SERVER-45408 Enable serialization of config "term" field

SERVER-45092 Remove “term” field of config document on downgrade

4 files changed, 91 insertions, 5 deletions

diff --git a/jstests/multiVersion/remove_config_term_on_replset_downgrade.js b/jstests/multiVersion/remove_config_term_on_replset_downgrade.js
new file mode 100644
index 00000000000..4315fdcc506
--- /dev/null
+++ b/jstests/multiVersion/remove_config_term_on_replset_downgrade.js
@@ -0,0 +1,73 @@
+/**
+ * Test the downgrade of a replica set from latest version to last-stable version succeeds and
+ * removes the "term" field of config document.
+ */
+
+(function() {
+'use strict';
+
+load('jstests/multiVersion/libs/multi_rs.js');
+load('jstests/libs/test_background_ops.js');
+
+let newVersion = "latest";
+let oldVersion = "last-stable";
+
+let nodes = {
+    n1: {binVersion: newVersion},
+    n2: {binVersion: newVersion},
+    n3: {binVersion: newVersion}
+};
+
+let rst = new ReplSetTest({nodes: nodes});
+rst.startSet();
+rst.initiate();
+
+let primary = rst.getPrimary();
+
+// The default FCV is latestFCV for non-shard replica sets.
+let primaryAdminDB = rst.getPrimary().getDB("admin");
+checkFCV(primaryAdminDB, latestFCV);
+
+// Reconfig in FCV 4.4.
+let originalConfig = rst.getReplSetConfigFromNode();
+originalConfig.version++;
+reconfig(rst, originalConfig);
+rst.awaitNodesAgreeOnConfigVersion();
+
+// Check that the term field exists in the config document on all nodes.
+rst.nodes.forEach(function(node) {
+    jsTestLog("Checking the config term on node " + tojson(node.host) + " before downgrade.");
+    let config = node.getDB("local").getCollection("system.replset").findOne();
+    assert(config.hasOwnProperty("term"));
+});
+
+// Remember the config from the primary before the downgrade.
+let configInNewVersion = rst.getReplSetConfigFromNode();
+
+jsTest.log("Downgrading FCV to 4.2");
+assert.commandWorked(primary.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
+rst.awaitReplication();
+// Check that the term field doesn't exist in the config document on all nodes.
+rst.nodes.forEach(function(node) {
+    jsTestLog("Checking the config term on node " + tojson(node.host) + " after FCV downgrade.");
+    let config = node.getDB("local").getCollection("system.replset").findOne();
+    assert(!config.hasOwnProperty("term"), tojson(config));
+    config.term = configInNewVersion.term;
+    assert.docEq(configInNewVersion, config);
+});
+
+jsTest.log("Downgrading replica set..");
+rst.upgradeSet({binVersion: oldVersion});
+jsTest.log("Downgrade complete.");
+
+// Check that the term field doesn't exist in the config document on all nodes.
+rst.nodes.forEach(function(node) {
+    reconnect(node);
+    jsTestLog("Checking the config term on node " + tojson(node.host) + " after binary downgrade.");
+    let config = node.getDB("local").getCollection("system.replset").findOne();
+    assert(!config.hasOwnProperty("term"), tojson(config));
+    config.term = configInNewVersion.term;
+    assert.docEq(configInNewVersion, config);
+});
+rst.stopSet();
+})();
diff --git a/jstests/replsets/reconfig_ignores_term_field.js b/jstests/replsets/reconfig_ignores_term_field.js
index bfe3d5b9d20..2cc48ac950d 100644
--- a/jstests/replsets/reconfig_ignores_term_field.js
+++ b/jstests/replsets/reconfig_ignores_term_field.js
@@ -25,7 +25,6 @@ assert.commandWorked(primary.getDB("admin").runCommand({replSetReconfig: config}
 replTest.awaitReplication();
 
 config = primary.getDB("local").system.replset.findOne();
-// TODO SERVER-45408: uncomment once we enable serialization of the term field.
-// assert.eq(config.term, 1);
+assert.eq(config.term, 1);
 replTest.stopSet();
 }());
\ No newline at end of file
diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index 48ed157c26f..c98457c1f86 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -162,6 +162,17 @@ void FeatureCompatibilityVersion::onInsertOrUpdate(OperationContext* opCtx, cons
               << FeatureCompatibilityVersionParser::toString(newVersion);
     }
 
+    // Remove term field of config document on downgrade.
+    if (newVersion == ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo42 &&
+        repl::ReplicationCoordinator::get(opCtx)->getReplicationMode() ==
+            repl::ReplicationCoordinator::modeReplSet) {
+        auto storageInterface = repl::StorageInterface::get(opCtx);
+        repl::UnreplicatedWritesBlock uwb(opCtx);
+        repl::TimestampedBSONObj update{BSON("$unset" << BSON("term" << 1)), Timestamp()};
+        uassertStatusOK(storageInterface->updateSingleton(
+            opCtx, NamespaceString::kSystemReplSetNamespace, {}, update));
+    }
+
     opCtx->recoveryUnit()->onCommit([opCtx, newVersion](boost::optional<Timestamp>) {
         serverGlobalParams.featureCompatibility.setVersion(newVersion);
         updateMinWireVersion();
diff --git a/src/mongo/db/repl/repl_set_config.cpp b/src/mongo/db/repl/repl_set_config.cpp
index 375e2a73d61..5094f610c8e 100644
--- a/src/mongo/db/repl/repl_set_config.cpp
+++ b/src/mongo/db/repl/repl_set_config.cpp
@@ -862,9 +862,12 @@ BSONObj ReplSetConfig::toBSON() const {
     BSONObjBuilder configBuilder;
     configBuilder.append(kIdFieldName, _replSetName);
     configBuilder.appendIntOrLL(kVersionFieldName, _version);
-    // TODO (SERVER-45408): Enable serialization of the config "term" field once we can handle it
-    // properly in upgrade/downgrade scenarios.
-    // configBuilder.appendIntOrLL(kTermFieldName, _term);
+
+    if (serverGlobalParams.featureCompatibility.isVersion(
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo44)) {
+        configBuilder.appendIntOrLL(kTermFieldName, _term);
+    }
+
     if (_configServer) {
         // Only include "configsvr" field if true
         configBuilder.append(kConfigServerFieldName, _configServer);