[SPARK-13747][SQL] Fix concurrent query with fork-join pool

## What changes were proposed in this pull request?

Fix this use case, which was already fixed in SPARK-10548 in 1.6 but was broken in master due to #9264:

```
(1 to 100).par.foreach { _ => sc.parallelize(1 to 5).map { i => (i, i) }.toDF("a", "b").count() }
```

This threw `IllegalArgumentException` consistently before this patch. For more detail, see the JIRA.

## How was this patch tested?

New test in `SQLExecutionSuite`.

Author: Andrew Or <andrew@databricks.com>

Closes #11586 from andrewor14/fix-concurrent-sql.

core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala

@@ -613,7 +613,12 @@ class DAGScheduler(
       properties: Properties): Unit = {
     val start = System.nanoTime
     val waiter = submitJob(rdd, func, partitions, callSite, resultHandler, properties)
-    Await.ready(waiter.completionFuture, atMost = Duration.Inf)
+    // Note: Do not call Await.ready(future) because that calls `scala.concurrent.blocking`,
+    // which causes concurrent SQL executions to fail if a fork-join pool is used. Note that
+    // due to idiosyncrasies in Scala, `awaitPermission` is not actually used anywhere so it's
+    // safe to pass in null here. For more detail, see SPARK-13747.
+    val awaitPermission = null.asInstanceOf[scala.concurrent.CanAwait]
+    waiter.completionFuture.ready(Duration.Inf)(awaitPermission)
     waiter.completionFuture.value.get match {
       case scala.util.Success(_) =>
         logInfo("Job %d finished: %s, took %f s".format

sql/core/src/test/scala/org/apache/spark/sql/execution/SQLExecutionSuite.scala

@@ -49,6 +49,20 @@ class SQLExecutionSuite extends SparkFunSuite {
     }
   }
 
+  test("concurrent query execution with fork-join pool (SPARK-13747)") {
+    val sc = new SparkContext("local[*]", "test")
+    val sqlContext = new SQLContext(sc)
+    import sqlContext.implicits._
+    try {
+      // Should not throw IllegalArgumentException
+      (1 to 100).par.foreach { _ =>
+        sc.parallelize(1 to 5).map { i => (i, i) }.toDF("a", "b").count()
+      }
+    } finally {
+      sc.stop()
+    }
+  }
+
   /**
    * Trigger SPARK-10548 by mocking a parent and its child thread executing queries concurrently.
    */