In order to avoid going through a full serialize/parse cycle for a LogicalStage during distributed planning, we'd like MongoExtensionDistributedPlanLogic to be able to return an array of variants, where the variant can be either a LogicalStage or a ParseNode.

• struct MongoExtensionDPLArray {
      size_t size;    
      struct MongoExtensionDPLArrayElement* const elements;
  };
  
  /** * Types of nodes which can be in an MongoExtensionDPLArray */
  typedef enum MongoExtensionDPLArrayElementType : uint32_t {    
      kParseNode = 0,    
      kLogicalStage = 1} 
  
  MongoExtensionDPLArrayElementType;
  struct MongoExtensionDistributedPlanLogicArrayElement {    
  // Type indicating what the union holds.    
  MongoExtensionDPLArrayElementType type;    
  union {         
  MongoExtensionAggregationStageParseNode* parseNode;         
  MongoExtensionAggregationLogicalStage* logicalStage;
      }};

Similar work was done for the expansion array in https://github.com/mongodb/mongo/commit/881a554ecb2bacc2b94931144cf59ab6c950ad35 

Once the DPLArray is populated by the extension, ownership is assumed to be transferred entirely to the Host.