Fix sort values coming from unrelated documents in certain multi-level sorts #31554

[dbevacqua]

Root filter of Nested returned from SortBuilder produces no docs due to incorrect MUST clause. By removing it and just using ToParentBlockJoinQuery on its own the correct docs are produced.

[elasticmachine]

Pinging @elastic/es-search-aggs

[martijnvg]

test this please

[dbevacqua]

I don't think the build failure is related to my commit. Failing test is an x-pack one:

10:56:05 > Task :x-pack:qa:rolling-upgrade:without-system-key:v6.4.0-SNAPSHOT#twoThirdsUpgradedTestRunner FAILED
10:56:05 Tests with failures:
10:56:05   - org.elasticsearch.upgrades.UpgradeClusterClientYamlTestSuiteIT.test {p0=mixed_cluster/30_ml_jobs_crud/Test get old cluster job}

[martijnvg]

@dbevacqua Yes, that is a ml test and is completely unrelated to the change. Can you merge master into your branch and push? This should trigger a new build.

[dbevacqua]

@martijnvg done (though no new build so far...)

[martijnvg]

test this please

[martijnvg]

@dbevacqua Thanks! I'm going to add some more tests, so that there is more test coverage for multi level nested sorting. Right now that test coverage is poor. I think your fix is good, but having more tests should also increase confidence in this fix and whether multi level nested sorting is actual working.

[martijnvg]

test this please

[martijnvg]

@jpountz Would you like to take a look at this when you have time?

[jimczi]

I am very sorry @dbevacqua, I tried to push to your branch to update this pr like @martijnvg did but I made something wrong and the pr got closed automatically. We discussed with @martijnvg about a simplification that should also fix #31783.
I opened a new pr with your commits and this additional change in #32204 which should add you as a co-author. Again sorry for the trouble.