[GitHub] [spark] HyukjinKwon opened a new pull request #23977: [WIP][SPARK-26923][SQL][R] Refactor ArrowRRunner and RRunner to share one BaseRRunner

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HyukjinKwon opened a new pull request #23977: [WIP][SPARK-26923][SQL][R] Refactor ArrowRRunner and RRunner to share one BaseRRunner
URL: https://github.com/apache/spark/pull/23977
 
 
   ## What changes were proposed in this pull request?
   
   This PR proposes to have one base R runner. 
   
   In the high level,
   
   Previously, it had `ArrowRRunner` and it inherited `RRunner`:
   
   ```
   └── RRunner
       └── ArrowRRunner
   ```
   
   After this PR, now it has a `BaseRRunner`, and `ArrowRRunner` and `RRunner` inherit `BaseRRunner`:
   
   ```
   └── BaseRRunner
       ├── ArrowRRunner
       └── RRunner
   ```
   
   This way is consistent with Python's.
   
   In more details, see below:
   
   ```scala
   class BaseRRunner[IN, OUT] {
   
     def compute: Iterator[OUT] = {
       ...
       newWriterThread(...)
       ...
       newReaderIterator(...)
       ...
     }
   
     // Make a thread that writes data from JVM to R process
     abstract protected def newWriterThread(..., iter: Iterator[IN], ...): WriterThread
   
     // Make an iterator that reads data from the R process to JVM
     abstract protected def newReaderIterator(...): ReaderIterator
   
     abstract class WriterThread(..., iter: Iterator[IN], ...) extends Thread {
       override def run(): Unit {
         ...
         writeIteratorToStream(...)
         ...
       }
   
       // Actually writing logic to the socket stream.
       abstract protected def writeIteratorToStream(dataOut: DataOutputStream): Unit
     }
   
     abstract class ReaderIterator extends Iterator[OUT] {
       override def hasNext: Boolean = {
   
       }
   
       override def next: OUT = {
         ...
         hasNext
         ...    
       }
   
       // Actually reading logic from the socket stream.
       abstract protected def read: OUT
     }
   }
   ```
   
   ```scala
   case RRunner extends BaseRRunner {
     override def newWriterThread(...) {
       new WriterThread(...) {
         override def writeIteratorToStream {
           ...
         }
       }
     }
   
     override def newReaderIterator(...) {
       new ReaderIterator(...) {
         override def read {
           ...
         }
       }
     }
   }
   ```
   
   ## How was this patch tested?
   
   Manually tested and existing tests should cover.
   

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