otterc commented on a change in pull request #32140:
URL: https://github.com/apache/spark/pull/32140#discussion_r656706415



##########
File path: 
core/src/main/scala/org/apache/spark/storage/PushBasedFetchHelper.scala
##########
@@ -0,0 +1,325 @@
+/*
+ * 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.spark.storage
+
+import java.util.concurrent.TimeUnit
+
+import scala.collection.mutable
+import scala.collection.mutable.ArrayBuffer
+import scala.util.{Failure, Success}
+
+import org.roaringbitmap.RoaringBitmap
+
+import org.apache.spark.MapOutputTracker
+import org.apache.spark.MapOutputTracker.SHUFFLE_PUSH_MAP_ID
+import org.apache.spark.internal.Logging
+import org.apache.spark.network.shuffle.{BlockStoreClient, MergedBlockMeta, 
MergedBlocksMetaListener}
+import org.apache.spark.storage.BlockManagerId.SHUFFLE_MERGER_IDENTIFIER
+import org.apache.spark.storage.ShuffleBlockFetcherIterator._
+
+/**
+ * Helper class for [[ShuffleBlockFetcherIterator]] that encapsulates all the 
push-based
+ * functionality to fetch push-merged block meta and shuffle chunks.
+ * A push-merged block contains multiple shuffle chunks where each shuffle 
chunk contains multiple
+ * shuffle blocks that belong to the common reduce partition and were merged 
by the ESS to that
+ * chunk.
+ */
+private class PushBasedFetchHelper(
+   private val iterator: ShuffleBlockFetcherIterator,
+   private val shuffleClient: BlockStoreClient,
+   private val blockManager: BlockManager,
+   private val mapOutputTracker: MapOutputTracker) extends Logging {
+
+  private[this] val startTimeNs = System.nanoTime()
+
+  private[storage] val localShuffleMergerBlockMgrId = BlockManagerId(
+    SHUFFLE_MERGER_IDENTIFIER, blockManager.blockManagerId.host,
+    blockManager.blockManagerId.port, blockManager.blockManagerId.topologyInfo)
+
+  /**
+   * A map for storing shuffle chunk bitmap.
+   */
+  private[this] val chunksMetaMap = new mutable.HashMap[ShuffleBlockChunkId, 
RoaringBitmap]()
+
+  /**
+   * Returns true if the address is for a push-merged block.
+   */
+  def isPushMergedShuffleBlockAddress(address: BlockManagerId): Boolean = {
+    SHUFFLE_MERGER_IDENTIFIER == address.executorId
+  }
+
+  /**
+   * Returns true if the address is of a remote push-merged block. false 
otherwise.
+   */
+  def isRemotePushMergedBlockAddress(address: BlockManagerId): Boolean = {
+    isPushMergedShuffleBlockAddress(address) && address.host != 
blockManager.blockManagerId.host
+  }
+
+  /**
+   * Returns true if the address is of a local push-merged block. false 
otherwise.
+   */
+  def isLocalPushMergedBlockAddress(address: BlockManagerId): Boolean = {
+    isPushMergedShuffleBlockAddress(address) && address.host == 
blockManager.blockManagerId.host
+  }
+
+  /**
+   * This is executed by the task thread when the `iterator.next()` is invoked 
and the iterator
+   * processes a response of type 
[[ShuffleBlockFetcherIterator.SuccessFetchResult]].
+   *
+   * @param blockId shuffle chunk id.
+   */
+  def removeChunk(blockId: ShuffleBlockChunkId): Unit = {
+    chunksMetaMap.remove(blockId)
+  }
+
+  /**
+   * This is executed by the task thread when the `iterator.next()` is invoked 
and the iterator
+   * processes a response of type 
[[ShuffleBlockFetcherIterator.PushMergedLocalMetaFetchResult]].
+   *
+   * @param blockId shuffle chunk id.
+   */
+  def addChunk(blockId: ShuffleBlockChunkId, chunkMeta: RoaringBitmap): Unit = 
{
+    chunksMetaMap(blockId) = chunkMeta
+  }
+
+  /**
+   * This is executed by the task thread when the `iterator.next()` is invoked 
and the iterator
+   * processes a response of type 
[[ShuffleBlockFetcherIterator.PushMergedRemoteMetaFetchResult]].
+   *
+   * @param shuffleId shuffle id.
+   * @param reduceId  reduce id.
+   * @param blockSize size of the push-merged block.
+   * @param numChunks number of chunks in the push-merged block.
+   * @param bitmaps   chunk bitmaps, where each bitmap contains all the mapIds 
that were merged
+   *                  to that chunk.
+   * @return  shuffle chunks to fetch.
+   */
+  def createChunkBlockInfosFromMetaResponse(
+      shuffleId: Int,
+      reduceId: Int,
+      blockSize: Long,
+      numChunks: Int,

Review comment:
       They can't be different. We have the assertion `assert (bitmaps.size() 
== numChunks)` in the `MergedBlockMeta.readChunkBitmaps()`. It comes here only 
when this assertion is true. Otherwise when it fails, then 
`PushMergedRemoteMetaFailedFetchResult` is posted which triggers the fallback.




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