[SPARK-20946][SQL] simplify the config setting logic in SparkSession.getOrCreate

## What changes were proposed in this pull request?

The current conf setting logic is a little complex and has duplication, this PR simplifies it.

## How was this patch tested?

existing tests.

Author: Wenchen Fan <[email protected]>

Closes #18172 from cloud-fan/session.

Author: cloud-fan

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

@@ -820,15 +820,13 @@ 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

@@ -757,6 +757,8 @@ object SparkSession {
 
     private[this] var userSuppliedContext: Option[SparkContext] = None
 
+    // The `SparkConf` inside the given `SparkContext` may get changed if you specify some options
+    // for this builder.
     private[spark] def sparkContext(sparkContext: SparkContext): Builder = synchronized {
       userSuppliedContext = Option(sparkContext)
       this
@@ -854,7 +856,7 @@ object SparkSession {
      *
      * @since 2.2.0
      */
-    def withExtensions(f: SparkSessionExtensions => Unit): Builder = {
+    def withExtensions(f: SparkSessionExtensions => Unit): Builder = synchronized {
       f(extensions)
       this
     }
@@ -899,22 +901,14 @@ 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) }
-          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)
-          }
-          sc
+          options.get("spark.master").foreach(sparkConf.setMaster)
+          // set a random app name if not given.
+          sparkConf.setAppName(options.getOrElse("spark.app.name",
+            java.util.UUID.randomUUID().toString))
+          SparkContext.getOrCreate(sparkConf)
         }
+        options.foreach { case (k, v) => sparkContext.conf.set(k, v) }
 
         // Initialize extensions if the user has defined a configurator class.
         val extensionConfOption = sparkContext.conf.get(StaticSQLConf.SPARK_SESSION_EXTENSIONS)
@@ -935,7 +929,6 @@ object SparkSession {
         }
 
         session = new SparkSession(sparkContext, None, None, extensions)
-        options.foreach { case (k, v) => session.sessionState.conf.setConfString(k, v) }
         defaultSession.set(session)
 
         // Register a successfully instantiated context to the singleton. This should be at the