mongo/jstests/multiVersion/genericSetFCVUsage/downgrade_replset.js

// Test the downgrade of a replica set from latest version
// to last-stable version succeeds, while reads and writes continue.

load('./jstests/multiVersion/libs/multi_rs.js');
load('./jstests/libs/test_background_ops.js');
load('./jstests/libs/feature_compatibility_version.js');

let newVersion = "latest";
let oldVersion = "last-stable";

let name = "replsetdowngrade";
let nodes = {
    n1: {binVersion: newVersion},
    n2: {binVersion: newVersion},
    n3: {binVersion: newVersion}
};

function runDowngradeTest() {
    let rst = new ReplSetTest({name: name, nodes: nodes, waitForKeys: true});
    rst.startSet();
    rst.initiate();

    let primary = rst.getPrimary();
    let coll = "test.foo";

    // The default FCV is latestFCV for non-shard replica sets.
    let primaryAdminDB = rst.getPrimary().getDB("admin");
    checkFCV(primaryAdminDB, latestFCV);

    // We wait for the feature compatibility version to be set to lastStableFCV on all nodes of the
    // replica set in order to ensure that all nodes can be successfully downgraded. This
    // effectively allows us to emulate upgrading to the latest version with existing data files and
    // then trying to downgrade back to lastStableFCV.
    assert.commandWorked(primary.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
    rst.awaitReplication();

    jsTest.log("Inserting documents into collection.");
    for (let i = 0; i < 10; i++) {
        primary.getCollection(coll).insert({_id: i, str: "hello world"});
    }

    function insertDocuments(rsURL, collParam) {
        let coll = new Mongo(rsURL).getCollection(collParam);
        let count = 10;
        while (!isFinished()) {
            assert.writeOK(coll.insert({_id: count, str: "hello world"}));
            count++;
        }
    }

    jsTest.log("Starting parallel operations during downgrade..");
    let joinFindInsert = startParallelOps(primary, insertDocuments, [rst.getURL(), coll]);

    jsTest.log("Downgrading replica set..");
    rst.upgradeSet({binVersion: oldVersion});
    jsTest.log("Downgrade complete.");

    // We save a reference to the old primary so that we can call reconnect() on it before
    // joinFindInsert() would attempt to send the node an update operation that signals the parallel
    // shell running the background operations to stop.
    let oldPrimary = primary;

    primary = rst.getPrimary();
    printjson(rst.status());

    // Since the old primary was restarted as part of the downgrade process, we explicitly reconnect
    // to it so that sending it an update operation silently fails with an unchecked NotMaster error
    // rather than a network error.
    reconnect(oldPrimary.getDB("admin"));
    joinFindInsert();
    rst.stopSet();
}

runDowngradeTest();