SERVER-60869 Re-add logic to set feature compatibility version on arbiters

Author: ali-mir

jstests/multiVersion/genericSetFCVUsage/arbiter_always_has_latest_fcv.js

@@ -0,0 +1,25 @@
+/*
+ * Tests that an arbiter will default to the latest FCV regardless of the FCV of the replica set.
+ */
+(function() {
+"use strict";
+
+function runTest(FCV) {
+    let rst = new ReplSetTest(
+        {nodes: [{}, {rsConfig: {arbiterOnly: true}}], nodeOpts: {binVersion: FCV}});
+    rst.startSet();
+    rst.initiate();
+
+    const arbiter = rst.getArbiter();
+    const res = assert.commandWorked(
+        arbiter.getDB("admin").runCommand({getParameter: 1, featureCompatibilityVersion: 1}));
+    assert.eq(res.featureCompatibilityVersion.version, latestFCV, tojson(res));
+    rst.stopSet();
+}
+
+runTest(latestFCV);
+runTest(lastLTSFCV);
+if (lastContinuousFCV != lastLTSFCV) {
+    runTest(lastContinuousFCV);
+}
+}());

src/mongo/db/repl/replication_coordinator_impl.cpp

@@ -4347,6 +4347,15 @@ ReplicationCoordinatorImpl::_updateMemberStateFromTopologyCoordinator(WithLock l
           "Replica set state transition",
           "newState"_attr = newState,
           "oldState"_attr = _memberState);
+
+    // Initializes the featureCompatibilityVersion to the latest value, because arbiters do not
+    // receive the replicated version. This is to avoid bugs like SERVER-32639.
+    if (newState.arbiter()) {
+        // (Generic FCV reference): This FCV check should exist across LTS binary versions.
+        serverGlobalParams.mutableFeatureCompatibility.setVersion(
+            multiversion::GenericFCV::kLatest);
+    }
+
     _memberState = newState;
 
     _cancelAndRescheduleElectionTimeout_inlock();