[SPARK-4470] Validate number of threads in local mode

When running Spark locally, if number of threads is specified as 0 (e.g., `spark-submit --master local[0] ...`), the job got stuck and does not run at all.
I think it's better to validate the parameter.

Fix for [SPARK-4470](https://issues.apache.org/jira/browse/SPARK-4470).

Author: Kenichi Maehashi <webmaster@kenichimaehashi.com>

Closes #3337 from kmaehashi/spark-4470 and squashes the following commits:

3ad76f3 [Kenichi Maehashi] fix code style
7716734 [Kenichi Maehashi] SPARK-4470: Validate number of threads in local mode

1 files changed, 3 insertions, 0 deletions

diff --git a/core/src/main/scala/org/apache/spark/SparkContext.scala b/core/src/main/scala/org/apache/spark/SparkContext.scala
index 37013121c5..ae8bbfb56f 100644
--- a/core/src/main/scala/org/apache/spark/SparkContext.scala
+++ b/core/src/main/scala/org/apache/spark/SparkContext.scala
@@ -1813,6 +1813,9 @@ object SparkContext extends Logging {
         def localCpuCount = Runtime.getRuntime.availableProcessors()
         // local[*] estimates the number of cores on the machine; local[N] uses exactly N threads.
         val threadCount = if (threads == "*") localCpuCount else threads.toInt
+        if (threadCount <= 0) {
+          throw new SparkException(s"Asked to run locally with $threadCount threads")
+        }
         val scheduler = new TaskSchedulerImpl(sc, MAX_LOCAL_TASK_FAILURES, isLocal = true)
         val backend = new LocalBackend(scheduler, threadCount)
         scheduler.initialize(backend)