[CI] ThreadPoolMergeSchedulerTests testSchedulerCloseWaitsForRunningMerge failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic-platform-support #8681 / windows-2022_checkpart1_platform-support-windows
• elasticsearch-periodic-platform-support #8681 / windows-2025_checkpart1_platform-support-windows
• elasticsearch-pull-request #74709 / part-1
• elasticsearch-periodic-platform-support #8681 / rocky-8_platform-support-unix
• elasticsearch-periodic-platform-support #8681 / almalinux-9_platform-support-unix
• elasticsearch-periodic-platform-support #8681 / oraclelinux-8_platform-support-unix

Reproduction Line:

gradlew ":server:test" --tests "org.elasticsearch.index.engine.ThreadPoolMergeSchedulerTests.testSchedulerCloseWaitsForRunningMerge" -Dtests.seed=A7E04417E8372109 -Dtests.locale=ur-PK -Dtests.timezone=Europe/Moscow -Druntime.java=24

Applicable branches:
9.0

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: 
Expected: iterable containing [<Mock for OneMerge, hashCode: 173950959>]
     but: no item was <Mock for OneMerge, hashCode: 173950959>

Issue Reasons:

• [9.0] 6 consecutive failures in test testSchedulerCloseWaitsForRunningMerge
• [9.0] 6 failures in test testSchedulerCloseWaitsForRunningMerge (54.5% fail rate in 11 executions)

Note:
This issue was created using new test triage automation. Please report issues or feedback to es-delivery.

[elasticsearchmachine]

This has been muted on branch main

Mute Reasons:

• [main] 13 failures in test testSchedulerCloseWaitsForRunningMerge (7.6% fail rate in 170 executions)
• [main] 2 failures in step part1 (9.1% fail rate in 22 executions)
• [main] 11 failures in step part-1 (13.6% fail rate in 81 executions)
• [main] 2 failures in pipeline elasticsearch-intake (9.1% fail rate in 22 executions)
• [main] 10 failures in pipeline elasticsearch-pull-request (13.5% fail rate in 74 executions)

Build Scans:

• elasticsearch-intake #20420 / part1
• elasticsearch-pull-request #63634 / part-1
• elasticsearch-pull-request #63614 / part-1
• elasticsearch-pull-request #63604 / part-1
• elasticsearch-pull-request #63574 / part-1
• elasticsearch-pull-request #63556 / part-1
• elasticsearch-pull-request #63554 / part-1
• elasticsearch-pull-request #63537 / part-1
• elasticsearch-pull-request #63519 / part-1
• elasticsearch-pull-request #63519 / part-1

[elasticsearchmachine]

Pinging @elastic/es-distributed-indexing (Team:Distributed Indexing)

[tlrx]

@albertzaharovits looks low priority to me, I wonder if the test should signal that the merge effectively started before closing the threadPoolMergeScheduler.