[SPARK-20946][SQL] Do not update conf for existing SparkContext in SparkSession.getOrCreate

## What changes were proposed in this pull request?

SparkContext is shared by all sessions, we should not update its conf for only one session.

## How was this patch tested?

existing tests

Author: Wenchen Fan <wenchen@databricks.com>

Closes #18536 from cloud-fan/config.

mllib/src/test/scala/org/apache/spark/ml/recommendation/ALSSuite.scala

@@ -818,15 +818,13 @@ class ALSCleanerSuite extends SparkFunSuite {
       FileUtils.listFiles(localDir, TrueFileFilter.INSTANCE, TrueFileFilter.INSTANCE).asScala.toSet
     try {
       conf.set("spark.local.dir", localDir.getAbsolutePath)
-      val sc = new SparkContext("local[2]", "test", conf)
+      val sc = new SparkContext("local[2]", "ALSCleanerSuite", conf)
       try {
         sc.setCheckpointDir(checkpointDir.getAbsolutePath)
         // Generate test data
         val (training, _) = ALSSuite.genImplicitTestData(sc, 20, 5, 1, 0.2, 0)
         // Implicitly test the cleaning of parents during ALS training
         val spark = SparkSession.builder
-          .master("local[2]")
-          .appName("ALSCleanerSuite")
           .sparkContext(sc)
           .getOrCreate()
         import spark.implicits._

mllib/src/test/scala/org/apache/spark/ml/tree/impl/TreeTests.scala

@@ -43,8 +43,6 @@ private[ml] object TreeTests extends SparkFunSuite {
       categoricalFeatures: Map[Int, Int],
       numClasses: Int): DataFrame = {
     val spark = SparkSession.builder()
-      .master("local[2]")
-      .appName("TreeTests")
       .sparkContext(data.sparkContext)
       .getOrCreate()
     import spark.implicits._

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

@@ -867,7 +867,7 @@ object SparkSession {
      *
      * @since 2.2.0
      */
-    def withExtensions(f: SparkSessionExtensions => Unit): Builder = {
+    def withExtensions(f: SparkSessionExtensions => Unit): Builder = synchronized {
       f(extensions)
       this
     }
@@ -912,21 +912,16 @@ object SparkSession {
 
         // No active nor global default session. Create a new one.
         val sparkContext = userSuppliedContext.getOrElse {
-          // set app name if not given
-          val randomAppName = java.util.UUID.randomUUID().toString
           val sparkConf = new SparkConf()
           options.foreach { case (k, v) => sparkConf.set(k, v) }
+
+          // set a random app name if not given.
           if (!sparkConf.contains("spark.app.name")) {
-            sparkConf.setAppName(randomAppName)
-          }
-          val sc = SparkContext.getOrCreate(sparkConf)
-          // maybe this is an existing SparkContext, update its SparkConf which maybe used
-          // by SparkSession
-          options.foreach { case (k, v) => sc.conf.set(k, v) }
-          if (!sc.conf.contains("spark.app.name")) {
-            sc.conf.setAppName(randomAppName)
+            sparkConf.setAppName(java.util.UUID.randomUUID().toString)
           }
-          sc
+
+          SparkContext.getOrCreate(sparkConf)
+          // Do not update `SparkConf` for existing `SparkContext`, as it's shared by all sessions.
         }
 
         // Initialize extensions if the user has defined a configurator class.

sql/core/src/test/scala/org/apache/spark/sql/SparkSessionBuilderSuite.scala

@@ -102,11 +102,9 @@ class SparkSessionBuilderSuite extends SparkFunSuite {
     assert(session.conf.get("key1") == "value1")
     assert(session.conf.get("key2") == "value2")
     assert(session.sparkContext == sparkContext2)
-    assert(session.sparkContext.conf.get("key1") == "value1")
-    // If the created sparkContext is not passed through the Builder's API sparkContext,
-    // the conf of this sparkContext will also contain the conf set through the API config.
-    assert(session.sparkContext.conf.get("key2") == "value2")
-    assert(session.sparkContext.conf.get("spark.app.name") == "test")
+    // We won't update conf for existing `SparkContext`
+    assert(!sparkContext2.conf.contains("key2"))
+    assert(sparkContext2.conf.get("key1") == "value1")
     session.stop()
   }