From 29497d3d585581d97bd28e90d995f14470cc9b93 Mon Sep 17 00:00:00 2001
From: ali-mir <ali.mir@mongodb.com>
Date: Mon, 25 Oct 2021 16:56:41 +0000
Subject: SERVER-60869 Re-add logic to set feature compatibility version on
 arbiters

(cherry picked from commit f094005b0b8a878bc039ddae540b8696f70848f7)
---
 .../arbiter_always_has_latest_fcv.js               | 25 ++++++++++++++++++++++
 src/mongo/db/repl/replication_coordinator_impl.cpp |  9 ++++++++
 2 files changed, 34 insertions(+)
 create mode 100644 jstests/multiVersion/genericSetFCVUsage/arbiter_always_has_latest_fcv.js

diff --git a/jstests/multiVersion/genericSetFCVUsage/arbiter_always_has_latest_fcv.js b/jstests/multiVersion/genericSetFCVUsage/arbiter_always_has_latest_fcv.js
new file mode 100644
index 00000000000..0e4d2fde530
--- /dev/null
+++ b/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);
+}
+}());
diff --git a/src/mongo/db/repl/replication_coordinator_impl.cpp b/src/mongo/db/repl/replication_coordinator_impl.cpp
index edd4a533241..a39a9dd61db 100644
--- a/src/mongo/db/repl/replication_coordinator_impl.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl.cpp
@@ -4345,6 +4345,15 @@ 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();
-- 
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