Overview

Implement DocumentSourceDocumentResultsAndMetadata::lowerToExecutable() and wire it into run_agg.cpp's prepareExecutorsForPipeline.

Background

\$_documentResultsAndMetadata is a transport container. During prepareExecutorsForPipeline in run_agg.cpp, if the pipeline begins with this stage it must be popped and replaced with directly executable stages before the pipeline runs.

Scope of Work

Implement DocumentSourceDocumentResultsAndMetadata::lowerToExecutable(Pipeline&):

• Pop the container from the pipeline front
• If _metadata == boost::none (elided): prepend _source directly — no Exchange needed
• Otherwise: construct an exec::agg::Exchange (policy kKeyRange on _streamType, two consumers) wrapping _source; create two DocumentSourceExchange consumers sharing the exchange via intrusive_ptr; append {{\$replaceRoot( {newRoot: "\$payload"}

  )}} after each consumer to unwrap user BSON from the {\$_streamType, payload} envelope

• For returnCursor = false (standalone): wrap consumer-1 and its \$replaceRoot in DocumentSourceSetVariableFromSubPipeline targeting Variables::kSearchMetaId; prepend consumer-0, \$replaceRoot, then \$setVar to the outer pipeline

Wire the call in run_agg.cpp:

if (auto* c = dynamic_cast(pipeline->peekFront()))
    c->lowerToExecutable(*pipeline);

Acceptance Criteria

• Metadata elided path: _source prepended with no Exchange overhead
• Standalone path: Exchange + $replaceRoot + $setVar wired correctly
• $$SEARCH_META is set before any downstream stage reads it
• Unit tests cover elided and non-elided paths