HDFS-17876: NameNode.stop() missing null check for tracer.close() can cause NPE during shutdown

[teamconfx]

Description of PR

This PR fix HDFS-17876

Summary of the change:

• File: hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/NameNode.java:1260
• Issue: tracer.close() was called without a null check, inconsistent with all other resources in the stop() method
• Fix: Added a defensive null check: if (tracer != null) { tracer.close(); }

This prevents a NullPointerException during shutdown if:

1. An exception occurs in the parent constructor before tracer is initialized
2. The stopAtException() method calls stop() early during initialization failures
3. Any future refactoring introduces paths where tracer isn't set before stop() is called

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?

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[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	21m 40s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		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	49m 33s		trunk passed
+1 💚	compile	1m 46s		trunk passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	compile	1m 46s		trunk passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	checkstyle	1m 16s		trunk passed
+1 💚	mvnsite	1m 57s		trunk passed
+1 💚	javadoc	1m 30s		trunk passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	javadoc	1m 29s		trunk passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	spotbugs	4m 24s		trunk passed
+1 💚	shadedclient	36m 51s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 25s		the patch passed
+1 💚	compile	1m 16s		the patch passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	javac	1m 16s		the patch passed
+1 💚	compile	1m 20s		the patch passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	javac	1m 20s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 46s		the patch passed
+1 💚	mvnsite	1m 30s		the patch passed
+1 💚	javadoc	0m 59s		the patch passed with JDK Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04
+1 💚	javadoc	1m 3s		the patch passed with JDK Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
+1 💚	spotbugs	4m 3s		the patch passed
+1 💚	shadedclient	35m 14s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
-1 ❌	unit	256m 44s	/patch-unit-hadoop-hdfs-project_hadoop-hdfs.txt	hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 49s		The patch does not generate ASF License warnings.
		425m 50s

Reason	Tests
Failed junit tests	hadoop.hdfs.tools.TestDFSAdmin
	hadoop.hdfs.server.namenode.ha.TestStandbyCheckpoints

Subsystem	Report/Notes
Docker	ClientAPI=1.52 ServerAPI=1.52 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8200/1/artifact/out/Dockerfile
GITHUB PR	#8200
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 226dfa97161c 5.15.0-164-generic #174-Ubuntu SMP Fri Nov 14 20:25:16 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 696a877
Default Java	Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
Multi-JDK versions	/usr/lib/jvm/java-21-openjdk-amd64:Ubuntu-21.0.7+6-Ubuntu-0ubuntu120.04 /usr/lib/jvm/java-17-openjdk-amd64:Ubuntu-17.0.15+6-Ubuntu-0ubuntu120.04
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8200/1/testReport/
Max. process+thread count	2351 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8200/1/console
versions	git=2.25.1 maven=3.9.11 spotbugs=4.9.7
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