commit 2df02dcdccfa22bbc700330249a53b6b46de65c4

Author: Devaraj Das <ddas@yahoo-inc.com>
Date:   Thu Oct 7 00:11:58 2010 -0700

    Added a comment around a doAs


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-0.20-security-patches@1077734 13f79535-47bb-0310-9956-ffa450edef68

src/mapred/org/apache/hadoop/mapred/JobLocalizer.java

@@ -382,6 +382,10 @@ public class JobLocalizer {
     //setup the distributed cache
     final long[] sizes = downloadPrivateCache(jConf);
     if (sizes != null) {
+      //the following doAs is required because the DefaultTaskController
+      //calls the localizeJobFiles method in the context of the TaskTracker
+      //process. The JVM authorization check would fail without this
+      //doAs. In the LinuxTC case, this doesn't harm.
       UserGroupInformation ugi = 
         UserGroupInformation.createRemoteUser(jobid.toString());
       ugi.doAs(new PrivilegedExceptionAction<Object>() {