SOLR-17269: Do not publish synthetic solr core (of Coordinator node) to ZK

[patsonluk]

https://issues.apache.org/jira/browse/SOLR-17269

Description

The synthetic collection/replica introduced in Solr 9 registers such collection to zookeeper. This might cause issues with other tools which monitor solr collections via zk and they might be mistaken the synthetic collection as a real collection.

Solution

We are introducing a new class SyntheticSolrCore which extends SolrCore, SyntheticSolrCore is only used in Coordinator node to support a subset of SolrCore functionalities required by Coordinator operations such as aggregating and writing out response and providing configset info. There will be one instance of SyntheticSolrCore per config set.

Such synthetic core is only registered to the CoreContainer but is no longer registering itself to zookeeper.

Take note that to minimize code changes, there will still be snapshot/index data directory written to the Coordinator node. We could have overridden initSnapshotMetaDataManager and initIndex to be no-op to avoid creating them, however, there is currently other code (and possibly future code too) that might rely on the existence of those fields. It is safer to just keep them as is.

The data folder for the synthetic core would NOT be loaded on QA node start-up as there's no core.properties in such directory. This is because instead of going through CoreContainer#create for such synthetic core, it's using SyntheticSolrCore#createAndRegisterCore, which bypass core.properties creation and zk registration

Tests

The existing TestCoordinatorRole has already provided good coverage of various scenarios. Take note that we have removed the test code that verifies the existence of cores from the synthetic collection since zk no longer keeps track of synthetic collections. Instead we added a check in TestCoordinatorRole#testSimple to ensure that the synthetic collection no longer exists in cluster state.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide