Use local segment fieldInfos to lookup tsdb merge stats

[dnhatn]

Merging shrink TSDB or LogsDB indices can fail in versions 8.19 or 9.1+.
When shrinking an index to a single shard, we use addIndexes, which can
add Lucene segments directly. In this case, FieldInfos can differ
between shards and the new segment. We should use the FieldInfos from
each segment to retrieve the merge stats, instead of the FieldInfos of
the merged segment.

Relates #125403

[elasticsearchmachine]

Hi @dnhatn, I've created a changelog YAML for you.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[martijnvg]

Thanks Nhat, very good find here. LGTM 👍

[martijnvg]

This is similar to this if statement:

var wrapped = perFieldReader.getDocValuesProducer(fieldInfo);
if (wrapped == null) {
   continue;
}

and so it is safe to continue here? (and we don't have to fallback to UNSUPPORTED)

[dnhatn]

Yes, it should be safe to continue with the merge optimization. If the FieldInfo is null in some segments, it is safe to skip those and use the merge stats from segments that have values.

[dnhatn]

Thanks Martijn!

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	8.19	Commit could not be cherrypicked due to conflicts
✅	9.1

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 132597

[dnhatn]

💚 All backports created successfully

Status	Branch	Result
✅	8.19

Questions ?

Please refer to the Backport tool documentation