[SPARK-15681][CORE] allow lowercase or mixed case log level string when calling sc.setLogLevel

## What changes were proposed in this pull request?
Currently `SparkContext API setLogLevel(level: String) `can not handle lower case or mixed case input string. But `org.apache.log4j.Level.toLevel` can take lowercase or mixed case.

This PR is to allow case-insensitive user input for the log level.

## How was this patch tested?
A unit testcase is added.

Author: Xin Wu <xinwu@us.ibm.com>

Closes #13422 from xwu0226/reset_loglevel.

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

@@ -20,7 +20,7 @@ package org.apache.spark
 import java.io._
 import java.lang.reflect.Constructor
 import java.net.URI
-import java.util.{Arrays, Properties, ServiceLoader, UUID}
+import java.util.{Arrays, Locale, Properties, ServiceLoader, UUID}
 import java.util.concurrent.ConcurrentMap
 import java.util.concurrent.atomic.{AtomicBoolean, AtomicInteger, AtomicReference}
 
@@ -356,12 +356,12 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
    * Valid log levels include: ALL, DEBUG, ERROR, FATAL, INFO, OFF, TRACE, WARN
    */
   def setLogLevel(logLevel: String) {
-    val validLevels = Seq("ALL", "DEBUG", "ERROR", "FATAL", "INFO", "OFF", "TRACE", "WARN")
-    if (!validLevels.contains(logLevel)) {
-      throw new IllegalArgumentException(
-        s"Supplied level $logLevel did not match one of: ${validLevels.mkString(",")}")
-    }
-    Utils.setLogLevel(org.apache.log4j.Level.toLevel(logLevel))
+    // let's allow lowcase or mixed case too
+    val upperCased = logLevel.toUpperCase(Locale.ENGLISH)
+    require(SparkContext.VALID_LOG_LEVELS.contains(upperCased),
+      s"Supplied level $logLevel did not match one of:" +
+        s" ${SparkContext.VALID_LOG_LEVELS.mkString(",")}")
+    Utils.setLogLevel(org.apache.log4j.Level.toLevel(upperCased))
   }
 
   try {
@@ -2179,6 +2179,8 @@ class SparkContext(config: SparkConf) extends Logging with ExecutorAllocationCli
  * various Spark features.
  */
 object SparkContext extends Logging {
+  private val VALID_LOG_LEVELS =
+    Set("ALL", "DEBUG", "ERROR", "FATAL", "INFO", "OFF", "TRACE", "WARN")
 
   /**
    * Lock that guards access to global variables that track SparkContext construction.

core/src/test/scala/org/apache/spark/SparkContextSuite.scala

@@ -363,4 +363,19 @@ class SparkContextSuite extends SparkFunSuite with LocalSparkContext {
     sc.stop()
     assert(result == null)
   }
+
+  test("log level case-insensitive and reset log level") {
+    sc = new SparkContext(new SparkConf().setAppName("test").setMaster("local"))
+    val originalLevel = org.apache.log4j.Logger.getRootLogger().getLevel
+    try {
+      sc.setLogLevel("debug")
+      assert(org.apache.log4j.Logger.getRootLogger().getLevel === org.apache.log4j.Level.DEBUG)
+      sc.setLogLevel("INfo")
+      assert(org.apache.log4j.Logger.getRootLogger().getLevel === org.apache.log4j.Level.INFO)
+    } finally {
+      sc.setLogLevel(originalLevel.toString)
+      assert(org.apache.log4j.Logger.getRootLogger().getLevel === originalLevel)
+      sc.stop()
+    }
+  }
 }