/**
 * 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;

const stageSpec = {
    "$listLocalSessions": {allUsers: false, users: [{user: "nobody", db: "nothing"}]}
};

// Use the test stage to create a pipeline that runs exclusively on mongoS.
const mongosOnlyPipeline = [stageSpec, {$match: {dummyField: 1}}];

// We expect the explain output to reflect the stage's spec.
const expectedExplainStages = [stageSpec, {$match: {dummyField: {$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(expectedExplainStages, explainPlan.mongos.stages);
assert.eq(explainPlan.mongos.host, mongosConn.name);
assert.isnull(explainPlan.splitPipeline);

st.stop();
})();