Revert inheritablethreadlocal and just restore taskcontext (apache-spark-on-k8s#216)

Author: robert3005

core/src/main/scala/org/apache/spark/TaskContext.scala

@@ -48,8 +48,7 @@ object TaskContext {
     }
   }
 
-  private[this] val taskContext: InheritableThreadLocal[TaskContext] =
-    new InheritableThreadLocal[TaskContext]
+  private[this] val taskContext: ThreadLocal[TaskContext] = new ThreadLocal[TaskContext]
 
   // Note: protected[spark] instead of private[spark] to prevent the following two from
   // showing up in JavaDoc.

core/src/test/scala/org/apache/spark/ThreadingSuite.scala

@@ -234,26 +234,6 @@ class ThreadingSuite extends SparkFunSuite with LocalSparkContext with Logging {
     assert(threadTestValue === originalTestValue)
   }
 
-  test("taskContext accessed from a thread pool") {
-    TaskContext.setTaskContext(TaskContext.empty())
-    assert(TaskContext.get() != null, "taskContext should have been defined")
-    var throwable: Option[Throwable] = None
-    val thread = new Thread() {
-      override def run() {
-        try {
-          assert(TaskContext.get() != null, "taskContext should have been inherited")
-        } catch {
-          case t: Throwable =>
-            throwable = Option(t)
-        }
-      }
-    }
-    thread.start()
-    thread.join()
-    throwable.foreach { t => throw improveStackTrace(t) }
-    TaskContext.unset()
-  }
-
   /**
    * Improve the stack trace of an error thrown from within a thread.
    * Otherwise it's difficult to tell which line in the test the error came from.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetFileFormat.scala

@@ -577,8 +577,10 @@ object ParquetFileFormat extends Logging {
     val parFiles = partFiles.par
     val pool = ThreadUtils.newForkJoinPool("readingParquetFooters", 8)
     parFiles.tasksupport = new ForkJoinTaskSupport(pool)
+    val tc = TaskContext.get()
     try {
       parFiles.flatMap { currentFile =>
+        TaskContext.setTaskContext(tc)
         try {
           // Skips row group information since we only need the schema.
           // ParquetFileReader.readFooter throws RuntimeException, instead of IOException,