gaborgsomogyi commented on code in PR #19372:
URL: https://github.com/apache/flink/pull/19372#discussion_r854030684


##########
flink-runtime/src/main/java/org/apache/flink/runtime/security/token/KerberosDelegationTokenManager.java:
##########
@@ -110,13 +139,62 @@ public void obtainDelegationTokens(Credentials 
credentials) {
      * task managers.
      */
     @Override
-    public void start() {
-        LOG.info("Starting renewal task");
+    public void start() throws Exception {
+        checkNotNull(scheduledExecutor, "Scheduled executor must not be null");
+        checkNotNull(executorService, "Executor service must not be null");
+        checkState(tgtRenewalFuture == null, "Manager is already started");
+
+        if (!kerberosRenewalPossibleProvider.isRenewalPossible()) {
+            LOG.info("Renewal is NOT possible, skipping to start renewal 
task");
+            return;
+        }
+
+        startTGTRenewal();
+    }
+
+    private void startTGTRenewal() throws IOException {
+        LOG.debug("Starting credential renewal task");
+
+        UserGroupInformation currentUser = 
UserGroupInformation.getCurrentUser();
+        if (currentUser.isFromKeytab()) {
+            // In Hadoop 2.x, renewal of the keytab-based login seems to be 
automatic, but in Hadoop
+            // 3.x, it is configurable (see 
hadoop.kerberos.keytab.login.autorenewal.enabled, added
+            // in HADOOP-9567). This task will make sure that the user stays 
logged in regardless of
+            // that configuration's value. Note that 
checkTGTAndReloginFromKeytab() is a no-op if
+            // the TGT does not need to be renewed yet.
+            long tgtRenewalPeriod = 
configuration.get(KERBEROS_RELOGIN_PERIOD).toMillis();
+            tgtRenewalFuture =
+                    scheduledExecutor.scheduleAtFixedRate(
+                            () ->
+                                    executorService.execute(
+                                            () -> {
+                                                try {
+                                                    LOG.debug("Renewing TGT");
+                                                    
currentUser.checkTGTAndReloginFromKeytab();

Review Comment:
   @dmvk did you have a chance to consider the situation in-depth? I've my own 
suggestion which may or may not intersect your opinion. Namely if it's not too 
horror complex upgrading to mockito 3.4.0 and choosing bullet point 1 is my 
preference. Though no idea why mockito is super old, maybe there was no 
agreement to upgrade that?! If you have some insights please share.
   
   If that would be an overkill then I would vote on bullet point 3 because all 
the other options would add hard to maintain and brittle solutions.
   
   WDYT?
   



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