mongo/jstests/multiVersion/genericSetFCVUsage/feature_compatibility_version_lagging_secondary.js

// Tests that a primary with upgrade featureCompatibilityVersion cannot connect with a secondary
// with a lower binary version.
import {ReplSetTest} from "jstests/libs/replsettest.js";
import {restartServerReplication, stopServerReplication} from "jstests/libs/write_concern_util.js";

const latest = "latest";

function runTest(downgradeVersion) {
    jsTestLog("Running test with downgradeVersion: " + downgradeVersion);
    const downgradeFCV = binVersionToFCV(downgradeVersion);
    // Start a new replica set with two latest version nodes.
    let rst = new ReplSetTest({
        nodes: [{binVersion: latest}, {binVersion: latest, rsConfig: {priority: 0}}],
        settings: {chainingAllowed: false},
    });
    rst.startSet();
    rst.initiate(null, null, {initiateWithDefaultElectionTimeout: true});

    let primary = rst.getPrimary();
    let latestSecondary = rst.getSecondary();

    // The default WC is majority and stopServerReplication will prevent satisfying any majority
    // writes.
    assert.commandWorked(
        primary.adminCommand({setDefaultRWConcern: 1, defaultWriteConcern: {w: 1}, writeConcern: {w: "majority"}}),
    );

    // Set the featureCompatibilityVersion to the downgrade version so that a downgrade node can
    // join the set.
    assert.commandWorked(
        primary.getDB("admin").runCommand({setFeatureCompatibilityVersion: downgradeFCV, confirm: true}),
    );

    // Add a downgrade node to the set.
    let downgradeSecondary = rst.add({binVersion: downgradeVersion, rsConfig: {priority: 0}});
    rst.reInitiate();

    // Wait for the downgrade secondary to finish initial sync.
    rst.awaitSecondaryNodes();
    rst.awaitReplication();
    rst.waitForAllNewlyAddedRemovals();

    // Stop replication on the downgrade secondary.
    stopServerReplication(downgradeSecondary);

    // Set the featureCompatibilityVersion to the upgrade version. This will not replicate to
    // the downgrade secondary, but the downgrade secondary will no longer be able to
    // communicate with the rest of the set.
    assert.commandWorked(primary.adminCommand({setFeatureCompatibilityVersion: latestFCV, confirm: true}));

    // Shut down the latest version secondary.
    rst.stop(latestSecondary);

    // The primary should step down, since it can no longer see a majority of the replica set.
    rst.awaitSecondaryNodes(null, [primary]);

    restartServerReplication(downgradeSecondary);
    rst.stopSet();
}

runTest("last-continuous");
runTest("last-lts");