[SPARK-20204] SQLConf should react to change in default timezone sett…

…ings

core/src/main/scala/org/apache/spark/internal/config/ConfigBuilder.scala

@@ -147,6 +147,15 @@ private[spark] class TypedConfigBuilder[T](
     }
   }
 
+  /** Creates a [[ConfigEntry]] with a function has a default value */
+  def createWithDefaultFunction(defaultFunc: () => T): ConfigEntry[T] = {
+    val entry =
+      new ConfigEntryWithDefaultFunction[T](parent.key, defaultFunc, converter,
+      stringConverter, parent._doc, parent._public)
+    parent._onCreate.foreach(_ (entry))
+    entry
+  }
+
   /**
    * Creates a [[ConfigEntry]] that has a default value. The default value is provided as a
    * [[String]] and must be a valid value for the entry.

core/src/main/scala/org/apache/spark/internal/config/ConfigEntry.scala

@@ -78,7 +78,24 @@ private class ConfigEntryWithDefault[T] (
   def readFrom(reader: ConfigReader): T = {
     reader.get(key).map(valueConverter).getOrElse(_defaultValue)
   }
+}
+
+private class ConfigEntryWithDefaultFunction[T] (
+     key: String,
+     _defaultFunction: () => T,
+     valueConverter: String => T,
+     stringConverter: T => String,
+     doc: String,
+     isPublic: Boolean)
+  extends ConfigEntry(key, valueConverter, stringConverter, doc, isPublic) {
+
+  override def defaultValue: Option[T] = Some(_defaultFunction())
 
+  override def defaultValueString: String = stringConverter(_defaultFunction())
+
+  def readFrom(reader: ConfigReader): T = {
+    reader.get(key).map(valueConverter).getOrElse(_defaultFunction())
+  }
 }
 
 private class ConfigEntryWithDefaultString[T] (

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -752,7 +752,7 @@ object SQLConf {
     buildConf("spark.sql.session.timeZone")
       .doc("""The ID of session local timezone, e.g. "GMT", "America/Los_Angeles", etc.""")
       .stringConf
-      .createWithDefault(TimeZone.getDefault().getID())
+      .createWithDefaultFunction(() => TimeZone.getDefault().getID())
 
   val WINDOW_EXEC_BUFFER_SPILL_THRESHOLD =
     buildConf("spark.sql.windowExec.buffer.spill.threshold")