[SPARK-19481] [REPL] [MAVEN] Avoid to leak SparkContext in Signaling.cancelOnInterrupt

## What changes were proposed in this pull request?

`Signaling.cancelOnInterrupt` leaks a SparkContext per call and it makes ReplSuite unstable.

This PR adds `SparkContext.getActive` to allow `Signaling.cancelOnInterrupt` to get the active `SparkContext` to avoid the leak.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16825 from zsxwing/SPARK-19481.

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

@@ -2489,6 +2489,13 @@ object SparkContext extends Logging {
     }
   }
 
+  /** Return the current active [[SparkContext]] if any. */
+  private[spark] def getActive: Option[SparkContext] = {
+    SPARK_CONTEXT_CONSTRUCTOR_LOCK.synchronized {
+      Option(activeContext.get())
+    }
+  }
+
   /**
    * Called at the beginning of the SparkContext constructor to ensure that no SparkContext is
    * running.  Throws an exception if a running context is detected and logs a warning if another

repl/scala-2.10/src/main/scala/org/apache/spark/repl/Main.scala

@@ -22,6 +22,7 @@ import org.apache.spark.internal.Logging
 object Main extends Logging {
 
   initializeLogIfNecessary(true)
+  Signaling.cancelOnInterrupt()
 
   private var _interp: SparkILoop = _
 

repl/scala-2.10/src/main/scala/org/apache/spark/repl/SparkILoop.scala

@@ -1027,7 +1027,6 @@ class SparkILoop(
       builder.getOrCreate()
     }
     sparkContext = sparkSession.sparkContext
-    Signaling.cancelOnInterrupt(sparkContext)
     sparkSession
   }
 

repl/scala-2.11/src/main/scala/org/apache/spark/repl/Main.scala

@@ -30,6 +30,7 @@ import org.apache.spark.util.Utils
 object Main extends Logging {
 
   initializeLogIfNecessary(true)
+  Signaling.cancelOnInterrupt()
 
   val conf = new SparkConf()
   val rootDir = conf.getOption("spark.repl.classdir").getOrElse(Utils.getLocalDir(conf))
@@ -108,7 +109,6 @@ object Main extends Logging {
       logInfo("Created Spark session")
     }
     sparkContext = sparkSession.sparkContext
-    Signaling.cancelOnInterrupt(sparkContext)
     sparkSession
   }
 

repl/src/main/scala/org/apache/spark/repl/Signaling.scala

@@ -28,15 +28,17 @@ private[repl] object Signaling extends Logging {
    * when no jobs are currently running.
    * This makes it possible to interrupt a running shell job by pressing Ctrl+C.
    */
-  def cancelOnInterrupt(ctx: SparkContext): Unit = SignalUtils.register("INT") {
-    if (!ctx.statusTracker.getActiveJobIds().isEmpty) {
-      logWarning("Cancelling all active jobs, this can take a while. " +
-        "Press Ctrl+C again to exit now.")
-      ctx.cancelAllJobs()
-      true
-    } else {
-      false
-    }
+  def cancelOnInterrupt(): Unit = SignalUtils.register("INT") {
+    SparkContext.getActive.map { ctx =>
+      if (!ctx.statusTracker.getActiveJobIds().isEmpty) {
+        logWarning("Cancelling all active jobs, this can take a while. " +
+          "Press Ctrl+C again to exit now.")
+        ctx.cancelAllJobs()
+        true
+      } else {
+        false
+      }
+    }.getOrElse(false)
   }
 
 }