MSQ worker: Support in-memory shuffles.

extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/exec/Controller.java

@@ -117,9 +117,9 @@ void resultsComplete(
   );
 
   /**
-   * Returns the current list of task ids, ordered by worker number. The Nth task has worker number N.
+   * Returns the current list of worker IDs, ordered by worker number. The Nth worker has worker number N.
    */
-  List<String> getTaskIds();
+  List<String> getWorkerIds();
 
   @Nullable
   TaskReport.ReportMap liveReports();

extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/exec/ControllerClient.java

@@ -23,20 +23,25 @@
 import org.apache.druid.msq.indexing.error.MSQErrorReport;
 import org.apache.druid.msq.kernel.StageDefinition;
 import org.apache.druid.msq.kernel.StageId;
+import org.apache.druid.msq.kernel.WorkOrder;
 import org.apache.druid.msq.statistics.PartialKeyStatisticsInformation;
 
 import javax.annotation.Nullable;
+import java.io.Closeable;
 import java.io.IOException;
 import java.util.List;
 
 /**
- * Client for the multi-stage query controller. Used by a Worker task.
+ * Client for the multi-stage query controller. Used by a {@link Worker}. Each instance is specific to a single query,
+ * meaning it communicates with a single controller.
  */
-public interface ControllerClient extends AutoCloseable
+public interface ControllerClient extends Closeable
 {
   /**
-   * Client side method to update the controller with partial key statistics information for a particular stage and worker.
-   * Controller's implementation collates all the information for a stage to fetch key statistics from workers.
+   * Client side method to update the controller with partial key statistics information for a particular stage
+   * and worker. The controller collates all the information for a stage to fetch key statistics from workers.
+   *
+   * Only used when {@link StageDefinition#mustGatherResultKeyStatistics()}.
    */
   void postPartialKeyStatistics(
       StageId stageId,
@@ -86,11 +91,16 @@ void postWorkerError(
   /**
    * Client side method to inform the controller about the warnings generated by the given worker.
    */
-  void postWorkerWarning(
-      List<MSQErrorReport> MSQErrorReports
-  ) throws IOException;
+  void postWorkerWarning(List<MSQErrorReport> MSQErrorReports) throws IOException;
 
-  List<String> getTaskList() throws IOException;
+  /**
+   * Client side method for retrieving the list of worker IDs from the controller. These IDs can be passed to
+   * {@link WorkerClient} methods to communicate with other workers. Not necessary when the {@link WorkOrder} has
+   * {@link WorkOrder#getWorkerIds()} set.
+   *
+   * @see Controller#getWorkerIds() for the controller side
+   */
+  List<String> getWorkerIds() throws IOException;
 
   /**
    * Close this client. Idempotent.

extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/exec/ControllerImpl.java

@@ -1172,7 +1172,7 @@ private List<SegmentIdWithShardSpec> generateSegmentIdsWithShardSpecsForReplace(
   }
 
   @Override
-  public List<String> getTaskIds()
+  public List<String> getWorkerIds()
   {
     if (workerManager == null) {
       return Collections.emptyList();
@@ -1261,7 +1261,7 @@ private void contactWorkersForStage(
   {
     // Sorted copy of target worker numbers to ensure consistent iteration order.
     final List<Integer> workersCopy = Ordering.natural().sortedCopy(workers);
-    final List<String> workerIds = getTaskIds();
+    final List<String> workerIds = getWorkerIds();
     final List<ListenableFuture<Void>> workerFutures = new ArrayList<>(workersCopy.size());
 
     try {
@@ -1486,7 +1486,7 @@ private List<Interval> findIntervalsToDrop(final Set<DataSegment> publishedSegme
   private CounterSnapshotsTree getCountersFromAllTasks()
   {
     final CounterSnapshotsTree retVal = new CounterSnapshotsTree();
-    final List<String> taskList = getTaskIds();
+    final List<String> taskList = getWorkerIds();
 
     final List<ListenableFuture<CounterSnapshotsTree>> futures = new ArrayList<>();
 
@@ -1506,7 +1506,7 @@ private CounterSnapshotsTree getCountersFromAllTasks()
 
   private void postFinishToAllTasks()
   {
-    final List<String> taskList = getTaskIds();
+    final List<String> taskList = getWorkerIds();
 
     final List<ListenableFuture<Void>> futures = new ArrayList<>();
 
@@ -2930,7 +2930,7 @@ private void startQueryResultsReader()
       }
 
       final StageId finalStageId = queryKernel.getStageId(queryDef.getFinalStageDefinition().getStageNumber());
-      final List<String> taskIds = getTaskIds();
+      final List<String> taskIds = getWorkerIds();
 
       final InputChannelFactory inputChannelFactory;
 

extensions-core/multi-stage-query/src/main/java/org/apache/druid/msq/exec/ListeningOutputChannelFactory.java

@@ -0,0 +1,74 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.msq.exec;
+
+import org.apache.druid.frame.processor.OutputChannel;
+import org.apache.druid.frame.processor.OutputChannelFactory;
+import org.apache.druid.frame.processor.PartitionedOutputChannel;
+
+import java.io.IOException;
+
+/**
+ * Decorator for {@link OutputChannelFactory} that notifies a {@link Listener} whenever a channel is opened.
+ */
+public class ListeningOutputChannelFactory implements OutputChannelFactory
+{
+  private final OutputChannelFactory delegate;
+  private final Listener listener;
+
+  public ListeningOutputChannelFactory(final OutputChannelFactory delegate, final Listener listener)
+  {
+    this.delegate = delegate;
+    this.listener = listener;
+  }
+
+  @Override
+  public OutputChannel openChannel(final int partitionNumber) throws IOException
+  {
+    return notifyListener(delegate.openChannel(partitionNumber));
+  }
+
+
+  @Override
+  public OutputChannel openNilChannel(final int partitionNumber)
+  {
+    return notifyListener(delegate.openNilChannel(partitionNumber));
+  }
+
+  @Override
+  public PartitionedOutputChannel openPartitionedChannel(
+      final String name,
+      final boolean deleteAfterRead
+  )
+  {
+    throw new UnsupportedOperationException("Listening to partitioned channels is not supported");
+  }
+
+  private OutputChannel notifyListener(OutputChannel channel)
+  {
+    listener.channelOpened(channel);
+    return channel;
+  }
+
+  public interface Listener
+  {
+    void channelOpened(OutputChannel channel);
+  }
+}