Overview

Fix $lookup and $unionWith to call lowerToExecutable() on $_documentResultsAndMetadata when it appears at the front of their sub-pipelines.

Background

run_agg.cpp's prepareExecutorsForPipeline only lowers $_documentResultsAndMetadata at the top-level pipeline. $lookup and $unionWith defer sub-pipeline preparation until doGetNext() at execution time, so the top-level lowering pass never recurses into them. If $search appears inside a $unionWith or $lookup sub-pipeline, the container will sit unexpanded when the sub-pipeline first executes.

Scope of Work

In DocumentSourceLookup::doGetNext() and DocumentSourceUnionWith::doGetNext(), before the sub-pipeline is first executed, check whether its front stage is a DocumentSourceDocumentResultsAndMetadata and if so call lowerToExecutable() on it

Acceptance Criteria

• $search inside a $unionWith sub-pipeline executes correctly with scoped $$SEARCH_META
• \$search inside a $lookup sub-pipeline executes correctly
• Outer $$SEARCH_META is not affected by inner-scope lowering
• Test coverage for $search-in-$unionWith and $search-in-$lookup