SERVER-25005 Use Pipeline to execute $lookup and $graphLookup.

Replaces the usages of DBDirectClient::query() in DocumentSourceLookUp
and DocumentSourceGraphLookUp to instead parse and execute a Pipeline.

Simplifies the registration process of the "inner" plan execution in an
aggregation pipeline. The DocumentSourceCursor class now owns its
PlanExecutor and the PipelineProxyStage class no longer has a
std::weak_ptr to it. The "inner" plan executor is registered with the
CursorManager of the underlying Collection and will receive invalidation
notifications if a catalog operation occurs.

1 files changed, 7 insertions, 1 deletions

diff --git a/src/mongo/db/pipeline/expression_context.h b/src/mongo/db/pipeline/expression_context.h
index 285318f7f7a..27708590b18 100644
--- a/src/mongo/db/pipeline/expression_context.h
+++ b/src/mongo/db/pipeline/expression_context.h
@@ -68,6 +68,12 @@ public:
         return _valueComparator;
     }
 
+    /**
+     * Returns an ExpressionContext that is identical to 'this' that can be used to execute a
+     * separate aggregation pipeline on 'ns'.
+     */
+    boost::intrusive_ptr<ExpressionContext> copyWith(NamespaceString ns) const;
+
     bool isExplain = false;
     bool inShard = false;
     bool inRouter = false;
@@ -81,7 +87,7 @@ public:
 
     // Collation requested by the user for this pipeline. Empty if the user did not request a
     // collation.
-    const BSONObj collation;
+    BSONObj collation;
 
     static const int kInterruptCheckPeriod = 128;
     int interruptCounter = kInterruptCheckPeriod;  // when 0, check interruptStatus