TEZ-4759: An external AM never re-registers after a ZooKeeper session loss

[deniskuzZ]

• ZkAMRegistry.add() wrote the AMRecord once, as a bare EPHEMERAL node; a session expiry deletes it and nothing re-creates it, so clients see an empty registry while the AM keeps running
• Hold a Curator PersistentNode per AMRecord: it re-creates the node after a session loss or a delete; remove() closes it
• add() fails when the node is not created within the ZooKeeper connection timeout
• TestZkAMRegistry: after the ZooKeeper server is replaced, the AMRecord node is back

[tez-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 8s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+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.
			_ master Compile Tests _
+1 💚	mvninstall	3m 25s		master passed
+1 💚	compile	2m 37s		master passed
+1 💚	checkstyle	0m 31s		master passed
+1 💚	javadoc	0m 21s		master passed
+0 🆗	spotbugs	1m 2s		tez-dag in master has 537 extant spotbugs warnings.
			_ Patch Compile Tests _
+1 💚	mvninstall	2m 13s		the patch passed
+1 💚	codespell	0m 50s		No new issues.
+1 💚	compile	2m 34s		the patch passed
+1 💚	javac	2m 34s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 23s		the patch passed
+1 💚	javadoc	0m 20s		the patch passed
+1 💚	spotbugs	1m 9s		the patch passed
			_ Other Tests _
+1 💚	unit	60m 51s		root in the patch passed.
+1 💚	asflicense	0m 25s		The patch does not generate ASF License warnings.
		77m 56s

Subsystem	Report/Notes
Docker	ClientAPI=1.56 ServerAPI=1.56 base: https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-543/2/artifact/out/Dockerfile
Optional Tests	dupname compile unit asflicense javac javadoc spotbugs checkstyle codespell detsecrets
uname	Linux fb13607be12e 5.15.0-190-generic #200-Ubuntu SMP Fri Aug 7 15:06:04 UTC 2026 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	tez-personality.sh
git revision	master / ae2756e
Default Java	Eclipse Adoptium-21.0.12.1+1-LTS
Test Results	https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-543/2/testReport/
Max. process+thread count	1396 (vs. ulimit of 5500)
modules	C: tez-dag U: tez-dag
Console output	https://ci-hadoop.apache.org/job/tez-multibranch/job/PR-543/2/console
versions	git=2.43.0 maven=3.9.15 spotbugs=4.9.3 codespell=2.4.1
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