[SPARK-23971] Should not leak Spark sessions across test suites

## What changes were proposed in this pull request?

Many suites currently leak Spark sessions (sometimes with stopped SparkContexts) via the thread-local active Spark session and default Spark session. We should attempt to clean these up and detect when this happens to improve the reproducibility of tests.

## How was this patch tested?

Existing tests

Author: Eric Liang <[email protected]>

Closes apache#21058 from ericl/clear-session.

Author: ericl

mllib/src/test/java/org/apache/spark/SharedSparkSession.java

@@ -42,7 +42,12 @@ public void setUp() throws IOException {
 
   @After
   public void tearDown() {
-    spark.stop();
-    spark = null;
+    try {
+      spark.stop();
+      spark = null;
+    } finally {
+      SparkSession.clearDefaultSession();
+      SparkSession.clearActiveSession();
+    }
   }
 }

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

@@ -44,7 +44,7 @@ import org.apache.spark.sql.sources.BaseRelation
 import org.apache.spark.sql.streaming._
 import org.apache.spark.sql.types.{DataType, StructType}
 import org.apache.spark.sql.util.ExecutionListenerManager
-import org.apache.spark.util.Utils
+import org.apache.spark.util.{CallSite, Utils}
 
 
 /**
@@ -81,6 +81,9 @@ class SparkSession private(
     @transient private[sql] val extensions: SparkSessionExtensions)
   extends Serializable with Closeable with Logging { self =>
 
+  // The call site where this SparkSession was constructed.
+  private val creationSite: CallSite = Utils.getCallSite()
+
   private[sql] def this(sc: SparkContext) {
     this(sc, None, None, new SparkSessionExtensions)
   }
@@ -763,7 +766,7 @@ class SparkSession private(
 
 
 @InterfaceStability.Stable
-object SparkSession {
+object SparkSession extends Logging {
 
   /**
    * Builder for [[SparkSession]].
@@ -1090,4 +1093,20 @@ object SparkSession {
     }
   }
 
+  private[spark] def cleanupAnyExistingSession(): Unit = {
+    val session = getActiveSession.orElse(getDefaultSession)
+    if (session.isDefined) {
+      logWarning(
+        s"""An existing Spark session exists as the active or default session.
+           |This probably means another suite leaked it. Attempting to stop it before continuing.
+           |This existing Spark session was created at:
+           |
+           |${session.get.creationSite.longForm}
+           |
+         """.stripMargin)
+      session.get.stop()
+      SparkSession.clearActiveSession()
+      SparkSession.clearDefaultSession()
+    }
+  }
 }

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

@@ -44,6 +44,8 @@ class SessionStateSuite extends SparkFunSuite {
     if (activeSession != null) {
       activeSession.stop()
       activeSession = null
+      SparkSession.clearActiveSession()
+      SparkSession.clearDefaultSession()
     }
     super.afterAll()
   }

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

@@ -60,6 +60,7 @@ trait SharedSparkSession
   protected implicit def sqlContext: SQLContext = _spark.sqlContext
 
   protected def createSparkSession: TestSparkSession = {
+    SparkSession.cleanupAnyExistingSession()
     new TestSparkSession(sparkConf)
   }
 
@@ -92,11 +93,22 @@ trait SharedSparkSession
    * Stop the underlying [[org.apache.spark.SparkContext]], if any.
    */
   protected override def afterAll(): Unit = {
-    super.afterAll()
-    if (_spark != null) {
-      _spark.sessionState.catalog.reset()
-      _spark.stop()
-      _spark = null
+    try {
+      super.afterAll()
+    } finally {
+      try {
+        if (_spark != null) {
+          try {
+            _spark.sessionState.catalog.reset()
+          } finally {
+            _spark.stop()
+            _spark = null
+          }
+        }
+      } finally {
+        SparkSession.clearActiveSession()
+        SparkSession.clearDefaultSession()
+      }
     }
   }