HADOOP-19502. Add RegexpSingleline module to checkstyle.xml.

[hfutatzhanghb]

Description of PR

Refer to #7505
This pr fix the bug of the original version.
The property of severity does not have warn value, we should use warning instead.

How was this patch tested?

[hfutatzhanghb]

@slfan1989 , have fixed the bug, please help review this pr when you have free time, thank you very much!

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	13m 33s		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.
+0 🆗	xmllint	0m 0s		xmllint 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	34m 58s		trunk passed
+1 💚	compile	0m 24s		trunk passed with JDK Ubuntu-11.0.26+4-post-Ubuntu-1ubuntu120.04
+1 💚	compile	0m 35s		trunk passed with JDK Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
+1 💚	mvnsite	0m 24s		trunk passed
+1 💚	javadoc	0m 31s		trunk passed with JDK Ubuntu-11.0.26+4-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	0m 34s		trunk passed with JDK Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
+1 💚	shadedclient	70m 49s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 26s		the patch passed
+1 💚	compile	0m 14s		the patch passed with JDK Ubuntu-11.0.26+4-post-Ubuntu-1ubuntu120.04
+1 💚	javac	0m 14s		the patch passed
+1 💚	compile	0m 24s		the patch passed with JDK Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
+1 💚	javac	0m 24s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	0m 14s		the patch passed
+1 💚	javadoc	0m 15s		the patch passed with JDK Ubuntu-11.0.26+4-post-Ubuntu-1ubuntu120.04
+1 💚	javadoc	0m 25s		the patch passed with JDK Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
+1 💚	shadedclient	35m 1s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	0m 30s		hadoop-build-tools in the patch passed.
+1 💚	asflicense	0m 37s		The patch does not generate ASF License warnings.
		123m 17s

Subsystem	Report/Notes
Docker	ClientAPI=1.48 ServerAPI=1.48 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7535/1/artifact/out/Dockerfile
GITHUB PR	#7535
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient codespell detsecrets xmllint
uname	Linux 2f84e2e91778 5.15.0-130-generic #140-Ubuntu SMP Wed Dec 18 17:59:53 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 0927728
Default Java	Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.26+4-post-Ubuntu-1ubuntu120.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_442-8u442-b06us1-0ubuntu120.04-b06
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7535/1/testReport/
Max. process+thread count	702 (vs. ulimit of 5500)
modules	C: hadoop-build-tools U: hadoop-build-tools
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-7535/1/console
versions	git=2.25.1 maven=3.6.3
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[slfan1989]

@pan3793 Could you please review this change? Thank you very much!

[pan3793]

@hfutatzhanghb Words like "fix the bug" make no sense, please explain the "why" of the change instead of "what" is the change.

[hfutatzhanghb]

Hi, @pan3793 . Thanks for reminding. Have updated in the description of this PR. Please see again~

[pan3793]

@hfutatzhanghb I do not get your point about why a new rule "RegexpSingleline" is required, what is the issue in the current trunk do you want to fix?

[hfutatzhanghb]

@hfutatzhanghb I do not get your point about why a new rule "RegexpSingleline" is required, what is the issue in the current trunk do you want to fix?

@pan3793 As described in #7505 . In my local, checkstyle tools use trunk's checkstyle.xml can not detect blanks.

[hfutatzhanghb]

@pan3793 Here is a screenshot, which can detail the problem.

[pan3793]

So this change intends to add a new checkstyle rule to forbid trailing spaces of lines, this generally is a good idea, but after I apply it (I change the severity to error to get the report) and check the current code base, there are more than 40k places that violate this rule

    <module name="RegexpSingleline">
        <property name="format" value="\s+$"/>
        <property name="severity" value="error"/>
    </module>

mvn checkstyle:check | grep RegexpSingleline | wc -l
   41890

If we add this rule, fixing those warnings will consume much time and introduce unnecessary commit history, but without real benefits.

Given such a situation, I suggest NOT adding such a rule, and just keeping things as-is.

[hfutatzhanghb]

@pan3793 Thanks for suggestioning. I got your concerns. Will close this PR.
Thanks @slfan1989 for helping.