SERVER-33718 Explain does not produce a plan for mongoS-only aggregation pipelines

1 files changed, 37 insertions, 0 deletions

diff --git a/jstests/sharding/mongos_local_explain.js b/jstests/sharding/mongos_local_explain.js
new file mode 100644
index 00000000000..0200c334d85
--- /dev/null
+++ b/jstests/sharding/mongos_local_explain.js
@@ -0,0 +1,37 @@
+/**
+ * Test that a mongos-only aggregation pipeline is explainable, and that the resulting explain plan
+ * confirms that the pipeline ran entirely on mongoS.
+ */
+(function() {
+    "use strict";
+
+    const st = new ShardingTest({name: "mongos_comment_test", mongos: 1, shards: 1});
+    const mongosConn = st.s;
+
+    // MongoS-only stages to be tested and the expected 'explain' output for that stage.
+    const testStages = {
+        "$listLocalSessions": {allUsers: false, users: [{user: "nobody", db: "nothing"}]},
+        "$listLocalCursors": {}
+    };
+
+    for (let stage in testStages) {
+        // Use the test stage to create a pipeline that runs exclusively on mongoS.
+        const mongosOnlyPipeline = [{[stage]: testStages[stage]}, {$match: {dummyField: 1}}];
+
+        // We expect the explain output to reflect the stage's spec.
+        const expectedExplainStages =
+            [{[stage]: testStages[stage]}, {$match: {dummyField: {$eq: 1}}}];
+
+        // Test that the mongoS-only pipeline is explainable.
+        const explainPlan = assert.commandWorked(mongosConn.getDB("admin").runCommand(
+            {aggregate: 1, pipeline: mongosOnlyPipeline, explain: true}));
+
+        // We expect the stages to appear under the 'mongos' heading, for 'splitPipeline' to be
+        // null, and for the 'mongos.host' field to be the hostname:port of the mongoS itself.
+        assert.docEq(explainPlan.mongos.stages, expectedExplainStages);
+        assert.eq(explainPlan.mongos.host, mongosConn.name);
+        assert.isnull(explainPlan.splitPipeline);
+    }
+
+    st.stop();
+})();
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