[SPARK-10812] [YARN] Spark hadoop util support switching to yarn

While this is likely not a huge issue for real production systems, for test systems which may setup a Spark Context and tear it down and stand up a Spark Context with a different master (e.g. some local mode & some yarn mode) tests this cane be an issue. Discovered during work on spark-testing-base on Spark 1.4.1, but seems like the logic that triggers it is present in master (see SparkHadoopUtil object). A valid work around for users encountering this issue is to fork a different JVM, however this can be heavy weight.

```
[info] SampleMiniClusterTest:
[info] Exception encountered when attempting to run a suite with class name: com.holdenkarau.spark.testing.SampleMiniClusterTest *** ABORTED ***
[info] java.lang.ClassCastException: org.apache.spark.deploy.SparkHadoopUtil cannot be cast to org.apache.spark.deploy.yarn.YarnSparkHadoopUtil
[info] at org.apache.spark.deploy.yarn.YarnSparkHadoopUtil$.get(YarnSparkHadoopUtil.scala:163)
[info] at org.apache.spark.deploy.yarn.Client.prepareLocalResources(Client.scala:257)
[info] at org.apache.spark.deploy.yarn.Client.createContainerLaunchContext(Client.scala:561)
[info] at org.apache.spark.deploy.yarn.Client.submitApplication(Client.scala:115)
[info] at org.apache.spark.scheduler.cluster.YarnClientSchedulerBackend.start(YarnClientSchedulerBackend.scala:57)
[info] at org.apache.spark.scheduler.TaskSchedulerImpl.start(TaskSchedulerImpl.scala:141)
[info] at org.apache.spark.SparkContext.<init>(SparkContext.scala:497)
[info] at com.holdenkarau.spark.testing.SharedMiniCluster$class.setup(SharedMiniCluster.scala:186)
[info] at com.holdenkarau.spark.testing.SampleMiniClusterTest.setup(SampleMiniClusterTest.scala:26)
[info] at com.holdenkarau.spark.testing.SharedMiniCluster$class.beforeAll(SharedMiniCluster.scala:103)
```

Author: Holden Karau <holden@pigscanfly.ca>

Closes #8911 from holdenk/SPARK-10812-spark-hadoop-util-support-switching-to-yarn.

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -1756,6 +1756,8 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
       }
       SparkEnv.set(null)
     }
+    // Unset YARN mode system env variable, to allow switching between cluster types.
+    System.clearProperty("SPARK_YARN_MODE")
     SparkContext.clearActiveContext()
     logInfo("Successfully stopped SparkContext")
   }

core/src/main/scala/org/apache/spark/deploy/SparkHadoopUtil.scala

@@ -385,20 +385,13 @@ class SparkHadoopUtil extends Logging {
 
 object SparkHadoopUtil {
 
-  private val hadoop = {
-    val yarnMode = java.lang.Boolean.valueOf(
-        System.getProperty("SPARK_YARN_MODE", System.getenv("SPARK_YARN_MODE")))
-    if (yarnMode) {
-      try {
-        Utils.classForName("org.apache.spark.deploy.yarn.YarnSparkHadoopUtil")
-          .newInstance()
-          .asInstanceOf[SparkHadoopUtil]
-      } catch {
-       case e: Exception => throw new SparkException("Unable to load YARN support", e)
-      }
-    } else {
-      new SparkHadoopUtil
-    }
+  private lazy val hadoop = new SparkHadoopUtil
+  private lazy val yarn = try {
+    Utils.classForName("org.apache.spark.deploy.yarn.YarnSparkHadoopUtil")
+      .newInstance()
+      .asInstanceOf[SparkHadoopUtil]
+  } catch {
+    case e: Exception => throw new SparkException("Unable to load YARN support", e)
   }
 
   val SPARK_YARN_CREDS_TEMP_EXTENSION = ".tmp"
@@ -406,6 +399,13 @@ object SparkHadoopUtil {
   val SPARK_YARN_CREDS_COUNTER_DELIM = "-"
 
   def get: SparkHadoopUtil = {
-    hadoop
+    // Check each time to support changing to/from YARN
+    val yarnMode = java.lang.Boolean.valueOf(
+        System.getProperty("SPARK_YARN_MODE", System.getenv("SPARK_YARN_MODE")))
+    if (yarnMode) {
+      yarn
+    } else {
+      hadoop
+    }
   }
 }

yarn/src/main/scala/org/apache/spark/deploy/yarn/Client.scala

@@ -86,7 +86,11 @@ private[spark] class Client(
   private val fireAndForget = isClusterMode &&
     !sparkConf.getBoolean("spark.yarn.submit.waitAppCompletion", true)
 
-  def stop(): Unit = yarnClient.stop()
+  def stop(): Unit = {
+    yarnClient.stop()
+    // Unset YARN mode system env variable, to allow switching between cluster types.
+    System.clearProperty("SPARK_YARN_MODE")
+  }
 
   /**
    * Submit an application running our ApplicationMaster to the ResourceManager.

yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala

@@ -30,6 +30,7 @@ import org.scalatest.Matchers
 import org.apache.hadoop.yarn.api.records.ApplicationAccessType
 
 import org.apache.spark.{Logging, SecurityManager, SparkConf, SparkException, SparkFunSuite}
+import org.apache.spark.deploy.SparkHadoopUtil
 import org.apache.spark.util.Utils
 
 
@@ -233,4 +234,15 @@ class YarnSparkHadoopUtilSuite extends SparkFunSuite with Matchers with Logging
       }
     assert(caught.getMessage === "Can't get Master Kerberos principal for use as renewer")
   }
+
+  test("check different hadoop utils based on env variable") {
+    try {
+      System.setProperty("SPARK_YARN_MODE", "true")
+      assert(SparkHadoopUtil.get.getClass === classOf[YarnSparkHadoopUtil])
+      System.setProperty("SPARK_YARN_MODE", "false")
+      assert(SparkHadoopUtil.get.getClass === classOf[SparkHadoopUtil])
+    } finally {
+      System.clearProperty("SPARK_YARN_MODE")
+    }
+  }
 }