HDFS-17955. Fix Checkpointer startup checkpoint on fresh VMs

[pan3793]

Description of PR

The Checkpointer triggers the first checkpoint when
shouldCheckpointAtStartup() returns true by leaving lastCheckpointTime = 0
and relying on now >= lastCheckpointTime + checkpointPeriodMSec to
evaluate true. This condition breaks when monotonicNow()
(System.nanoTime based) returns a value smaller than
checkpointPeriodMSec (3600000ms = 1 hour), which happens on freshly
booted VMs such as GitHub-hosted runners. The checkpoint never fires,
and any caller waiting for it hangs indefinitely.

Add an explicit lastCheckpointTime == 0 check so the first checkpoint
fires immediately regardless of the monotonic clock value. After the
first checkpoint, lastCheckpointTime is set to the current time and
normal period-based scheduling resumes.

This is a latent bug present since the Checkpointer logic was introduced
in 2011 (a196766). It was exposed by the GitHub-hosted runner image
ubuntu24/20260720.247, which provisions VMs fast enough that host
uptime is below 1 hour at test execution. A BackupNode that restarts
within 1 hour of boot on any deployment has the same bug.

How was this patch tested?

Verified via GitHub Actions CI on a fork branch. Before the fix,
TestBackupNode hangs indefinitely (fork timeout after 900s). After the
fix, the Checkpointer fires the startup checkpoint immediately and
TestBackupNode passes.

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (HDFS-17955)?
• 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?

AI Tooling

If an AI tool was used:

• The PR includes the phrase "Contains content generated by <tool>"
  where <tool> is the name of the AI tool used.
• My use of AI contributions follows the ASF legal policy
  https://www.apache.org/legal/generative-tooling.html

Contains content generated by GLM 5.2

[pan3793]

cc @Hexiaoqiao @ajfabbri @slfan1989
this should fix the root cause of the recent consistently failed "Test hdfs - other", and HADOOP-19950 (#8651) and HDFS-17956 (#8650) are used to diagnose this issue

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	19m 39s		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	46m 17s		trunk passed
+1 💚	compile	1m 44s		trunk passed with JDK Ubuntu-21.0.11+10-1-24.04.2-Ubuntu
+1 💚	compile	1m 52s		trunk passed with JDK Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
+1 💚	checkstyle	1m 48s		trunk passed
+1 💚	mvnsite	1m 56s		trunk passed
+1 💚	javadoc	1m 28s		trunk passed with JDK Ubuntu-21.0.11+10-1-24.04.2-Ubuntu
+1 💚	javadoc	1m 28s		trunk passed with JDK Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
+1 💚	spotbugs	4m 18s		trunk passed
+1 💚	shadedclient	37m 43s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 25s		the patch passed
+1 💚	compile	1m 17s		the patch passed with JDK Ubuntu-21.0.11+10-1-24.04.2-Ubuntu
+1 💚	javac	1m 17s		the patch passed
+1 💚	compile	1m 21s		the patch passed with JDK Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
+1 💚	javac	1m 21s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	1m 19s		the patch passed
+1 💚	mvnsite	1m 28s		the patch passed
+1 💚	javadoc	0m 57s		the patch passed with JDK Ubuntu-21.0.11+10-1-24.04.2-Ubuntu
+1 💚	javadoc	1m 4s		the patch passed with JDK Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
+1 💚	spotbugs	4m 7s		the patch passed
+1 💚	shadedclient	36m 46s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	260m 34s		hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 53s		The patch does not generate ASF License warnings.
		427m 23s

Subsystem	Report/Notes
Docker	ClientAPI=1.55 ServerAPI=1.55 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8649/1/artifact/out/Dockerfile
GITHUB PR	#8649
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux 19fb92f5b7d5 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 / ea1f862
Default Java	Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
Multi-JDK versions	/usr/lib/jvm/java-21-openjdk-amd64:Ubuntu-21.0.11+10-1-24.04.2-Ubuntu /usr/lib/jvm/java-17-openjdk-amd64:Ubuntu-17.0.19+10-1-24.04.2-Ubuntu
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-8649/1/testReport/
Max. process+thread count	2275 (vs. ulimit of 10000)
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-8649/1/console
versions	git=2.43.0 maven=3.9.15 spotbugs=4.9.7
Powered by	Apache Yetus 0.14.1 https://yetus.apache.org

This message was automatically generated.

[Hexiaoqiao]

LGTM. +1. Thanks @pan3793 .

[slfan1989]

LGTM

[pan3793]

thanks, merged to trunk/3.5/3.4