Limit scope of mutable set

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

@@ -361,10 +361,15 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
     override protected def initialValue(): Properties = new Properties()
   }
 
+  // Keys of local properties that should not be inherited by children threads
+  private val nonInheritedLocalProperties: HashSet[String] = new HashSet[String]
+
   /**
-   * Keys of local properties that should not be inherited by children threads.
+   * Mark a local property such that its values are never inherited across the thread hierarchy.
    */
-  private[spark] val nonInheritedLocalProperties: HashSet[String] = new HashSet[String]
+  private[spark] def markLocalPropertyNonInherited(key: String): Unit = {
+    nonInheritedLocalProperties += key
+  }
 
   /* ------------------------------------------------------------------------------------- *
    | Initialization. This code initializes the context in a manner that is exception-safe. |

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

@@ -214,7 +214,7 @@ class ThreadingSuite extends SparkFunSuite with LocalSparkContext with Logging {
 
   test("inheritance exclusions (SPARK-10548)") {
     sc = new SparkContext("local", "test")
-    sc.nonInheritedLocalProperties.add("do-not-inherit-me")
+    sc.markLocalPropertyNonInherited("do-not-inherit-me")
     sc.setLocalProperty("do-inherit-me", "parent")
     sc.setLocalProperty("do-not-inherit-me", "parent")
     var throwable: Option[Throwable] = None

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

@@ -81,7 +81,7 @@ class SQLContext(@transient val sparkContext: SparkContext)
   // Ensure query execution IDs are not inherited across the thread hierarchy, which is
   // the default behavior for SparkContext local properties. Otherwise, we may confuse
   // the listener as to which query is being executed. (SPARK-10548)
-  sparkContext.nonInheritedLocalProperties.add(SQLExecution.EXECUTION_ID_KEY)
+  sparkContext.markLocalPropertyNonInherited(SQLExecution.EXECUTION_ID_KEY)
 
   /**
    * Set Spark SQL configuration properties.