address comments

Author: cloud-fan

sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala

@@ -575,6 +575,10 @@ final class DataFrameWriter[T] private[sql](ds: Dataset[T]) {
     format("csv").save(path)
   }
 
+  /**
+   * Wrap a DataFrameWriter action to track the QueryExecution and time cost, then report to the
+   * user-registered callback functions.
+   */
   private def runCommand(session: SparkSession, name: String)(command: LogicalPlan): Unit = {
     val qe = session.sessionState.executePlan(command)
     try {

sql/core/src/test/scala/org/apache/spark/sql/util/DataFrameCallbackSuite.scala

@@ -166,7 +166,6 @@ class DataFrameCallbackSuite extends QueryTest with SharedSQLContext {
     val commands = ArrayBuffer.empty[(String, LogicalPlan)]
     val exceptions = ArrayBuffer.empty[(String, Exception)]
     val listener = new QueryExecutionListener {
-      // Only test successful case here, so no need to implement `onFailure`
       override def onFailure(funcName: String, qe: QueryExecution, exception: Exception): Unit = {
         exceptions += funcName -> exception
       }