Github user vanzin commented on a diff in the pull request:

    https://github.com/apache/spark/pull/4688#discussion_r25914836
  
    --- Diff: 
yarn/src/main/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtil.scala ---
    @@ -82,6 +102,180 @@ class YarnSparkHadoopUtil extends SparkHadoopUtil {
         if (credentials != null) credentials.getSecretKey(new Text(key)) else 
null
       }
     
    +  private[spark] override def scheduleLoginFromKeytab(): Unit = {
    +    val principal = System.getenv("SPARK_PRINCIPAL")
    +    val keytab = System.getenv("SPARK_KEYTAB")
    +    if (principal != null) {
    +      val delegationTokenRenewerThread =
    +        new Runnable {
    +          override def run(): Unit = {
    +            if (!loggedInViaKeytab) {
    +              // Keytab is copied by YARN to the working directory of the 
AM, so full path is
    +              // not needed.
    +              loggedInUGI = 
UserGroupInformation.loginUserFromKeytabAndReturnUGI(
    +                principal, keytab)
    +              loggedInViaKeytab = true
    +            }
    +            val nns = getNameNodesToAccess(sparkConf)
    +            val newCredentials = loggedInUGI.getCredentials
    +            obtainTokensForNamenodes(nns, conf, newCredentials)
    +            val remoteFs = FileSystem.get(conf)
    +            val stagingDirPath =
    +              new Path(remoteFs.getHomeDirectory, 
System.getenv("SPARK_YARN_STAGING_DIR"))
    +            val tokenPathStr = sparkConf.get("spark.yarn.credentials.file")
    +            val tokenPath = new Path(stagingDirPath.toString, tokenPathStr)
    +            val tempTokenPath = new Path(stagingDirPath.toString, 
tokenPathStr + ".tmp")
    +            val stream = remoteFs.create(tempTokenPath, true)
    +            // Now write this out to HDFS
    +            newCredentials.writeTokenStorageToStream(stream)
    +            stream.hflush()
    +            stream.close()
    +            // HDFS does reads by inodes now, so just doing a rename 
should be fine. But I could
    +            // not find a clear explanation of when the blocks on HDFS are 
deleted. Ideally, we
    +            // would not need this, but just be defensive to ensure we 
don't mess up the
    +            // credentials. So create a file to show that we are currently 
updating - if the
    +            // reader sees this file, they go away and come back later. 
Then delete old token and
    +            // rename the old to new.
    +            val updatingPath = new Path(stagingDirPath, "_UPDATING")
    +            if (remoteFs.exists(updatingPath)) {
    +              remoteFs.delete(updatingPath, true)
    --- End diff --
    
    Seems unnecessary to delete the file just to re-create it on the next line?
    
    Also, feels like the next few lines need some exception-safety love.


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