Small breaking change upgrading to 8.17 with synthetic source

[henrikno]

Elasticsearch Version

8.17

Installed Plugins

No response

Java Version

bundled

OS Version

Docker

Problem Description

One our our integration tests discovered a change in the output format of documents when using synthetic source/tsds and upgrading to 8.17.
The test was looking for a field in the source _source['queuestat']['active'] but it changed to _source['queuestat.active'] after upgrading.
This seems like a breaking change, but I think it only happens if the original document has . in the fieldname, corresponding to this synthetic source modification + synthetic source or TSDS. It might be worth mentioning it as a caveat, since it currently says "There are no breaking changes in Elasticsearch 8.17."

Steps to Reproduce

Gist to reproduce: https://gist.github.com/henrikno/dc6430542549fd773fcbcda8084358fb

Logs (if relevant)

No response

[elasticsearchmachine]

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

[lkts]

We document this here https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-source-field.html#synthetic-source-modifications-field-names but indeed it is not marked as breaking change.

[lkts]

Nevermind, i was thinking of a different thing. Looking at the repro i actually don't really understand why that change to source format is necessary.

[lkts]

I tried this on 8.17 and i can't reproduce it. I don't believe upgrading part really matters given that data stream is rolled over and new document is indexed into a new write index? Is it possible that these new fields are ignored in your test due to field limit?