningScheduledExecutorService exec = null;
-  private volatile ListenableFuture<?> future = null;
-
-  private volatile long retryStartTime = 0;
+  /** The number of times this SQLMetadataRuleManager was started. */
+  private long startCount = 0;
+  /**
+   * Equal to the current {@link #startCount} value, if the SQLMetadataRuleManager is currently started; -1 if
+   * currently stopped.
+   *
+   * This field is used to implement a simple stamp mechanism instead of just a boolean "started" flag to prevent
+   * the theoretical situation of two tasks scheduled in {@link #start()} calling {@link #poll()} concurrently, if
+   * the sequence of {@link #start()} - {@link #stop()} - {@link #start()} actions occurs quickly.
+   */
+  private long currentStartOrder = -1;
+  private ScheduledExecutorService exec = null;
+  private long retryStartTime = 0;
 
 Review comment:
   nit: looks that this can be a local variable.

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