Use Case

I have several aggregations using $out or $merge ("write stages") where I want to read from a secondary using readPreference: 'secondary'.

The provided readPreference is not used if the operation has a write stage in execeute_operation because topology.commonWireVersion is 0, resulting in the wrong custom server selector being returned in server_selection where it falls back to using ReadPreference.primary.

Root cause is probably commonWireVersion being initialised with 0 instead of null in topology_description_ctor and then not being updated in topology_description_update

User Experience

• Desired outcome: The provided read preference is used when operation has write stage.
• Impact: All customers using aggregations with write stages and read preference 'secondary'. Falls back to primary read preference.

Dependencies

• n/a

Risks/Unknowns

• n/a

Acceptance Criteria

Implementation Requirements

• functional reqs, potential snafus to avoid, performance targets, etc

Testing Requirements

• unit test, spec test sync, etc

Documentation Requirements

• DOCSP ticket, API docs, etc

Follow Up Requirements

• additional tickets to file, required releases, etc
• if node behavior differs/will differ from other drivers, confirm with dbx devs what standard to aim for and what plan, if any, exists to reconcile the diverging behavior moving forward