[SPARK-22938] Assert that SQLConf.get is accessed only on the driver.

## What changes were proposed in this pull request?

Assert if code tries to access SQLConf.get on executor.
This can lead to hard to detect bugs, where the executor will read fallbackConf, falling back to default config values, ignoring potentially changed non-default configs.
If a config is to be passed to executor code, it needs to be read on the driver, and passed explicitly.

## How was this patch tested?

Check in existing tests.

Author: Juliusz Sompolski <[email protected]>

Closes #20136 from juliuszsompolski/SPARK-22938.

Author: juliuszsompolski

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

@@ -27,11 +27,13 @@ import scala.util.matching.Regex
 
 import org.apache.hadoop.fs.Path
 
+import org.apache.spark.{SparkContext, SparkEnv}
 import org.apache.spark.internal.Logging
 import org.apache.spark.internal.config._
 import org.apache.spark.network.util.ByteUnit
 import org.apache.spark.sql.catalyst.analysis.Resolver
 import org.apache.spark.sql.catalyst.expressions.codegen.CodeGenerator
+import org.apache.spark.util.Utils
 
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 // This file defines the configuration options for Spark SQL.
@@ -70,7 +72,7 @@ object SQLConf {
    * Default config. Only used when there is no active SparkSession for the thread.
    * See [[get]] for more information.
    */
-  private val fallbackConf = new ThreadLocal[SQLConf] {
+  private lazy val fallbackConf = new ThreadLocal[SQLConf] {
     override def initialValue: SQLConf = new SQLConf
   }
 
@@ -1087,6 +1089,12 @@ object SQLConf {
 class SQLConf extends Serializable with Logging {
   import SQLConf._
 
+  if (Utils.isTesting && SparkEnv.get != null) {
+    // assert that we're only accessing it on the driver.
+    assert(SparkEnv.get.executorId == SparkContext.DRIVER_IDENTIFIER,
+      "SQLConf should only be created and accessed on the driver.")
+  }
+
   /** Only low degree of contention is expected for conf, thus NOT using ConcurrentHashMap. */
   @transient protected[spark] val settings = java.util.Collections.synchronizedMap(
     new java.util.HashMap[String, String]())