SOLR-17725: CoreAdmin API to upgrade an index in-place to facilitate Solr upgrade across major versions

[rahulgoswami]

https://issues.apache.org/jira/browse/SOLR-17725

Description

Provide a /admin/cores api (action=UPGRADECOREINDEX) to upgrade older lucene segments to latest version (by targeted reindexing). Calling this REST endpoint on an index created in version X-1 (assuming you are on version X) would help prepare the index for when Solr is upgraded to X+1 without having to recreate the index from source.

Solution

Please provide a short description of the approach taken to implement your solution.

Tests

Please describe the tests you've developed or run to confirm this patch implements the feature or solves the problem.

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, not available for branches on forks living under an organisation)
• 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
• I have added a changelog entry for my change

[rahulgoswami]

First (half-baked) draft. WIP.

[rahulgoswami]

Looking good. Basic (manual) tests with /admin/cores?action=UPGRADECOREINDEX working fine. Will publish response with segment status before and after index upgrade. Also need to add tests.