`WeightRampingUpStrategy` uses locks to avoid occasional selection failure for `XdsEndpointGroup`

[jrhee17]

Motivation:

https://github.com/line/armeria/issues/5749

While re-investigating this issue, I found that the probably cause is the changes introduced by https://github.com/line/armeria/pull/5693

Previously, when updateEndpoints is called the endpointSelector was initialized from the same thread

https://github.com/line/armeria/blob/da7f76a7eebf9b03a51ff68a9245269eee4a5a6d/core/src/main/java/com/linecorp/armeria/client/endpoint/WeightRampingUpStrategy.java#L147-L150

However after the PR above, updateEndpoints doesn't update the endpointSelector immediately and instead schedules an update task

https://github.com/line/armeria/blob/191fb7db981d1d47de8ce50be0b0f6d5d9dfa8ee/core/src/main/java/com/linecorp/armeria/client/endpoint/WeightRampingUpStrategy.java#L150

For this reason, even if updateEndpoints is called the endpointSelector will remain as the EMPTY_SELECTOR.

https://github.com/line/armeria/blob/191fb7db981d1d47de8ce50be0b0f6d5d9dfa8ee/core/src/main/java/com/linecorp/armeria/client/endpoint/WeightRampingUpStrategy.java#L128

Normally, this isn't an issue since refreshEndpoints will be called after the call to updateEndpoints which will in turn complete the select call. However, XdsEndpointGroup is composed of multiple internal EndpointGroups and only selectNow is called. Because the initialization event is never propagated to the XdsEndpointSelector occasional failures can occur.

https://github.com/line/armeria/blob/191fb7db981d1d47de8ce50be0b0f6d5d9dfa8ee/xds/src/main/java/com/linecorp/armeria/xds/client/endpoint/DefaultLoadBalancer.java#L61-L84

Modifications:

• Use locks instead of event loops for updateEndpoints

Result:

• Endpoint selection calls to WeightRampingUpStrategy succeeds before the first call to updateEndpoints for XdsEndpointGroup.

[github-actions[bot]]

🔍 Build Scan® (commit: d8c97f1ca91777f81b2c184a39408c56b60c4642)

Job name	Status	Build Scan®
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build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/2ukke44ngsgla
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/d5y4zdclnk6ui
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