HDFS-16303. Fix edge case where DatanodeAdminDefaultMonitor doubly en…

[KevinWikant]

Description of PR

A rare edge case was noticed in DatanodeAdminDefaultMonitor which causes a DatanodeDescriptor to be added twice to the pendingNodes queue.

• a datanode is unhealthy so it gets added to "unhealthyDns"
• an exception is thrown which causes this catch block to execute
• the datanode is added to "pendingNodes"
• under certain conditions the datanode can be added again from "unhealthyDns" to "pendingNodes" here

How was this patch tested?

All "TestDecommission" & "DatanodeAdminManager" tests pass when run locally

Same manual testing from #3675 applies.

For code changes:

• Does the title or this PR starts with the corresponding JIRA issue id (e.g. 'HADOOP-17799. Your PR title ...')?
• [n/a ] Object storage: have the integration tests been executed and the endpoint declared according to the connector-specific documentation?
• [n/a] If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• [n/a] If applicable, have you updated the LICENSE, LICENSE-binary, NOTICE-binary files?

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 50s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 1s		codespell 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	32m 31s		trunk passed
+1 💚	compile	1m 28s		trunk passed with JDK Ubuntu-11.0.13+8-Ubuntu-0ubuntu1.20.04
+1 💚	compile	1m 17s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	checkstyle	1m 2s		trunk passed
+1 💚	mvnsite	1m 30s		trunk passed
+1 💚	javadoc	1m 5s		trunk passed with JDK Ubuntu-11.0.13+8-Ubuntu-0ubuntu1.20.04
+1 💚	javadoc	1m 32s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	3m 12s		trunk passed
+1 💚	shadedclient	22m 13s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 15s		the patch passed
+1 💚	compile	1m 19s		the patch passed with JDK Ubuntu-11.0.13+8-Ubuntu-0ubuntu1.20.04
+1 💚	javac	1m 19s		the patch passed
+1 💚	compile	1m 13s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	javac	1m 13s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 52s		the patch passed
+1 💚	mvnsite	1m 19s		the patch passed
+1 💚	javadoc	0m 50s		the patch passed with JDK Ubuntu-11.0.13+8-Ubuntu-0ubuntu1.20.04
+1 💚	javadoc	1m 24s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	3m 12s		the patch passed
+1 💚	shadedclient	22m 1s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	unit	234m 26s		hadoop-hdfs in the patch passed.
+1 💚	asflicense	0m 47s		The patch does not generate ASF License warnings.
		333m 17s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3923/1/artifact/out/Dockerfile
GITHUB PR	#3923
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell
uname	Linux f38f562a168b 4.15.0-58-generic #64-Ubuntu SMP Tue Aug 6 11:12:41 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 1fecaff
Default Java	Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Ubuntu-11.0.13+8-Ubuntu-0ubuntu1.20.04 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3923/1/testReport/
Max. process+thread count	3702 (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-3923/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[jojochuang]

@KevinWikant while this is a small patch, please file a new jira to make issue tracking easier. Thank you
We'd like to keep one commit per issue.

[KevinWikant]

Thanks @jojochuang! I have moved the PR here [https://github.com//pull/3942] as per your request