SERVER-40429 Add merge stage to write output to existing collection

5 files changed, 10 insertions, 14 deletions

diff --git a/jstests/libs/override_methods/implicitly_wrap_pipelines_in_facets.js b/jstests/libs/override_methods/implicitly_wrap_pipelines_in_facets.js
index c5c26b6e8b1..84da15b1b8f 100644
--- a/jstests/libs/override_methods/implicitly_wrap_pipelines_in_facets.js
+++ b/jstests/libs/override_methods/implicitly_wrap_pipelines_in_facets.js
@@ -31,14 +31,8 @@
             return originalRunCommand.apply(this, arguments);
         }
 
-        const stagesDisallowedInsideFacet = [
-            '$changeStream',
-            '$collStats',
-            '$facet',
-            '$geoNear',
-            '$indexStats',
-            '$out',
-        ];
+        const stagesDisallowedInsideFacet =
+            ['$changeStream', '$collStats', '$facet', '$geoNear', '$indexStats', '$merge', '$out'];
         for (let stageSpec of originalPipeline) {
             // Skip wrapping the pipeline in a $facet stage if it has an invalid stage
             // specification.
diff --git a/jstests/libs/override_methods/network_error_and_txn_override.js b/jstests/libs/override_methods/network_error_and_txn_override.js
index 526e343c8c7..9394ca836cc 100644
--- a/jstests/libs/override_methods/network_error_and_txn_override.js
+++ b/jstests/libs/override_methods/network_error_and_txn_override.js
@@ -386,7 +386,7 @@
                 shouldForceReadConcern = false;
             }
 
-            if (OverrideHelpers.isAggregationWithOutStage(cmdName, cmdObj)) {
+            if (OverrideHelpers.isAggregationWithOutOrMergeStage(cmdName, cmdObj)) {
                 // The $out stage can only be used with readConcern={level: "local"}.
                 shouldForceReadConcern = false;
             } else {
diff --git a/jstests/libs/override_methods/override_helpers.js b/jstests/libs/override_methods/override_helpers.js
index 94d7f72cb06..bd49e0d7191 100644
--- a/jstests/libs/override_methods/override_helpers.js
+++ b/jstests/libs/override_methods/override_helpers.js
@@ -25,7 +25,7 @@ var OverrideHelpers = (function() {
         };
     }
 
-    function isAggregationWithOutStage(commandName, commandObj) {
+    function isAggregationWithOutOrMergeStage(commandName, commandObj) {
         if (commandName !== "aggregate" || typeof commandObj !== "object" || commandObj === null) {
             return false;
         }
@@ -39,7 +39,8 @@ var OverrideHelpers = (function() {
             return false;
         }
 
-        return Object.keys(lastStage)[0] === "$out";
+        const lastStageName = Object.keys(lastStage)[0];
+        return lastStageName === "$out" || lastStageName === "$merge";
     }
 
     function isMapReduceWithInlineOutput(commandName, commandObj) {
@@ -99,7 +100,7 @@ var OverrideHelpers = (function() {
     return {
         isAggregationWithListLocalSessionsStage:
             makeIsAggregationWithFirstStage("$listLocalSessions"),
-        isAggregationWithOutStage: isAggregationWithOutStage,
+        isAggregationWithOutOrMergeStage: isAggregationWithOutOrMergeStage,
         isMapReduceWithInlineOutput: isMapReduceWithInlineOutput,
         prependOverrideInParallelShell: prependOverrideInParallelShell,
         overrideRunCommand: overrideRunCommand,
diff --git a/jstests/libs/override_methods/set_read_and_write_concerns.js b/jstests/libs/override_methods/set_read_and_write_concerns.js
index e8c37681f25..4b9adfb23fa 100644
--- a/jstests/libs/override_methods/set_read_and_write_concerns.js
+++ b/jstests/libs/override_methods/set_read_and_write_concerns.js
@@ -68,7 +68,8 @@
                 shouldForceReadConcern = false;
             }
 
-            if (OverrideHelpers.isAggregationWithOutStage(commandName, commandObjUnwrapped)) {
+            if (OverrideHelpers.isAggregationWithOutOrMergeStage(commandName,
+                                                                 commandObjUnwrapped)) {
                 // The $out stage can only be used with readConcern={level: "local"} or
                 // readConcern={level: "majority"}
                 if (TestData.defaultReadConcernLevel === "linearizable") {
diff --git a/jstests/libs/override_methods/set_read_preference_secondary.js b/jstests/libs/override_methods/set_read_preference_secondary.js
index 3646198fff0..70dfc5737a5 100644
--- a/jstests/libs/override_methods/set_read_preference_secondary.js
+++ b/jstests/libs/override_methods/set_read_preference_secondary.js
@@ -110,7 +110,7 @@
         }
 
         let shouldForceReadPreference = kCommandsSupportingReadPreference.has(commandName);
-        if (OverrideHelpers.isAggregationWithOutStage(commandName, commandObjUnwrapped)) {
+        if (OverrideHelpers.isAggregationWithOutOrMergeStage(commandName, commandObjUnwrapped)) {
             // An aggregation with a $out stage must be sent to the primary.
             shouldForceReadPreference = false;
         } else if ((commandName === "mapReduce" || commandName === "mapreduce") &&