Fix race condition when initializing HealthCheckedEndpointGroup

[hyunw9]

Motivation

Fixes #6018

A race condition can occur when adding a listener immediately after creating a HealthCheckedEndpointGroup.
Because the endpoint group initialization is asynchronous, the listener may receive endpoints in an incomplete or intermediate state.

Modification

Explicitly await the completion of HealthCheckedEndpointGroup initialization using whenReady().join() before adding a listener.
This ensures the listener will always receive a fully initialized endpoint list, eliminating any race conditions.

Result
• Race condition is fixed.
• Listener always receives a fully initialized EndpointGroup, ensuring reliable and predictable behavior.

[github-actions]

🔍 Build Scan® (commit: 64a9e31)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/v5bemumj5tula
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/2pw37ttap7dmy
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/xlyjqfxr3bqae
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/pdlhju2i7jfrc
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/n3aftswk7kcja
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/oh7ohizucmktm
build-macos-latest-jdk-21	✅	https://ge.armeria.dev/s/prrqjvygo72fy

[jrhee17]

It seems like the test in question has failed again: https://github.com/line/armeria/actions/runs/14173631131/job/39703141791?pr=6188

[hyunw9]

I noticed that the test failure in HealthCheckedEndpointGroupTest#cacheReflectsAttributeChanges seemed to stem from an unexpected ordering issue during initialization.

In this test, registering the endpoint is essentially part of the setup phase. However, the internal initialization of HealthCheckedEndpointGroup is asynchronous, and addListener(..., true) can synchronously invoke listener.accept(latest) before the system is ready, which introduces a race condition.

Initially, I thought calling whenReady().join() would fully synchronize the group’s initialization. However, after digging into the code, I realized that although join() ensures the group is initialized, it doesn’t control the timing of the listener callback.

Internally, addListener() acquires a lock to update the listener list, but the actual call to listener.accept(latest) (triggered by notifyLatestValue = true) occurs outside the lock. That means a race condition can still occur between listener.accept(...) and the external setup logic that expects a stable state.

To address this, I changed the approach:
1. I registered the listener using addListener(endpointListener, false) to avoid immediate invocation.
2. Then, after the group was fully initialized via whenReady().join(), I manually called listener.accept(...) to reflect the initial state in a safe and deterministic manner.

This ensures that the state propagation is explicitly controlled and prevents any premature callback from breaking the test setup.

[minwoox]

Thanks!

[minwoox]

Can't we do this?

Suggested change

	endpointGroup.addListener(endpointsListener, false);
	endpointsListener.accept(endpointGroup.endpoints());
	endpointGroup.addListener(endpointsListener, true);

[hyunw9]

it works ! I think Fix duplicate endpoint handling in HealthCheckedEndpointGroup
this change helps the new group include endpoints from the previous group temporarily, which avoids any brief disappearance during the transition.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.53%. Comparing base (8150425) to head (64a9e31).
Report is 52 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6188      +/-   ##
============================================
+ Coverage     74.46%   74.53%   +0.06%     
- Complexity    22234    22364     +130     
============================================
  Files          1963     1969       +6     
  Lines         82437    82767     +330     
  Branches      10764    10777      +13     
============================================
+ Hits          61385    61688     +303     
- Misses        15918    15949      +31     
+ Partials       5134     5130       -4     

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