Fix PercolatorQuerySearchIT#testPercolatorQueryExistingDocument test

[talevy]

The PercolatorQuerySearchIT tests do not support SMILE since
it cannot create valid UTF-8 which the Percolator queries want.

This failed on my backport PR, guess it was missed:
https://gradle-enterprise.elastic.co/s/otxkawsvp66ze

Should fix this test failure.

[elasticmachine]

Pinging @elastic/es-search (:Search/Percolator)

[romseygeek]

LGTM. Do we know why this has suddenly started failing?

[martijnvg]

Does this problem only occur when percolating an existing document?
If so then I wonder whether this is a real bug.

When documents are provided as part of the percolate query then the docs get copied and parsed as json (see PercolateQueryBuilder#parseDocument(...) method). However the document to percolate is fetched from a get response (see PercolateQueryBuilder#doRewrite(...)) and if the _source is stored as smile then this could cause the assertion error in PercolateQuery#toString(). I think we should also reparse the document to be percolated as json when percolating an existing document?

[talevy]

failed on master here: https://gradle-enterprise.elastic.co/s/ck7zbtbf7tceu/tests/vmlk5cn6cneke-ftftgx32uquuu.

I think we should also reparse the document to be percolated as json when percolating an existing document?

It does seem like that would solve the problem. I'm going to go ahead and merge this so fewer CI runs fail for now!

[talevy]

@romseygeek thanks for the review! This issue in PercolateQueries always existed, but it never showed up because of a quirk in our testing infrastructure that would create an external ES Cluster in Gradle for testing, and that cluster never had any coordinating nodes (if I remember correctly). For other reasons, I "fixed" this external cluster quirk and forced it to create an internal test cluster. These internal test clusters have more diverse topologies/configurations that bubble up this bug that document sources stored in SMILE break in toString since they do not support UTF-8 well.