FunctionScoreQuery have a direct reference to QueryShardContext

[jimczi]

FunctionScoreQuery can be cached on a per-segment basis in the Lucene query cache. However a recent refactoring made them depend on the QueryShardContext so the query cache can now keep references to old index readers.
That should be considered as a leak since this memory is not taken into account in the cache size.

[elasticmachine]

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

[jimczi]

One additional comment here is whether we should allow FunctionScoreQuery to be cached. They are meant to produce a score so seeing them in the query cache would be surprising.

[romseygeek]

I think we have two options here:

• add an isCacheable() method on score functions so that we can choose whether or not to cache a score function query depending on what function it's using
• just disable cacheing entirely

Note that we already have some tests for whether or not this is cacheable at the request layer, but that checks for things like whether NOW has been accessed on the execution context so is at a different level.