[SPARK-10001] Consolidate Signaling and SignalLogger.

## What changes were proposed in this pull request?
This is a follow-up to #12557, with the following changes:

1. Fixes some of the style issues.
2. Merges Signaling and SignalLogger into a new class called SignalUtils. It was pretty confusing to have Signaling and Signal in one file, and it was also confusing to have two classes named Signaling and one called the other.
3. Made logging registration idempotent.

## How was this patch tested?
N/A.

Author: Reynold Xin <rxin@databricks.com>

Closes #12605 from rxin/SPARK-10001.

5 files changed, 58 insertions, 77 deletions

diff --git a/core/src/main/scala/org/apache/spark/util/SignalLogger.scala b/core/src/main/scala/org/apache/spark/util/SignalLogger.scala
deleted file mode 100644
index a793c9135e..0000000000
--- a/core/src/main/scala/org/apache/spark/util/SignalLogger.scala
+++ /dev/null
@@ -1,36 +0,0 @@
-/*
- * Licensed to the Apache Software Foundation (ASF) under one or more
- * contributor license agreements.  See the NOTICE file distributed with
- * this work for additional information regarding copyright ownership.
- * The ASF licenses this file to You under the Apache License, Version 2.0
- * (the "License"); you may not use this file except in compliance with
- * the License.  You may obtain a copy of the License at
- *
- *    http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.apache.spark.util
-
-import org.slf4j.Logger
-
-/**
- * Used to log signals received. This can be very useful in debugging crashes or kills.
- */
-private[spark] object SignalLogger {
-
-  private var registered = false
-
-  /** Register a signal handler to log signals on UNIX-like systems. */
-  def register(log: Logger): Unit = Seq("TERM", "HUP", "INT").foreach{ sig =>
-    Signaling.register(sig) {
-      log.error("RECEIVED SIGNAL " + sig)
-      false
-    }
-  }
-}
diff --git a/core/src/main/scala/org/apache/spark/util/Signaling.scala b/core/src/main/scala/org/apache/spark/util/SignalUtils.scala
index 2075cc45a9..9479d8f74d 100644
--- a/core/src/main/scala/org/apache/spark/util/Signaling.scala
+++ b/core/src/main/scala/org/apache/spark/util/SignalUtils.scala
@@ -17,28 +17,69 @@
 
 package org.apache.spark.util
 
-import java.util.{Collections, LinkedList}
+import java.util.Collections
 
 import scala.collection.JavaConverters._
-import scala.collection.mutable.HashMap
 
 import org.apache.commons.lang3.SystemUtils
+import org.slf4j.Logger
 import sun.misc.{Signal, SignalHandler}
 
 import org.apache.spark.internal.Logging
 
-
 /**
  * Contains utilities for working with posix signals.
  */
-private[spark] object Signaling extends Logging {
+private[spark] object SignalUtils extends Logging {
+
+  /** A flag to make sure we only register the logger once. */
+  private var loggerRegistered = false
+
+  /** Register a signal handler to log signals on UNIX-like systems. */
+  def registerLogger(log: Logger): Unit = synchronized {
+    if (!loggerRegistered) {
+      Seq("TERM", "HUP", "INT").foreach { sig =>
+        SignalUtils.register(sig) {
+          log.error("RECEIVED SIGNAL " + sig)
+          false
+        }
+      }
+      loggerRegistered = true
+    }
+  }
+
+  /**
+   * Adds an action to be run when a given signal is received by this process.
+   *
+   * Note that signals are only supported on unix-like operating systems and work on a best-effort
+   * basis: if a signal is not available or cannot be intercepted, only a warning is emitted.
+   *
+   * All actions for a given signal are run in a separate thread.
+   */
+  def register(signal: String)(action: => Boolean): Unit = synchronized {
+    if (SystemUtils.IS_OS_UNIX) {
+      try {
+        val handler = handlers.getOrElseUpdate(signal, {
+          logInfo("Registered signal handler for " + signal)
+          new ActionHandler(new Signal(signal))
+        })
+        handler.register(action)
+      } catch {
+        case ex: Exception => logWarning(s"Failed to register signal handler for " + signal, ex)
+      }
+    }
+  }
 
   /**
    * A handler for the given signal that runs a collection of actions.
    */
   private class ActionHandler(signal: Signal) extends SignalHandler {
 
-    private val actions = Collections.synchronizedList(new LinkedList[() => Boolean])
+    /**
+     * List of actions upon the signal; the callbacks should return true if the signal is "handled",
+     * i.e. should not escalate to the next callback.
+     */
+    private val actions = Collections.synchronizedList(new java.util.LinkedList[() => Boolean])
 
     // original signal handler, before this handler was attached
     private val prevHandler: SignalHandler = Signal.handle(signal, this)
@@ -51,11 +92,10 @@ private[spark] object Signaling extends Logging {
       // register old handler, will receive incoming signals while this handler is running
       Signal.handle(signal, prevHandler)
 
-      val escalate = actions.asScala forall { action =>
-        !action()
-      }
-
-      if(escalate) {
+      // run all actions, escalate to parent handler if no action catches the signal
+      // (i.e. all actions return false)
+      val escalate = actions.asScala.forall { action => !action() }
+      if (escalate) {
         prevHandler.handle(sig)
       }
 
@@ -64,36 +104,13 @@ private[spark] object Signaling extends Logging {
     }
 
     /**
-     * Add an action to be run by this handler.
+     * Adds an action to be run by this handler.
      * @param action An action to be run when a signal is received. Return true if the signal
-     * should be stopped with this handler, false if it should be escalated.
+     *               should be stopped with this handler, false if it should be escalated.
      */
     def register(action: => Boolean): Unit = actions.add(() => action)
-
-  }
-
-  // contains association of signals to their respective handlers
-  private val handlers = new HashMap[String, ActionHandler]
-
-  /**
-   * Adds an action to be run when a given signal is received by this process.
-   *
-   * Note that signals are only supported on unix-like operating systems and work on a best-effort
-   * basis: if a signal is not available or cannot be intercepted, only a warning is emitted.
-   *
-   * All actions for a given signal are run in a separate thread.
-   */
-  def register(signal: String)(action: => Boolean): Unit = synchronized {
-    if (SystemUtils.IS_OS_UNIX) try {
-      val handler = handlers.getOrElseUpdate(signal, {
-        val h = new ActionHandler(new Signal(signal))
-        logInfo("Registered signal handler for " + signal)
-        h
-      })
-      handler.register(action)
-    } catch {
-      case ex: Exception => logWarning(s"Failed to register signal handler for " + signal, ex)
-    }
   }
 
+  /** Mapping from signal to their respective handlers. */
+  private val handlers = new scala.collection.mutable.HashMap[String, ActionHandler]
 }
diff --git a/core/src/main/scala/org/apache/spark/util/Utils.scala b/core/src/main/scala/org/apache/spark/util/Utils.scala
index 848f7d7adb..ea49991493 100644
--- a/core/src/main/scala/org/apache/spark/util/Utils.scala
+++ b/core/src/main/scala/org/apache/spark/util/Utils.scala
@@ -2284,7 +2284,7 @@ private[spark] object Utils extends Logging {
    */
   def initDaemon(log: Logger): Unit = {
     log.info(s"Started daemon with process name: ${Utils.getProcessName()}")
-    SignalLogger.register(log)
+    SignalUtils.registerLogger(log)
   }
 }
 
diff --git a/repl/src/main/scala/org/apache/spark/repl/Signaling.scala b/repl/src/main/scala/org/apache/spark/repl/Signaling.scala
index c305ed545c..202febf144 100644
--- a/repl/src/main/scala/org/apache/spark/repl/Signaling.scala
+++ b/repl/src/main/scala/org/apache/spark/repl/Signaling.scala
@@ -19,7 +19,7 @@ package org.apache.spark.repl
 
 import org.apache.spark.SparkContext
 import org.apache.spark.internal.Logging
-import org.apache.spark.util.{Signaling => USignaling}
+import org.apache.spark.util.SignalUtils
 
 private[repl] object Signaling extends Logging {
 
@@ -28,7 +28,7 @@ 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 = USignaling.register("INT") {
+  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.")
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 5bb63500c8..4df90d7b6b 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
@@ -716,7 +716,7 @@ object ApplicationMaster extends Logging {
   private var master: ApplicationMaster = _
 
   def main(args: Array[String]): Unit = {
-    SignalLogger.register(log)
+    SignalUtils.registerLogger(log)
     val amArgs = new ApplicationMasterArguments(args)
     SparkHadoopUtil.get.runAsSparkUser { () =>
       master = new ApplicationMaster(amArgs, new YarnRMClient)