1 files changed, 42 insertions, 7 deletions

diff --git a/src/mongo/db/pipeline/cluster_aggregation_planner.h b/src/mongo/db/pipeline/cluster_aggregation_planner.h
index f62f158f6ca..3b3aaa63df4 100644
--- a/src/mongo/db/pipeline/cluster_aggregation_planner.h
+++ b/src/mongo/db/pipeline/cluster_aggregation_planner.h
@@ -28,31 +28,66 @@
 
 #pragma once
 
+#include "mongo/db/pipeline/lite_parsed_pipeline.h"
 #include "mongo/db/pipeline/pipeline.h"
+#include "mongo/s/query/cluster_client_cursor_impl.h"
 
 namespace mongo {
 namespace cluster_aggregation_planner {
 
 /**
- * Performs optimizations with the aim of reducing computing time and network traffic when a
- * pipeline has been split into two pieces. Modifies 'shardPipeline' and 'mergingPipeline' such that
- * they may contain different stages, but still compute the same results when executed.
+ * Represents the two halves of a pipeline that will execute in a sharded cluster. 'shardsPipeline'
+ * will execute in parallel on each shard, and 'mergePipeline' will execute on the merge host -
+ * either one of the shards or a mongos.
  */
-void performSplitPipelineOptimizations(Pipeline* shardPipeline, Pipeline* mergingPipeline);
+struct SplitPipeline {
+    SplitPipeline(std::unique_ptr<Pipeline, PipelineDeleter> shardsPipeline,
+                  std::unique_ptr<Pipeline, PipelineDeleter> mergePipeline,
+                  boost::optional<BSONObj> shardCursorsSortSpec)
+        : shardsPipeline(std::move(shardsPipeline)),
+          mergePipeline(std::move(mergePipeline)),
+          shardCursorsSortSpec(std::move(shardCursorsSortSpec)) {}
+
+    std::unique_ptr<Pipeline, PipelineDeleter> shardsPipeline;
+    std::unique_ptr<Pipeline, PipelineDeleter> mergePipeline;
+
+    // If set, the cursors from the shards are expected to be sorted according to this spec, and to
+    // have populated a "$sortKey" metadata field which can be used to compare the results.
+    boost::optional<BSONObj> shardCursorsSortSpec;
+};
 
 /**
- * Rips off an initial $sort stage that can be handled by cursor merging machinery. Returns the
- * sort key pattern of such a $sort stage if there was one, and boost::none otherwise.
+ * Split the current Pipeline into a Pipeline for each shard, and a Pipeline that combines the
+ * results within a merging process. This call also performs optimizations with the aim of reducing
+ * computing time and network traffic when a pipeline has been split into two pieces.
+ *
+ * The 'mergePipeline' returned as part of the SplitPipeline here is not ready to execute until the
+ * 'shardsPipeline' has been sent to the shards and cursors have been established. Once cursors have
+ * been established, the merge pipeline can be made executable by calling 'addMergeCursorsSource()'
  */
-boost::optional<BSONObj> popLeadingMergeSort(Pipeline* mergePipeline);
+SplitPipeline splitPipeline(std::unique_ptr<Pipeline, PipelineDeleter> pipeline);
 
 /**
  * Creates a new DocumentSourceMergeCursors from the provided 'remoteCursors' and adds it to the
  * front of 'mergePipeline'.
  */
 void addMergeCursorsSource(Pipeline* mergePipeline,
+                           const LiteParsedPipeline&,
+                           BSONObj cmdSentToShards,
                            std::vector<RemoteCursor> remoteCursors,
+                           const std::vector<ShardId>& targetedShards,
+                           boost::optional<BSONObj> shardCursorsSortSpec,
                            executor::TaskExecutor*);
 
+/**
+ * Builds a ClusterClientCursor which will execute 'pipeline'. If 'pipeline' consists entirely of
+ * $skip and $limit stages, the pipeline is eliminated entirely and replaced with a RouterExecStage
+ * tree that does same thing but will avoid using a RouterStagePipeline. Avoiding a
+ * RouterStagePipeline will remove an expensive conversion from BSONObj -> Document for each result.
+ */
+ClusterClientCursorGuard buildClusterCursor(OperationContext* opCtx,
+                                            std::unique_ptr<Pipeline, PipelineDeleter> pipeline,
+                                            ClusterClientCursorParams&&);
+
 }  // namespace cluster_aggregation_planner
 }  // namespace mongo