SERVER-43860 Always upsert exact source document for pipeline-insert $merger4.2.2-rc0

(cherry picked from commit 23e55cb3d041236f399f7095df31cd3e3da491cc)
(cherry picked from commit 70ef84cd14388602b3d763b12c6f8448ba62a9d8)

1 files changed, 5 insertions, 0 deletions

diff --git a/src/mongo/db/pipeline/expression_context.cpp b/src/mongo/db/pipeline/expression_context.cpp
index 9f65c12669f..781c0cc96c1 100644
--- a/src/mongo/db/pipeline/expression_context.cpp
+++ b/src/mongo/db/pipeline/expression_context.cpp
@@ -67,6 +67,10 @@ ExpressionContext::ExpressionContext(OperationContext* opCtx,
     } else {
         variables.setDefaultRuntimeConstants(opCtx);
     }
+
+    // Any request which did not originate from a mongoS, or which did originate from a mongoS but
+    // has the 'useNewUpsert' flag set, can use the new upsertSupplied mechanism for $merge.
+    useNewUpsert = request.getUseNewUpsert() || !request.isFromMongos();
 }
 
 ExpressionContext::ExpressionContext(OperationContext* opCtx,
@@ -164,6 +168,7 @@ intrusive_ptr<ExpressionContext> ExpressionContext::copyWith(
     expCtx->subPipelineDepth = subPipelineDepth;
 
     expCtx->tempDir = tempDir;
+    expCtx->useNewUpsert = useNewUpsert;
 
     expCtx->opCtx = opCtx;