Github user holdenk commented on a diff in the pull request:

    https://github.com/apache/spark/pull/14467#discussion_r75384836
  
    --- Diff: core/src/main/scala/org/apache/spark/api/python/PythonRDD.scala 
---
    @@ -889,21 +892,42 @@ private class PythonAccumulatorParam(@transient 
private val serverHost: String,
         socket
       }
     
    -  override def zero(value: JList[Array[Byte]]): JList[Array[Byte]] = new 
JArrayList
    +  override def reset(): Unit = {
    +    this._acc = Collections.synchronizedList(new JArrayList[Array[Byte]])
    +  }
    +
    +  override def isZero: Boolean = {
    +    this._acc.isEmpty
    +  }
    +
    +  override def copyAndReset(): PythonAccumulatorV2 = new 
PythonAccumulatorV2(serverHost, serverPort)
    +
    +  override def copy(): PythonAccumulatorV2 = {
    +    val newAcc = new PythonAccumulatorV2(serverHost, serverPort)
    +    newAcc._acc.addAll(this._acc)
    +    newAcc
    +  }
    +
    +  // This happens on the worker node, where we just want to remember all 
the updates
    +  override def add(val2: JList[Array[Byte]]): Unit = {
    +    _acc.addAll(val2)
    +  }
    +
     
    -  override def addInPlace(val1: JList[Array[Byte]], val2: 
JList[Array[Byte]])
    -      : JList[Array[Byte]] = synchronized {
    +  override def merge(other: AccumulatorV2[JList[Array[Byte]], 
JList[Array[Byte]]]): Unit = {
    +    val otherPythonAccumulator = other.asInstanceOf[PythonAccumulatorV2]
    +    // This conditional isn't strictly speaking needed - merging only 
currently happens on the
    +    // driver program - but that isn't gauranteed so incase this changes.
         if (serverHost == null) {
    -      // This happens on the worker node, where we just want to remember 
all the updates
    -      val1.addAll(val2)
    -      val1
    +      // We are on the worker
    +      add(otherPythonAccumulator._acc)
         } else {
           // This happens on the master, where we pass the updates to Python 
through a socket
           val socket = openSocket()
           val in = socket.getInputStream
           val out = new DataOutputStream(new 
BufferedOutputStream(socket.getOutputStream, bufferSize))
    -      out.writeInt(val2.size)
    -      for (array <- val2.asScala) {
    +      out.writeInt(otherPythonAccumulator._acc.size)
    --- End diff --
    
    Sure I can do that - just on the other hand we already don't do this inside 
of CollectionAccumulator (it uses a raw ArrayList without any syncrhonization) 
- so if this assumption were to change we would also break all collection 
accumulators.


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