[SPARK-17240][CORE] Make SparkConf serializable again.

Make the config reader transient, and initialize it lazily so that
serialization works with both java and kryo (and hopefully any other
custom serializer).

Added unit test to make sure SparkConf remains serializable and the
reader works with both built-in serializers.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #14813 from vanzin/SPARK-17240.

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

@@ -56,10 +56,13 @@ class SparkConf(loadDefaults: Boolean) extends Cloneable with Logging with Seria
 
   private val settings = new ConcurrentHashMap[String, String]()
 
-  private val reader = new ConfigReader(new SparkConfigProvider(settings))
-  reader.bindEnv(new ConfigProvider {
-    override def get(key: String): Option[String] = Option(getenv(key))
-  })
+  @transient private lazy val reader: ConfigReader = {
+    val _reader = new ConfigReader(new SparkConfigProvider(settings))
+    _reader.bindEnv(new ConfigProvider {
+      override def get(key: String): Option[String] = Option(getenv(key))
+    })
+    _reader
+  }
 
   if (loadDefaults) {
     loadFromSystemProperties(false)

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

@@ -26,8 +26,9 @@ import scala.util.{Random, Try}
 
 import com.esotericsoftware.kryo.Kryo
 
+import org.apache.spark.internal.config._
 import org.apache.spark.network.util.ByteUnit
-import org.apache.spark.serializer.{KryoRegistrator, KryoSerializer}
+import org.apache.spark.serializer.{JavaSerializer, KryoRegistrator, KryoSerializer}
 import org.apache.spark.util.{ResetSystemProperties, RpcUtils}
 
 class SparkConfSuite extends SparkFunSuite with LocalSparkContext with ResetSystemProperties {
@@ -283,6 +284,25 @@ class SparkConfSuite extends SparkFunSuite with LocalSparkContext with ResetSyst
     assert(conf.contains("spark.io.compression.lz4.blockSize"))
     assert(conf.contains("spark.io.unknown") === false)
   }
+
+  val serializers = Map(
+    "java" -> new JavaSerializer(new SparkConf()),
+    "kryo" -> new KryoSerializer(new SparkConf()))
+
+  serializers.foreach { case (name, ser) =>
+    test(s"SPARK-17240: SparkConf should be serializable ($name)") {
+      val conf = new SparkConf()
+      conf.set(DRIVER_CLASS_PATH, "${" + DRIVER_JAVA_OPTIONS.key + "}")
+      conf.set(DRIVER_JAVA_OPTIONS, "test")
+
+      val serializer = ser.newInstance()
+      val bytes = serializer.serialize(conf)
+      val deser = serializer.deserialize[SparkConf](bytes)
+
+      assert(conf.get(DRIVER_CLASS_PATH) === deser.get(DRIVER_CLASS_PATH))
+    }
+  }
+
 }
 
 class Class1 {}