SERVER-47211 Remove 4.2 style reconfig behavior on FCV downgrade

6 files changed, 4 insertions, 119 deletions

diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index ad455262237..69062283c85 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -253,49 +253,11 @@ public:
 
             FeatureCompatibilityVersion::setTargetDowngrade(opCtx);
 
-            // Safe reconfig introduces a new "term" field in the config document. If the user tries
-            // to downgrade the replset to FCV42, the primary will initiate a reconfig without the
-            // term and wait for it to be replicated on all nodes.
-            auto replCoord = repl::ReplicationCoordinator::get(opCtx);
-            const bool isReplSet =
-                replCoord->getReplicationMode() == repl::ReplicationCoordinator::modeReplSet;
-            if (isReplSet &&
-                replCoord->getConfig().getConfigTerm() != repl::OpTime::kUninitializedTerm) {
-                // Force reconfig with term -1 to remove the 4.2 incompatible "term" field.
-                auto getNewConfig = [&](const repl::ReplSetConfig& oldConfig, long long term) {
-                    auto newConfig = oldConfig;
-                    newConfig.setConfigTerm(repl::OpTime::kUninitializedTerm);
-                    newConfig.setConfigVersion(newConfig.getConfigVersion() + 1);
-                    return newConfig;
-                };
-
-                // "force" reconfig in order to skip safety checks. This is safe since the content
-                // of config is the same.
-                LOGV2(4628800, "Downgrading replica set config.");
-                auto status = replCoord->doReplSetReconfig(opCtx, getNewConfig, true /* force */);
-                uassertStatusOKWithContext(status, "Failed to downgrade the replica set config");
-
-                LOGV2(4628801,
-                      "Waiting for the downgraded replica set config to propagate to all nodes");
-                // If a write concern is given, we'll use its wTimeout. It's kNoTimeout by default.
-                WriteConcernOptions writeConcern(repl::ReplSetConfig::kConfigAllWriteConcernName,
-                                                 WriteConcernOptions::SyncMode::NONE,
-                                                 opCtx->getWriteConcern().wTimeout);
-                writeConcern.checkCondition = WriteConcernOptions::CheckCondition::Config;
-                repl::OpTime fakeOpTime(Timestamp(1, 1), replCoord->getTerm());
-                uassertStatusOKWithContext(
-                    replCoord->awaitReplication(opCtx, fakeOpTime, writeConcern).status,
-                    "Failed to wait for the downgraded replica set config to propagate to all "
-                    "nodes");
-                LOGV2(4628802,
-                      "The downgraded replica set config has been propagated to all nodes");
-            }
-
             {
                 // Take the global lock in S mode to create a barrier for operations taking the
                 // global IX or X locks. This ensures that either
                 //   - The global IX/X locked operation will start after the FCV change, see the
-                //     downgrading to 4.2 FCV and act accordingly.
+                //     downgrading to 4.4 FCV and act accordingly.
                 //   - The global IX/X locked operation began prior to the FCV change, is acting on
                 //     that assumption and will finish before downgrade procedures begin right after
                 //     this.
@@ -308,7 +270,7 @@ public:
             if (serverGlobalParams.clusterRole == ClusterRole::ShardServer) {
                 LOGV2(20502, "Downgrade: dropping config.rangeDeletions collection");
                 migrationutil::dropRangeDeletionsCollection(opCtx);
-            } else if (isReplSet || serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
+            } else if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
                 // The default rwc document should only be deleted on plain replica sets and the
                 // config server replica set, not on shards or standalones.
                 deletePersistedDefaultRWConcernDocument(opCtx);
diff --git a/src/mongo/db/repl/repl_set_heartbeat_args_v1.cpp b/src/mongo/db/repl/repl_set_heartbeat_args_v1.cpp
index e63d39611da..d97b1d15759 100644
--- a/src/mongo/db/repl/repl_set_heartbeat_args_v1.cpp
+++ b/src/mongo/db/repl/repl_set_heartbeat_args_v1.cpp
@@ -172,12 +172,7 @@ void ReplSetHeartbeatArgsV1::addToBSON(BSONObjBuilder* builder) const {
         builder->append(kCheckEmptyFieldName, _checkEmpty);
     }
     builder->appendIntOrLL(kConfigVersionFieldName, _configVersion);
-    // The configTerm field is new in 4.4 and cannot be parsed by MongoDB 4.2. Therefore omit it if
-    // we have a 4.2-style replica set config with no "term". This permits us to downgrade by first
-    // removing the replica set config's term, then downgrading to 4.2.
-    if (_configTerm != OpTime::kUninitializedTerm) {
-        builder->appendIntOrLL(kConfigTermFieldName, _configTerm);
-    }
+    builder->appendIntOrLL(kConfigTermFieldName, _configTerm);
     if (_hasHeartbeatVersion) {
         builder->appendIntOrLL(kHeartbeatVersionFieldName, _hasHeartbeatVersion);
     }
diff --git a/src/mongo/db/repl/repl_set_request_votes_args.cpp b/src/mongo/db/repl/repl_set_request_votes_args.cpp
index 0dff3a3e22c..b685bb0ab4e 100644
--- a/src/mongo/db/repl/repl_set_request_votes_args.cpp
+++ b/src/mongo/db/repl/repl_set_request_votes_args.cpp
@@ -83,8 +83,6 @@ Status ReplSetRequestVotesArgs::initialize(const BSONObj& argsObj) {
     if (!status.isOK())
         return status;
 
-    // In order to be compatible with FCV 4.2, default the config term to -1 if we are unable
-    // parse a configTerm field from the args.
     status = bsonExtractIntegerFieldWithDefault(
         argsObj, kConfigTermFieldName, OpTime::kUninitializedTerm, &_cfgTerm);
     if (!status.isOK())
diff --git a/src/mongo/db/repl/replication_coordinator_impl.cpp b/src/mongo/db/repl/replication_coordinator_impl.cpp
index a13520f2b7c..005629813eb 100644
--- a/src/mongo/db/repl/replication_coordinator_impl.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl.cpp
@@ -3278,8 +3278,7 @@ Status ReplicationCoordinatorImpl::doReplSetReconfig(OperationContext* opCtx,
         // Make sure that the latest committed optime from the previous config is committed in the
         // current config. If this is the initial reconfig, then we don't need to check this
         // condition, since there were no prior configs. Also, for force reconfigs we bypass this
-        // safety check condition. In any FCV < 4.4 we also bypass it to preserve client facing
-        // behavior in mixed version sets.
+        // safety check condition.
         auto isInitialReconfig = (_rsConfig.getConfigVersion() == 1);
         // If our config was installed via a "force" reconfig, we bypass the oplog commitment check.
         auto leavingForceConfig = (_rsConfig.getConfigTerm() == OpTime::kUninitializedTerm);
diff --git a/src/mongo/db/repl/topology_coordinator.cpp b/src/mongo/db/repl/topology_coordinator.cpp
index fc8fbdb487d..a4dc8a1dc73 100644
--- a/src/mongo/db/repl/topology_coordinator.cpp
+++ b/src/mongo/db/repl/topology_coordinator.cpp
@@ -1268,28 +1268,6 @@ StatusWith<bool> TopologyCoordinator::setLastOptime(const UpdatePositionArgs::Up
                 "appliedOpTime"_attr = args.appliedOpTime,
                 "durableOpTime"_attr = args.durableOpTime);
 
-    // If we're in FCV 4.4, allow replSetUpdatePosition commands between config versions.
-    if (!serverGlobalParams.featureCompatibility.isVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo44)) {
-        if (args.cfgver != _rsConfig.getConfigVersion()) {
-            static constexpr char errmsg[] =
-                "Received replSetUpdatePosition for node whose config version doesn't match our "
-                "config version";
-            LOGV2_DEBUG(21813,
-                        1,
-                        errmsg,
-                        "memberId"_attr = memberId,
-                        "memberConfigVersion"_attr = args.cfgver,
-                        "ourConfigVersion"_attr = _rsConfig.getConfigVersion());
-            *configVersion = _rsConfig.getConfigVersion();
-            return Status(ErrorCodes::InvalidReplicaSetConfig,
-                          str::stream()
-                              << errmsg << ", memberId: " << memberId
-                              << ", member config version: " << args.cfgver
-                              << ", our config version: " << _rsConfig.getConfigVersion());
-        }
-    }
-
     // While we can accept replSetUpdatePosition commands across config versions, we still do not
     // allow receiving them from a node that is not in our config.
     auto* memberData = _findMemberDataByMemberId(memberId.getData());
@@ -2908,24 +2886,6 @@ bool TopologyCoordinator::shouldChangeSyncSource(const HostAndPort& currentSourc
         return true;
     }
 
-    // If we're in FCV 4.4, allow data replication between config versions. Otherwise, change
-    // our sync source.
-    if (!serverGlobalParams.featureCompatibility.isVersion(
-            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo44)) {
-        if (replMetadata.getConfigVersion() != _rsConfig.getConfigVersion()) {
-            LOGV2(
-                21830,
-                "Choosing new sync source because the config version supplied by {currentSource}, "
-                "{syncSourceConfigVersion}, does not match ours, {configVersion}",
-                "Choosing new sync source because the config version supplied by the current sync "
-                "source does not match ours",
-                "currentSource"_attr = currentSource,
-                "syncSourceConfigVersion"_attr = replMetadata.getConfigVersion(),
-                "configVersion"_attr = _rsConfig.getConfigVersion());
-            return true;
-        }
-    }
-
     // While we can allow data replication across config versions, we still do not allow syncing
     // from a node that is not in our config.
     const int currentSourceIndex = _rsConfig.findMemberIndexByHostAndPort(currentSource);
diff --git a/src/mongo/db/repl/topology_coordinator_v1_test.cpp b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
index 87914e55c43..a77c4f8423c 100644
--- a/src/mongo/db/repl/topology_coordinator_v1_test.cpp
+++ b/src/mongo/db/repl/topology_coordinator_v1_test.cpp
@@ -2830,35 +2830,6 @@ TEST_F(PrepareHeartbeatResponseV1Test,
     ASSERT_EQUALS(HostAndPort("h2"), response.getSyncingTo());
 }
 
-TEST_F(TopoCoordTest, OmitUninitializedConfigTermFromHeartbeat) {
-    // A MongoDB 4.2 replica set config with no term.
-    updateConfig(BSON("_id"
-                      << "rs0"
-                      << "version" << 1 << "members"
-                      << BSON_ARRAY(BSON("_id" << 10 << "host"
-                                               << "hself")
-                                    << BSON("_id" << 20 << "host"
-                                                  << "h2"))),
-                 0);
-
-    // A member with this config omits configTerm from heartbeat requests.
-    auto [request, timeout] =
-        getTopoCoord().prepareHeartbeatRequestV1(Date_t(), "rs0", HostAndPort("h2", 27017));
-    (void)timeout;  // Unused.
-
-    ASSERT_EQUALS(request.getConfigTerm(), -1);
-    ASSERT_EQUALS(request.getConfigVersion(), 1);
-    ASSERT_FALSE(request.toBSON().hasField("configTerm"_sd));
-
-    // A member with this config omits configTerm from heartbeat responses.
-    ReplSetHeartbeatArgsV1 args;
-    args.setSetName("rs0");
-    args.setSenderId(20);
-    ReplSetHeartbeatResponse response;
-    ASSERT_OK(getTopoCoord().prepareHeartbeatResponseV1(now()++, args, "rs0", &response));
-    ASSERT_FALSE(response.toBSON().hasField("configTerm"_sd));
-}
-
 TEST_F(TopoCoordTest, RespondToHeartbeatsWithNullLastAppliedAndLastDurableWhileInInitialSync) {
     ASSERT_TRUE(TopologyCoordinator::Role::kFollower == getTopoCoord().getRole());
     ASSERT_EQUALS(MemberState::RS_STARTUP, getTopoCoord().getMemberState().s);