[SPARK-9446] Clear Active SparkContext in stop() method

In thread 'stopped SparkContext remaining active' on mailing list, Andres observed the following in driver log:
```
15/07/29 15:17:09 WARN YarnSchedulerBackend$YarnSchedulerEndpoint: ApplicationMaster has disassociated: <address removed>
15/07/29 15:17:09 INFO YarnClientSchedulerBackend: Shutting down all executors
Exception in thread "Yarn application state monitor" org.apache.spark.SparkException: Error asking standalone scheduler to shut down executors
        at org.apache.spark.scheduler.cluster.CoarseGrainedSchedulerBackend.stopExecutors(CoarseGrainedSchedulerBackend.scala:261)
        at org.apache.spark.scheduler.cluster.CoarseGrainedSchedulerBackend.stop(CoarseGrainedSchedulerBackend.scala:266)
        at org.apache.spark.scheduler.cluster.YarnClientSchedulerBackend.stop(YarnClientSchedulerBackend.scala:158)
        at org.apache.spark.scheduler.TaskSchedulerImpl.stop(TaskSchedulerImpl.scala:416)
        at org.apache.spark.scheduler.DAGScheduler.stop(DAGScheduler.scala:1411)
        at org.apache.spark.SparkContext.stop(SparkContext.scala:1644)
        at org.apache.spark.scheduler.cluster.YarnClientSchedulerBackend$$anon$1.run(YarnClientSchedulerBackend.scala:139)
Caused by: java.lang.InterruptedException
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.tryAcquireSharedNanos(AbstractQueuedSynchronizer.java:1325)
        at scala.concurrent.impl.Promise$DefaultPromise.tryAwait(Promise.scala:208)
        at scala.concurrent.impl.Promise$DefaultPromise.ready(Promise.scala:218)
        at scala.concurrent.impl.Promise$DefaultPromise.result(Promise.scala:223)
        at scala.concurrent.Await$$anonfun$result$1.apply(package.scala:190)
        at scala.concurrent.BlockContext$DefaultBlockContext$.blockOn(BlockContext.scala:53)
        at scala.concurrent.Await$.result(package.scala:190)15/07/29 15:17:09 INFO YarnClientSchedulerBackend: Asking each executor to shut down

        at org.apache.spark.rpc.RpcEndpointRef.askWithRetry(RpcEndpointRef.scala:102)
        at org.apache.spark.rpc.RpcEndpointRef.askWithRetry(RpcEndpointRef.scala:78)
        at org.apache.spark.scheduler.cluster.CoarseGrainedSchedulerBackend.stopExecutors(CoarseGrainedSchedulerBackend.scala:257)
        ... 6 more
```
Effect of the above exception is that a stopped SparkContext is returned to user since SparkContext.clearActiveContext() is not called.

Author: tedyu <yuzhihong@gmail.com>

Closes #7756 from tedyu/master and squashes the following commits:

7339ff2 [tedyu] Move null assignment out of tryLogNonFatalError block
6e02cd9 [tedyu] Use Utils.tryLogNonFatalError to guard resource release
f5fb519 [tedyu] Clear Active SparkContext in stop() method using finally

1 files changed, 37 insertions, 13 deletions

diff --git a/core/src/main/scala/org/apache/spark/SparkContext.scala b/core/src/main/scala/org/apache/spark/SparkContext.scala
index ac6ac6c216..2d8aa25d81 100644
--- a/core/src/main/scala/org/apache/spark/SparkContext.scala
+++ b/core/src/main/scala/org/apache/spark/SparkContext.scala
@@ -1689,33 +1689,57 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
       Utils.removeShutdownHook(_shutdownHookRef)
     }
 
-    postApplicationEnd()
-    _ui.foreach(_.stop())
+    Utils.tryLogNonFatalError {
+      postApplicationEnd()
+    }
+    Utils.tryLogNonFatalError {
+      _ui.foreach(_.stop())
+    }
     if (env != null) {
-      env.metricsSystem.report()
+      Utils.tryLogNonFatalError {
+        env.metricsSystem.report()
+      }
     }
     if (metadataCleaner != null) {
-      metadataCleaner.cancel()
+      Utils.tryLogNonFatalError {
+        metadataCleaner.cancel()
+      }
+    }
+    Utils.tryLogNonFatalError {
+      _cleaner.foreach(_.stop())
+    }
+    Utils.tryLogNonFatalError {
+      _executorAllocationManager.foreach(_.stop())
     }
-    _cleaner.foreach(_.stop())
-    _executorAllocationManager.foreach(_.stop())
     if (_dagScheduler != null) {
-      _dagScheduler.stop()
+      Utils.tryLogNonFatalError {
+        _dagScheduler.stop()
+      }
       _dagScheduler = null
     }
     if (_listenerBusStarted) {
-      listenerBus.stop()
-      _listenerBusStarted = false
+      Utils.tryLogNonFatalError {
+        listenerBus.stop()
+        _listenerBusStarted = false
+      }
+    }
+    Utils.tryLogNonFatalError {
+      _eventLogger.foreach(_.stop())
     }
-    _eventLogger.foreach(_.stop())
     if (env != null && _heartbeatReceiver != null) {
-      env.rpcEnv.stop(_heartbeatReceiver)
+      Utils.tryLogNonFatalError {
+        env.rpcEnv.stop(_heartbeatReceiver)
+      }
+    }
+    Utils.tryLogNonFatalError {
+      _progressBar.foreach(_.stop())
     }
-    _progressBar.foreach(_.stop())
     _taskScheduler = null
     // TODO: Cache.stop()?
     if (_env != null) {
-      _env.stop()
+      Utils.tryLogNonFatalError {
+        _env.stop()
+      }
       SparkEnv.set(null)
     }
     SparkContext.clearActiveContext()