Updated code as per the comments

conf/livy.conf.template

@@ -32,7 +32,7 @@
 # livy.impersonation.enabled = true
 
 # Logs size livy can cache for each session/batch. 0 means don't cache the logs.
-# livy.spark.logs.size = 200 
+# livy.cache-log.size = 200
 
 # Comma-separated list of Livy RSC jars. By default Livy will upload jars from its installation
 # directory every time a session is started. By caching these files in HDFS, for example, startup

server/src/main/scala/com/cloudera/livy/LivyConf.scala

@@ -116,7 +116,7 @@ object LivyConf {
   val RECOVERY_STATE_STORE_URL = Entry("livy.server.recovery.state-store.url", "")
 
   // Livy will cache the max no of logs specified. 0 means don't cache the logs.
-  val SPARK_LOGS_SIZE = Entry("livy.spark.logs.size", 200)
+  val SPARK_LOGS_SIZE = Entry("livy.cache-log.size", 200)
 
   // If Livy can't find the yarn app within this time, consider it lost.
   val YARN_APP_LOOKUP_TIMEOUT = Entry("livy.server.yarn.app-lookup-timeout", "60s")

server/src/main/scala/com/cloudera/livy/server/interactive/InteractiveSession.scala

@@ -380,9 +380,9 @@ class InteractiveSession(
   heartbeat()
 
   private val app = mockApp.orElse {
-      val driverProcess = client.flatMap { c => Option(c.getDriverProcess) }
+    val driverProcess = client.flatMap { c => Option(c.getDriverProcess) }
         .map(new LineBufferedProcess(_, livyConf.getInt(LivyConf.SPARK_LOGS_SIZE)))
-      Option(SparkApp.create(appTag, appId, driverProcess, livyConf, Some(this)))
+    driverProcess.map { _ => SparkApp.create(appTag, appId, driverProcess, livyConf, Some(this)) }
   }
 
   if (client.isEmpty) {

server/src/main/scala/com/cloudera/livy/utils/LineBufferedStream.scala

@@ -38,11 +38,11 @@ class LineBufferedStream(inputStream: InputStream, logSize: Int) extends Logging
   private val thread = new Thread {
     override def run() = {
       val lines = Source.fromInputStream(inputStream).getLines()
-       for (line <- lines) {
+      for (line <- lines) {
+        info(s"stdout: $line")
         _lock.lock()
         try {
-          info(s"stdout: $line")
-          if(logSize > 0) _lines add line
+          _lines.add(line)
           _condition.signalAll()
         } finally {
           _lock.unlock()
@@ -61,7 +61,12 @@ class LineBufferedStream(inputStream: InputStream, logSize: Int) extends Logging
   thread.setDaemon(true)
   thread.start()
 
-  def lines: IndexedSeq[String] = IndexedSeq.empty[String] ++ _lines.toArray(Array.empty[String])
+  def lines: IndexedSeq[String] = {
+    _lock.lock()
+    val lines = IndexedSeq.empty[String] ++ _lines.toArray(Array.empty[String])
+    _lock.unlock()
+    lines
+  }
 
   def iterator: Iterator[String] = {
     new LinesIterator

server/src/main/scala/com/cloudera/livy/utils/SparkApp.scala

@@ -92,8 +92,7 @@ object SparkApp {
     if (livyConf.isRunningOnYarn()) {
       new SparkYarnApp(uniqueAppTag, appId, process, listener, livyConf)
     } else {
-      // process is None in recovery mode
-      // require(process.isDefined, "process must not be None when Livy master is not YARN.")
+      require(process.isDefined, "process must not be None when Livy master is not YARN.")
       new SparkProcApp(process.get, listener)
     }
   }

server/src/main/scala/com/cloudera/livy/utils/SparkProcApp.scala

@@ -42,6 +42,7 @@ class SparkProcApp (
 
   override def log(): IndexedSeq[String] =
     ("stdout: " +: process.inputLines) ++ ("\nstderr: " +: process.errorLines)
+
   private def changeState(newState: SparkApp.State.Value) = {
     if (state != newState) {
       listener.foreach(_.stateChanged(state, newState))