[SPARK-14508][BUILD] Add a new ScalaStyle Rule `OmitBracesInCase`

## What changes were proposed in this pull request?

According to the [Spark Code Style Guide](https://cwiki.apache.org/confluence/display/SPARK/Spark+Code+Style+Guide) and [Scala Style Guide](http://docs.scala-lang.org/style/control-structures.html#curlybraces), we had better enforce the following rule.
  ```
  case: Always omit braces in case clauses.
  ```
This PR makes a new ScalaStyle rule, 'OmitBracesInCase', and enforces it to the code.

## How was this patch tested?

Pass the Jenkins tests (including Scala style checking)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12280 from dongjoon-hyun/SPARK-14508.

4 files changed, 12 insertions, 24 deletions

diff --git a/yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala b/yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala
index 9e8453429c..d447a59937 100644
--- a/yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala
+++ b/yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala
@@ -374,7 +374,7 @@ private[spark] class ApplicationMaster(
             failureCount = 0
           } catch {
             case i: InterruptedException =>
-            case e: Throwable => {
+            case e: Throwable =>
               failureCount += 1
               // this exception was introduced in hadoop 2.4 and this code would not compile
               // with earlier versions if we refer it directly.
@@ -390,7 +390,6 @@ private[spark] class ApplicationMaster(
               } else {
                 logWarning(s"Reporter thread fails $failureCount time(s) in a row.", e)
               }
-            }
           }
           try {
             val numPendingAllocate = allocator.getPendingAllocate.size
diff --git a/yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocator.scala b/yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocator.scala
index b0bfe855e9..23742eab62 100644
--- a/yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocator.scala
+++ b/yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnAllocator.scala
@@ -148,11 +148,10 @@ private[yarn] class YarnAllocator(
         classOf[Array[String]], classOf[Array[String]], classOf[Priority], classOf[Boolean],
         classOf[String]))
     } catch {
-      case e: NoSuchMethodException => {
+      case e: NoSuchMethodException =>
         logWarning(s"Node label expression $expr will be ignored because YARN version on" +
           " classpath does not support it.")
         None
-      }
     }
   }
 
diff --git a/yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnSchedulerBackend.scala b/yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnSchedulerBackend.scala
index 8720ee57fe..6b3c831e60 100644
--- a/yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnSchedulerBackend.scala
+++ b/yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnSchedulerBackend.scala
@@ -223,17 +223,15 @@ private[spark] abstract class YarnSchedulerBackend(
           val lossReasonRequest = GetExecutorLossReason(executorId)
           val future = am.ask[ExecutorLossReason](lossReasonRequest, askTimeout)
           future onSuccess {
-            case reason: ExecutorLossReason => {
+            case reason: ExecutorLossReason =>
               driverEndpoint.askWithRetry[Boolean](RemoveExecutor(executorId, reason))
-            }
           }
           future onFailure {
-            case NonFatal(e) => {
+            case NonFatal(e) =>
               logWarning(s"Attempted to get executor loss reason" +
                 s" for executor id ${executorId} at RPC address ${executorRpcAddress}," +
                 s" but got no response. Marking as slave lost.", e)
               driverEndpoint.askWithRetry[Boolean](RemoveExecutor(executorId, SlaveLost()))
-            }
             case t => throw t
           }
         case None =>
diff --git a/yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala b/yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala
index de14e36f4e..fe09808ae5 100644
--- a/yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala
+++ b/yarn/src/test/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtilSuite.scala
@@ -101,22 +101,18 @@ class YarnSparkHadoopUtilSuite extends SparkFunSuite with Matchers with Logging
     val modifyAcls = acls.get(ApplicationAccessType.MODIFY_APP)
 
     viewAcls match {
-      case Some(vacls) => {
+      case Some(vacls) =>
         val aclSet = vacls.split(',').map(_.trim).toSet
         assert(aclSet.contains(System.getProperty("user.name", "invalid")))
-      }
-      case None => {
+      case None =>
         fail()
-      }
     }
     modifyAcls match {
-      case Some(macls) => {
+      case Some(macls) =>
         val aclSet = macls.split(',').map(_.trim).toSet
         assert(aclSet.contains(System.getProperty("user.name", "invalid")))
-      }
-      case None => {
+      case None =>
         fail()
-      }
     }
   }
 
@@ -135,26 +131,22 @@ class YarnSparkHadoopUtilSuite extends SparkFunSuite with Matchers with Logging
     val modifyAcls = acls.get(ApplicationAccessType.MODIFY_APP)
 
     viewAcls match {
-      case Some(vacls) => {
+      case Some(vacls) =>
         val aclSet = vacls.split(',').map(_.trim).toSet
         assert(aclSet.contains("user1"))
         assert(aclSet.contains("user2"))
         assert(aclSet.contains(System.getProperty("user.name", "invalid")))
-      }
-      case None => {
+      case None =>
         fail()
-      }
     }
     modifyAcls match {
-      case Some(macls) => {
+      case Some(macls) =>
         val aclSet = macls.split(',').map(_.trim).toSet
         assert(aclSet.contains("user3"))
         assert(aclSet.contains("user4"))
         assert(aclSet.contains(System.getProperty("user.name", "invalid")))
-      }
-      case None => {
+      case None =>
         fail()
-      }
     }
 
   }