fix: Correctly pickup MRT value for msearch's search requests

[pawankartik-elastic]

Here's the before code:

parseMultiLineRequest(restRequest, multiRequest.indicesOptions(), allowExplicitIndex, (searchRequest, parser) -> {
    // ...
    if (searchRequest.pointInTimeBuilder() != null) {
        // ...
    } else {
        searchRequest.setCcsMinimizeRoundtrips( // <-- This is problematic; by now parsing and reading are done!
            SearchParamsParser.parseCcsMinimizeRoundtrips(crossProjectEnabled, restRequest, searchRequest.isCcsMinimizeRoundtrips())
        );
    }
    multiRequest.add(searchRequest);
}, /* ... */ );

parseMultiLineRequest() is responsible for parsing the msearch's body. Within this method, we instantiate a new SearchRequest object for each request and set its MRT to whatever's mentioned in the query parameter (the default fallback value is true). Then, the body of each request is read by readMultiLineFormat(), which extracts the MRT value in the request's body (if present) and sets it by overwriting the previously set value. This way, the MRT value in the request body takes precedence over the query parameter.

However, when the parsing and reading are done, we end up setting the query param value again before adding the request to the msearch object (ref above code and the code comment). Whatever value we read from the request's body is now lost; effectively, we're ignoring whatever value is present in the request's body.

The fix is to drop the problematic MRT setting op after the request's parsing is complete.

[pawankartik-elastic]

Will mark it as ready once the CI issue is resolved.

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[elasticsearchmachine]

Hi @pawankartik-elastic, I've created a changelog YAML for you.

[quux00]

Why does it default to true?
Is that because _msearch always uses _search rather than _async_search and _search defaults to true?

[pawankartik-elastic]

Right. And when the SearchRequest object is instantiated, MRT defaults to true.

[quux00]

LGTM

[elasticsearchmachine]

💔 Backport failed

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

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 138583

[pawankartik-elastic]

💚 All backports created successfully

Status	Branch	Result
✅	9.2
✅	9.1
✅	8.19

Questions ?

Please refer to the Backport tool documentation