HADOOP-19935. Windows: make preferIPv4Stack overridable, matching hadoop-functions.sh

[smengcl]

Description of PR

Prompted by HADOOP-16943, I noticed that Windows hadoop-config.cmd still disables IPv6.

On Unix the IPv4 pin has been overridable since HADOOP-9902: hadoop-functions.sh uses ${HADOOP_OPTS:-"-Djava.net.preferIPv4Stack=true"}, so setting HADOOP_OPTS selects dual-stack or IPv6. hadoop-config.cmd never matched it. It appended the pin unconditionally, so IPv6 could not be enabled on Windows at all.

Apply the pin only when HADOOP_OPTS is unset, mirroring the Unix default. IPv4 stays the default when HADOOP_OPTS is unset; operators who set it can now choose dual-stack or IPv6.

How was this patch tested?

n/a. There's no Windows CI.

For code changes:

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[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 59s		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	42m 10s		trunk passed
+1 💚	mvnsite	2m 2s		trunk passed
+1 💚	shadedclient	72m 50s		branch has no errors when building and testing our client artifacts.
-0 ⚠️	patch	73m 22s		Used diff version of patch file. Binary files and potentially other changes not applied. Please rebase and squash commits if necessary.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 14s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	1m 28s		the patch passed
+1 💚	shadedclient	28m 20s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	1m 15s		hadoop-common in the patch passed.
+1 💚	asflicense	0m 37s		The patch does not generate ASF License warnings.
		106m 58s

Subsystem	Report/Notes
Docker	ClientAPI=1.55 ServerAPI=1.55 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8592/1/artifact/out/Dockerfile
Optional Tests	dupname asflicense mvnsite unit codespell detsecrets
uname	Linux 7909dddfb10b 5.15.0-181-generic #191-Ubuntu SMP Fri May 22 19:09:02 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 2fd406b
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8592/1/testReport/
Max. process+thread count	636 (vs. ulimit of 10000)
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-8592/1/console
versions	git=2.43.0 maven=3.9.15
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