Introduce virtual host snapshots

[jrhee17]

Motivation:

We are planning on deprecating XdsEndpointGroup and introducing XdsPreprocessor at #6096.

Previously, XdsEndpointGroup created sub-EndpointGroups which consume resources. (e.g. HealthCheckedEndpointGroup, DnsEndpointGroup, etc..) assuming that all xDS clients will share a common EndpointGroup.

However, XdsPreprocessor cannot be shared among all client types.
For instance, thrift clients will need to use RpcPreprocessor whereas web clients will need to use normal Preprocessors.

I propose that XdsBootstrap contains a single ClusterManager which caches all EndpointGroups derived from Clusters. This ClusterManager is aligned with the xDS API cluster_manager and is aligned with the upstream implementation as well.

One prerequisite for the introduction of ClusterManager is the ability to cache Clusters. This is difficult since the current implementation of ClusterSnapshot also contains a reference to the containing Route.

armeria/xds/src/main/java/com/linecorp/armeria/xds/ClusterSnapshot.java

Line 82 in 533f798

* The {@link Route} this {@link Cluster} belongs to.

I propose that VirtualHostSnapshot and RouteEntry is introduced, and snapshot mappings are unidirectional. This direction aligns with a possible future implementation of VHDS as well.

As this is a refactor of the previous implementation, there is no expected change in behavior except for subset load balancing (which actually matches upstream now)

Modifications:

• Introduced VirtualHostSnapshot which is equivalent to VirtualHost, and RouteEntry which is equivalent to Route.
• Refactored ClusterManager and SubsetLoadBalancer to not use ClusterSnapshot#route
  • SubsetLoadBalancer now is aligned with the upstream implementation (ref)
  • The previous test case for skipping failure on empty metadata is removed as it does not match the upstream implementation.
• (Misc) Cleaned up the ConfigSourceMapper implementation by removing unused fields and the corresponding logic.

Result:

• The ClusterSnapshot API is prepared for the introduction of a central ClusterManager maintained by XdsBootstrapImpl

[github-actions]

🔍 Build Scan® (commit: 0d23073)

Job name	Status	Build Scan®
build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/rtiwyyifw4oby
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/3komghgkwawds
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/abtuufusbwto2
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/7ljtftgpnamqk
build-ubicloud-standard-16-jdk-17-leak	✅	https://ge.armeria.dev/s/tzp7k2je5orei
build-ubicloud-standard-16-jdk-11	✅	https://ge.armeria.dev/s/5m7wxhs3cxj3u
build-macos-latest-jdk-21	✅	https://ge.armeria.dev/s/lk2sv5wlevbrs

[minwoox]

👍 👍 👍

[minwoox]

In RouteResourceNode, we call virtualHostSnapshots.clear(); where as we call pending.clear(); here. Is this intentional?

[jrhee17]

Good point, updated both implementations to just clear both data structures

[trustin]

Mostly nits. Looks good!

[jrhee17]

Let me go ahead and merge this PR, thanks for the review all