Summary

The execution-level ChangeStreamAddPostImageStage currently does two unrelated jobs: the updateLookup enrichment (fullDocument: "updateLookup" - a lookup against the user collection) and the post-image-from-pre-image computation (applying the oplog delta on top of the pre-image, for the required / whenAvailable modes). Split it into two single-responsibility stages.

Scope

• Extract a dedicated ChangeStreamUpdateLookupStage handling only the updateLookup user-collection lookup.
• Slim ChangeStreamAddPostImageStage down to only the post-image-from-pre-image+delta path (required / whenAvailable).
• Have DocumentSourceChangeStreamAddPostImage build the correct execution stage per fullDocument mode.

Files

• src/mongo/db/exec/agg/change_stream_update_lookup_stage.h / .cpp (new)
• src/mongo/db/exec/agg/change_stream_add_post_image_stage.h / .cpp
• src/mongo/db/exec/agg/change_stream_add_post_image_stage_fn.cpp
• src/mongo/db/pipeline/document_source_change_stream_add_post_image.h / .cpp

Acceptance criteria

• Behaviour-preserving: no change-stream output change, no feature flag.
• Existing change_streams and the sharded / whole-db passthrough suites stay green.
• Unit tests covering per-fullDocument-mode stage selection.