[SPARK-3646][SQL] Copy SQL configuration from SparkConf when a SQLContext is created.

This will allow us to take advantage of things like the spark.defaults file.

Author: Michael Armbrust <michael@databricks.com>

Closes #2493 from marmbrus/copySparkConf and squashes the following commits:

0bd1377 [Michael Armbrust] Copy SQL configuration from SparkConf when a SQLContext is created.

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

@@ -75,6 +75,11 @@ class SQLContext(@transient val sparkContext: SparkContext)
   protected[sql] def executePlan(plan: LogicalPlan): this.QueryExecution =
     new this.QueryExecution { val logical = plan }
 
+  sparkContext.getConf.getAll.foreach {
+    case (key, value) if key.startsWith("spark.sql") => setConf(key, value)
+    case _ =>
+  }
+
   /**
    * :: DeveloperApi ::
    * Allows catalyst LogicalPlans to be executed as a SchemaRDD.  Note that the LogicalPlan

sql/core/src/main/scala/org/apache/spark/sql/test/TestSQLContext.scala

@@ -22,7 +22,11 @@ import org.apache.spark.sql.{SQLConf, SQLContext}
 
 /** A SQLContext that can be used for local testing. */
 object TestSQLContext
-  extends SQLContext(new SparkContext("local[2]", "TestSQLContext", new SparkConf())) {
+  extends SQLContext(
+    new SparkContext(
+      "local[2]",
+      "TestSQLContext",
+      new SparkConf().set("spark.sql.testkey", "true"))) {
 
   /** Fewer partitions to speed up testing. */
   override private[spark] def numShufflePartitions: Int =

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

@@ -17,16 +17,25 @@
 
 package org.apache.spark.sql
 
+import org.scalatest.FunSuiteLike
+
 import org.apache.spark.sql.test._
 
 /* Implicits */
 import TestSQLContext._
 
-class SQLConfSuite extends QueryTest {
+class SQLConfSuite extends QueryTest with FunSuiteLike {
 
   val testKey = "test.key.0"
   val testVal = "test.val.0"
 
+  test("propagate from spark conf") {
+    // We create a new context here to avoid order dependence with other tests that might call
+    // clear().
+    val newContext = new SQLContext(TestSQLContext.sparkContext)
+    assert(newContext.getConf("spark.sql.testkey", "false") == "true")
+  }
+
   test("programmatic ways of basic setting and getting") {
     clear()
     assert(getAllConfs.size === 0)