HADOOP-19290. Operating on / in ChecksumFileSystem throws NPE.

[ayushtkn]

Description of PR

Avoid fetching the checksum file for /, / is a directory anyway we don't have checksum files for directories & parent of root is null which leads to NPE failing the entire call

How was this patch tested?

UT

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?

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 21s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 1s		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 appears to include 1 new or modified test files.
			_ trunk Compile Tests _
+1 💚	mvninstall	32m 18s		trunk passed
+1 💚	compile	9m 4s		trunk passed with JDK Ubuntu-11.0.24+8-post-Ubuntu-1ubuntu320.04
+1 💚	compile	8m 16s		trunk passed with JDK Private Build-1.8.0_422-8u422-b05-1~20.04-b05
+1 💚	checkstyle	0m 40s		trunk passed
+1 💚	mvnsite	1m 1s		trunk passed
+1 💚	javadoc	0m 48s		trunk passed with JDK Ubuntu-11.0.24+8-post-Ubuntu-1ubuntu320.04
+1 💚	javadoc	0m 34s		trunk passed with JDK Private Build-1.8.0_422-8u422-b05-1~20.04-b05
+1 💚	spotbugs	1m 35s		trunk passed
+1 💚	shadedclient	21m 24s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 33s		the patch passed
+1 💚	compile	10m 4s		the patch passed with JDK Ubuntu-11.0.24+8-post-Ubuntu-1ubuntu320.04
+1 💚	javac	10m 4s		the patch passed
+1 💚	compile	9m 25s		the patch passed with JDK Private Build-1.8.0_422-8u422-b05-1~20.04-b05
+1 💚	javac	9m 25s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 43s		the patch passed
+1 💚	mvnsite	0m 55s		the patch passed
+1 💚	javadoc	0m 36s		the patch passed with JDK Ubuntu-11.0.24+8-post-Ubuntu-1ubuntu320.04
+1 💚	javadoc	0m 38s		the patch passed with JDK Private Build-1.8.0_422-8u422-b05-1~20.04-b05
+1 💚	spotbugs	1m 38s		the patch passed
+1 💚	shadedclient	21m 27s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	16m 53s		hadoop-common in the patch passed.
+1 💚	asflicense	0m 41s		The patch does not generate ASF License warnings.
		140m 5s

Subsystem	Report/Notes
Docker	ClientAPI=1.47 ServerAPI=1.47 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7074/1/artifact/out/Dockerfile
GITHUB PR	#7074
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 9d44f90a7e48 5.15.0-116-generic #126-Ubuntu SMP Mon Jul 1 10:14:24 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 0eae378
Default Java	Private Build-1.8.0_422-8u422-b05-1~20.04-b05
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.24+8-post-Ubuntu-1ubuntu320.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_422-8u422-b05-1~20.04-b05
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7074/1/testReport/
Max. process+thread count	1273 (vs. ulimit of 5500)
modules	C: hadoop-common-project/hadoop-common U: hadoop-common-project/hadoop-common
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7074/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[steveloughran]

+1
We have seen this happening enough elsewhere that maybe a new contract test is needed.