SERVER-46579 Permit multinode reconfigs in FCV 4.2

2 files changed, 20 insertions, 1 deletions

diff --git a/src/mongo/db/repl/repl_set_config_checks.cpp b/src/mongo/db/repl/repl_set_config_checks.cpp
index 9c7deaad980..bf72b9cca0c 100644
--- a/src/mongo/db/repl/repl_set_config_checks.cpp
+++ b/src/mongo/db/repl/repl_set_config_checks.cpp
@@ -361,7 +361,9 @@ StatusWith<int> validateConfigForReconfig(ReplicationCoordinatorExternalState* e
 
     // For non-force reconfigs, verify that the reconfig only adds or removes a single node. This
     // ensures that all quorums of the new config overlap with all quorums of the old config.
-    if (!force) {
+    if (!force &&
+        serverGlobalParams.featureCompatibility.getVersion() ==
+            ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo44) {
         status = validateSingleNodeChange(oldConfig, newConfig);
         if (!status.isOK()) {
             return StatusWith<int>(status);
diff --git a/src/mongo/db/repl/repl_set_config_checks_test.cpp b/src/mongo/db/repl/repl_set_config_checks_test.cpp
index 22c17222c11..63c33f7c1de 100644
--- a/src/mongo/db/repl/repl_set_config_checks_test.cpp
+++ b/src/mongo/db/repl/repl_set_config_checks_test.cpp
@@ -38,12 +38,15 @@
 #include "mongo/db/server_options.h"
 #include "mongo/db/service_context.h"
 #include "mongo/db/service_context_test_fixture.h"
+#include "mongo/unittest/ensure_fcv.h"
 #include "mongo/unittest/unittest.h"
 
 namespace mongo {
 namespace repl {
 namespace {
 
+using unittest::EnsureFCV;
+
 TEST_F(ServiceContextTest, ValidateConfigForInitiate_VersionMustBe1) {
     ReplicationCoordinatorExternalStateMock rses;
     rses.addSelf(HostAndPort("h1"));
@@ -1100,6 +1103,20 @@ TEST_F(ServiceContextTest, ValidateForReconfig_MultiNodeRemovalDisallowed) {
                   validateMemberReconfig(oldMembers, newMembers, m1));
 }
 
+TEST_F(ServiceContextTest, ValidateForReconfig_MultiNodeAdditionAllowedFCV42) {
+    EnsureFCV fcv{ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo42};
+    BSONArray oldMembers = BSON_ARRAY(m1 << m2);
+    BSONArray newMembers = BSON_ARRAY(m1 << m2 << m3 << m4);  // add 2 voting nodes.
+    ASSERT_OK(validateMemberReconfig(oldMembers, newMembers, m1));
+}
+
+TEST_F(ServiceContextTest, ValidateForReconfig_MultiNodeRemovalAllowedFCV42) {
+    EnsureFCV fcv{ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo42};
+    BSONArray oldMembers = BSON_ARRAY(m1 << m2 << m3 << m4);
+    BSONArray newMembers = BSON_ARRAY(m1 << m2);  // remove 2 voting nodes.
+    ASSERT_OK(validateMemberReconfig(oldMembers, newMembers, m1));
+}
+
 TEST_F(ServiceContextTest, ValidateForReconfig_MultiNodeAdditionOfNonVotingNodesAllowed) {
     BSONArray oldMembers = BSON_ARRAY(m1 << m2);
     BSONArray newMembers =