YARN-11384: NPE in DelegationTokenRenewer causes all subsequent apps to fail with "Timer already cancelled"

[pan3793]

Description of PR

As described in YARN-11384, DelegationTokenRenewer#removeFailedDelegationToken may throw NPE due to appTokens.get(applicationId) returning null (I'm not sure how this happened, but it does happen), such an uncaught exception stops the global renewalTimer and causes all subsequent apps to fail with "Timer already cancelled", this effectively leads the entire YARN cluster unavailable.

This patch simply adds a null check to prevent the NPE.

How was this patch tested?

Change is straightforward I'm not sure whether it requires a UT with plenty of mocked stuff.

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[pan3793]

cc @slfan1989 @Hexiaoqiao

[slfan1989]

@pan3793 LGTM. I need to research the deeper reasons behind this issue.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 35s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 1s		codespell was not available.
+0 🆗	detsecrets	0m 1s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 💚	mvninstall	37m 6s		trunk passed
+1 💚	compile	0m 59s		trunk passed with JDK Ubuntu-11.0.26+4-post-Ubuntu-1ubuntu120.04
+1 💚	compile	0m 52s		trunk passed with JDK Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
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+1 💚	javadoc	0m 49s		trunk passed with JDK Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
+1 💚	spotbugs	1m 55s		trunk passed
+1 💚	shadedclient	38m 9s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 46s		the patch passed
+1 💚	compile	0m 51s		the patch passed with JDK Ubuntu-11.0.26+4-post-Ubuntu-1ubuntu120.04
+1 💚	javac	0m 51s		the patch passed
+1 💚	compile	0m 44s		the patch passed with JDK Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
+1 💚	javac	0m 44s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 41s		the patch passed
+1 💚	mvnsite	0m 48s		the patch passed
+1 💚	javadoc	0m 43s		the patch passed with JDK Ubuntu-11.0.26+4-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	0m 39s		the patch passed with JDK Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
+1 💚	spotbugs	1m 55s		the patch passed
+1 💚	shadedclient	36m 12s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	0m 55s		hadoop-yarn-server-resourcemanager in the patch passed.
+1 💚	asflicense	0m 36s		The patch does not generate ASF License warnings.
		128m 3s

Subsystem	Report/Notes
Docker	ClientAPI=1.47 ServerAPI=1.47 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7404/1/artifact/out/Dockerfile
GITHUB PR	#7404
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
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modules	C: hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager U: hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager
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[slfan1989]

@pan3793 Thanks for the contribution! Merged into trunk.