groupBy v2: Fix dangling references. (#3500)

Acquiring references in the processing task prevents dangling references
caused by canceled processing tasks.

processing/src/main/java/io/druid/query/groupby/epinephelinae/GroupByMergingQueryRunnerV2.java

@@ -205,17 +205,16 @@ public class GroupByMergingQueryRunnerV2 implements QueryRunner
                                 );
                               }
 
-                              final Releaser bufferReleaser = mergeBufferHolder.increment();
-                              try {
-                                final Releaser grouperReleaser = grouperHolder.increment();
-                                try {
                               return exec.submit(
                                   new AbstractPrioritizedCallable<Boolean>(priority)
                                   {
                                     @Override
                                     public Boolean call() throws Exception
                                     {
-                                          try {
+                                      try (
+                                          Releaser bufferReleaser = mergeBufferHolder.increment();
+                                          Releaser grouperReleaser = grouperHolder.increment()
+                                      ) {
                                         final Object retVal = input.run(queryForRunners, responseContext)
                                                                    .accumulate(grouper, accumulator);
 
@@ -229,26 +228,10 @@ public class GroupByMergingQueryRunnerV2 implements QueryRunner
                                         log.error(e, "Exception with one of the sequences!");
                                         throw Throwables.propagate(e);
                                       }
-                                          finally {
-                                            grouperReleaser.close();
-                                            bufferReleaser.close();
-                                          }
                                     }
                                   }
                               );
                             }
-                                catch (Exception e) {
-                                  // Exception caught while submitting the task; release resources.
-                                  grouperReleaser.close();
-                                  throw e;
-                                }
-                              }
-                              catch (Exception e) {
-                                // Exception caught while submitting the task; release resources.
-                                bufferReleaser.close();
-                                throw e;
-                              }
-                            }
                           }
                       )
                   )