Hello, I am using Apache Solr 7.7.2 with indexes which were originally created on 4.8 and upgraded ever since. I recently tried upgrading to 8.x using the lucene IndexUpgrader tool and the upgrade fails. I know that lucene 8.x prevents opening any segment which was touched by <= 6.x at any point in the past. I also know the general recommendation is to reindex upon migration to another major release, however it is not always feasible.
So I tried to remove the check for LATEST-1 in SegmentInfos.java ( https://github.com/apache/lucene-solr/blob/releases/lucene-solr/8.11.1/lucene/core/src/java/org/apache/lucene/index/SegmentInfos.java#L321) and also checked for other references to IndexFormatTooOldException. Turns out that removing this check and rebuilding lucene-core lets the upgrade go through fine. I ran a full sequence of index upgrades from 5.x -> 6.x -> 7.x ->8.x. which went through fine. Also search/update operations work without any issues in 8.x. I could not find any JIRAs which talk about the technical reason behind imposing this restriction, and would like to know the nitty-gritties. Also would like to know about any potential pitfalls that I might be overlooking with the above hack. Thanks, Rahul
