/**
 * Ensure that unique indexes are not updated on running an empty collMod command with FCV=4.0.
 * Creates a unique index, downgrades the FCV, and runs collMod on the collection.
 * @tags: [requires_replication, requires_wiredtiger]
 */
(function() {
    'use strict';

    const newIndexFormatVersion = 12;
    const rst = new ReplSetTest({nodes: [{}, {rsConfig: {priority: 0, votes: 0}}]});
    const nodes = rst.startSet();
    rst.initiate();

    let dbName = 'test';
    let collName = 't';
    const primary = rst.getPrimary();
    const primaryDb = primary.getDB(dbName);
    const secondary = rst.getSecondary();
    const coll = primaryDb.getCollection(collName);

    assert.commandWorked(coll.createIndex({a: 1}, {unique: true}));
    assert.writeOK(coll.insert({_id: 0, a: 1}));
    assert.commandWorked(primaryDb.adminCommand({setFeatureCompatibilityVersion: '4.0'}));
    assert.commandWorked(primaryDb.runCommand({collMod: coll.getName()}));

    // Wait for replication of the index creation.
    rst.awaitReplication();
    const secondaryDb = secondary.getDB(dbName);
    const coll_secondary = secondaryDb.getCollection(collName);
    const index = coll_secondary.getIndexes();
    assert.eq(index[1].unique, true, "Expected a unique index: " + tojson(index[1]));
    // Validate that the unique index is not updated on the secondary after an empty collMod
    // command.
    const indexFormatVersion = coll_secondary.aggregate({$collStats: {storageStats: {}}})
                                   .next()
                                   .storageStats.indexDetails[index[1].name]
                                   .metadata.formatVersion;
    assert.eq(indexFormatVersion,
              newIndexFormatVersion,
              "Expected index format version 12 for the unique index: " + tojson(index[1]));

    rst.stopSet();
})();