[SPARK-25076][SQL] SQLConf should not be retrieved from a stopped SparkSession

## What changes were proposed in this pull request?

When a `SparkSession` is stopped, `SQLConf.get` should use the fallback conf to avoid weird issues like
```
sbt.ForkMain$ForkError: java.lang.IllegalStateException: LiveListenerBus is stopped.
	at org.apache.spark.scheduler.LiveListenerBus.addToQueue(LiveListenerBus.scala:97)
	at org.apache.spark.scheduler.LiveListenerBus.addToStatusQueue(LiveListenerBus.scala:80)
	at org.apache.spark.sql.internal.SharedState.<init>(SharedState.scala:93)
	at org.apache.spark.sql.SparkSession$$anonfun$sharedState$1.apply(SparkSession.scala:120)
	at org.apache.spark.sql.SparkSession$$anonfun$sharedState$1.apply(SparkSession.scala:120)
	at scala.Option.getOrElse(Option.scala:121)
...
```

## How was this patch tested?

a new test suite

Closes #22056 from cloud-fan/session.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Xiao Li <gatorsmile@gmail.com>

Author: cloud-fan

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

@@ -92,7 +92,8 @@ class SparkSession private(
 
   // If there is no active SparkSession, uses the default SQL conf. Otherwise, use the session's.
   SQLConf.setSQLConfGetter(() => {
-    SparkSession.getActiveSession.map(_.sessionState.conf).getOrElse(SQLConf.getFallbackConf)
+    SparkSession.getActiveSession.filterNot(_.sparkContext.isStopped).map(_.sessionState.conf)
+      .getOrElse(SQLConf.getFallbackConf)
   })
 
   /**

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

@@ -36,19 +36,14 @@ trait LocalSparkSession extends BeforeAndAfterEach with BeforeAndAfterAll { self
 
   override def afterEach() {
     try {
-      resetSparkContext()
+      LocalSparkSession.stop(spark)
       SparkSession.clearActiveSession()
       SparkSession.clearDefaultSession()
+      spark = null
     } finally {
       super.afterEach()
     }
   }
-
-  def resetSparkContext(): Unit = {
-    LocalSparkSession.stop(spark)
-    spark = null
-  }
-
 }
 
 object LocalSparkSession {

sql/core/src/test/scala/org/apache/spark/sql/internal/SQLConfGetterSuite.scala

@@ -0,0 +1,33 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.internal
+
+import org.apache.spark.SparkFunSuite
+import org.apache.spark.sql.{LocalSparkSession, SparkSession}
+
+class SQLConfGetterSuite extends SparkFunSuite with LocalSparkSession {
+
+  test("SPARK-25076: SQLConf should not be retrieved from a stopped SparkSession") {
+    spark = SparkSession.builder().master("local").getOrCreate()
+    assert(SQLConf.get eq spark.sessionState.conf,
+      "SQLConf.get should get the conf from the active spark session.")
+    spark.stop()
+    assert(SQLConf.get eq SQLConf.getFallbackConf,
+      "SQLConf.get should not get conf from a stopped spark session.")
+  }
+}