SERVER-32519 clean up WireVersion enum uses in the code to use LATEST_WIRE_VERSION

6 files changed, 166 insertions, 99 deletions

diff --git a/src/mongo/client/scoped_db_conn_test.cpp b/src/mongo/client/scoped_db_conn_test.cpp
index c117b73a0c7..13a9bb0cf7f 100644
--- a/src/mongo/client/scoped_db_conn_test.cpp
+++ b/src/mongo/client/scoped_db_conn_test.cpp
@@ -132,7 +132,7 @@ private:
 
         // We need to handle the isMaster received during connection.
         if (request.getCommandName() == "isMaster") {
-            commandResponse.append("maxWireVersion", WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+            commandResponse.append("maxWireVersion", WireVersion::LATEST_WIRE_VERSION);
             commandResponse.append("minWireVersion", WireVersion::RELEASE_2_4_AND_BEFORE);
         }
 
diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index 4e4867ed49d..589620d1acb 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -409,13 +409,8 @@ void FeatureCompatibilityVersion::updateMinWireVersion() {
             spec.outgoing.minWireVersion = LATEST_WIRE_VERSION;
             return;
         case ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34:
-            // It would be preferable to set 'incomingInternalClient.minWireVersion' and
-            // 'outgoing.minWireVersion' to LATEST_WIRE_VERSION - 1, but this is not possible due to
-            // a bug in 3.4, where if the receiving node says it supports wire version range
-            // [COMMANDS_ACCEPT_WRITE_CONCERN, SUPPORTS_OP_MSG], the initiating node will think it
-            // only supports OP_QUERY.
-            spec.incomingInternalClient.minWireVersion = RELEASE_2_4_AND_BEFORE;
-            spec.outgoing.minWireVersion = RELEASE_2_4_AND_BEFORE;
+            spec.incomingInternalClient.minWireVersion = LATEST_WIRE_VERSION - 1;
+            spec.outgoing.minWireVersion = LATEST_WIRE_VERSION - 1;
             return;
         case ServerGlobalParams::FeatureCompatibility::Version::kUnsetDefault34Behavior:
             // getVersion() does not return this value.
diff --git a/src/mongo/rpc/protocol_test.cpp b/src/mongo/rpc/protocol_test.cpp
index 2c760cdc8e8..f3f8ba51f97 100644
--- a/src/mongo/rpc/protocol_test.cpp
+++ b/src/mongo/rpc/protocol_test.cpp
@@ -127,56 +127,148 @@ TEST(Protocol, parseProtocolSetFromIsMasterReply) {
     } while (0);
 
 TEST(Protocol, validateWireVersion) {
-    // Base Test
+    /*
+     * Test communication with a MongoD latest binary server with downgraded FCV.
+     */
+
+    // MongoD 'latest' client -> MongoD 'latest' server
+    VALIDATE_WIRE_VERSION(ASSERT_OK,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION);
+
+    // MongoD 'latest' client -> MongoD downgraded 'last-stable' server
     VALIDATE_WIRE_VERSION(ASSERT_OK,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 2,
+                          WireVersion::LATEST_WIRE_VERSION - 1);
 
-    // Allowed during upgrade
-    // MongoD 3.4 client -> MongoD 3.4 server
+    // MongoD 'latest' client -> MongoD upgraded 'last-stable' server
     VALIDATE_WIRE_VERSION(ASSERT_OK,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1);
 
-    // MongoD 3.4 client -> MongoD 3.2 server
+    // MongoD downgraded 'last-stable' client -> MongoD 'latest' server
     VALIDATE_WIRE_VERSION(ASSERT_OK,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::FIND_COMMAND);
+                          WireVersion::LATEST_WIRE_VERSION - 2,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION);
 
-    // MongoD 3.2 client -> MongoD 3.4 server
+    // MongoD upgraded 'last-stable' client -> MongoD 'latest' server
     VALIDATE_WIRE_VERSION(ASSERT_OK,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION);
 
-    // MongoS 3.4 client -> MongoD 3.4 server
+    // MongoS 'latest' client -> MongoD 'latest' server
     VALIDATE_WIRE_VERSION(ASSERT_OK,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION);
 
-    // MongoS 3.2 client -> MongoD 3.4 server
+    // MongoS 'last-stable' client -> MongoD 'latest' server
     VALIDATE_WIRE_VERSION(ASSERT_OK,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::FIND_COMMAND,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION);
+    /*
+     * Test communication with a MongoD latest binary server with upgraded FCV.
+     */
+
+    // MongoD 'latest' client -> MongoD 'latest' server
+    VALIDATE_WIRE_VERSION(ASSERT_OK,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION);
+
+    // MongoD 'latest' client -> MongoD downgraded 'last-stable' server
+    VALIDATE_WIRE_VERSION(ASSERT_NOT_OK,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 2,
+                          WireVersion::LATEST_WIRE_VERSION - 1);
+
+    // MongoD 'latest' client -> MongoD upgraded 'last-stable' server
+    VALIDATE_WIRE_VERSION(ASSERT_NOT_OK,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1);
+
+    // MongoD downgraded 'last-stable' client -> MongoD 'latest' server
+    VALIDATE_WIRE_VERSION(ASSERT_NOT_OK,
+                          WireVersion::LATEST_WIRE_VERSION - 2,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION);
+
+    // MongoD upgraded 'last-stable' client -> MongoD 'latest' server
+    VALIDATE_WIRE_VERSION(ASSERT_NOT_OK,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION);
+
+    // MongoS 'latest' client -> MongoD 'latest' server
+    VALIDATE_WIRE_VERSION(ASSERT_OK,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION);
+
+    // MongoS 'last-stable' client -> MongoD 'latest' server
+    VALIDATE_WIRE_VERSION(ASSERT_NOT_OK,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION);
+
+    /*
+     * Test communication between MongoD latest binary servers where one has upgraded FCV and the
+     * other downgraded FCV.
+     */
+
+    // MongoD upgraded 'latest' client -> MongoD downgraded 'latest' server
+    VALIDATE_WIRE_VERSION(ASSERT_OK,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION);
+
+    // MongoD downgraded 'latest' client -> MongoD upgraded 'latest' server
+    VALIDATE_WIRE_VERSION(ASSERT_OK,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION);
+
+    /*
+     * Test that it is disallowed for MongoS to communicate with a lower binary server, regardless
+     * of FCV.
+     */
+
+    // MongoS 'latest' -> MongoD downgraded 'last-stable' server
+    VALIDATE_WIRE_VERSION(ASSERT_NOT_OK,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 2,
+                          WireVersion::LATEST_WIRE_VERSION - 1);
 
-    // Disallowed
-    // MongoS 3.4 -> MongoDB 3.2 server
+    // MongoS 'latest' -> MongoD upgraded 'last-stable' server
     VALIDATE_WIRE_VERSION(ASSERT_NOT_OK,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN,
-                          WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN,
-                          WireVersion::RELEASE_2_4_AND_BEFORE,
-                          WireVersion::FIND_COMMAND);
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION,
+                          WireVersion::LATEST_WIRE_VERSION - 1,
+                          WireVersion::LATEST_WIRE_VERSION - 1);
 }
 
 }  // namespace
diff --git a/src/mongo/s/catalog/sharding_catalog_add_shard_test.cpp b/src/mongo/s/catalog/sharding_catalog_add_shard_test.cpp
index 5ce39eba1bf..e3695912e20 100644
--- a/src/mongo/s/catalog/sharding_catalog_add_shard_test.cpp
+++ b/src/mongo/s/catalog/sharding_catalog_add_shard_test.cpp
@@ -140,7 +140,7 @@ protected:
             ASSERT_EQ(request.dbname, "admin");
             ASSERT_BSONOBJ_EQ(request.cmdObj,
                               BSON("setFeatureCompatibilityVersion"
-                                   << "3.4"));
+                                   << "3.6"));
 
             return response;
         });
@@ -401,7 +401,7 @@ TEST_F(AddShardTest, StandaloneBasicSuccess) {
     });
 
     BSONObj commandResponse = BSON("ok" << 1 << "ismaster" << true << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     // Get databases list from new shard
@@ -486,7 +486,7 @@ TEST_F(AddShardTest, StandaloneGenerateName) {
     });
 
     BSONObj commandResponse = BSON("ok" << 1 << "ismaster" << true << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     // Get databases list from new shard
@@ -610,34 +610,6 @@ TEST_F(AddShardTest, AddMongosAsShard) {
     future.timed_get(kLongFutureTimeout);
 }
 
-// Attempt to add a pre-v3.4 mongod.
-TEST_F(AddShardTest, AddVersion32Shard) {
-    std::unique_ptr<RemoteCommandTargeterMock> targeter(
-        stdx::make_unique<RemoteCommandTargeterMock>());
-    HostAndPort shardTarget("StandaloneHost:12345");
-    targeter->setConnectionStringReturnValue(ConnectionString(shardTarget));
-    targeter->setFindHostReturnValue(shardTarget);
-
-    targeterFactory()->addTargeterToReturn(ConnectionString(shardTarget), std::move(targeter));
-    std::string expectedShardName = "StandaloneShard";
-
-    auto future = launchAsync([this, &expectedShardName, &shardTarget] {
-        Client::initThreadIfNotAlready();
-        auto status =
-            ShardingCatalogManager::get(operationContext())
-                ->addShard(
-                    operationContext(), &expectedShardName, ConnectionString(shardTarget), 100);
-        ASSERT_EQUALS(ErrorCodes::IncompatibleServerVersion, status);
-    });
-
-    // The maxWireVersion indicates that this is a v3.2 shard.
-    BSONObj commandResponse =
-        BSON("ok" << 1 << "ismaster" << true << "maxWireVersion" << WireVersion::FIND_COMMAND);
-    expectIsMaster(shardTarget, commandResponse);
-
-    future.timed_get(kLongFutureTimeout);
-}
-
 // A replica set name was found for the host but no name was provided with the host.
 TEST_F(AddShardTest, AddReplicaSetShardAsStandalone) {
     std::unique_ptr<RemoteCommandTargeterMock> targeter(
@@ -662,7 +634,7 @@ TEST_F(AddShardTest, AddReplicaSetShardAsStandalone) {
     BSONObj commandResponse = BSON("ok" << 1 << "ismaster" << true << "setName"
                                         << "myOtherSet"
                                         << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     future.timed_get(kLongFutureTimeout);
@@ -690,7 +662,7 @@ TEST_F(AddShardTest, AddStandaloneHostShardAsReplicaSet) {
     });
 
     BSONObj commandResponse = BSON("ok" << 1 << "ismaster" << true << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     future.timed_get(kLongFutureTimeout);
@@ -720,7 +692,7 @@ TEST_F(AddShardTest, ReplicaSetMistmatchedReplicaSetName) {
     BSONObj commandResponse = BSON("ok" << 1 << "ismaster" << true << "setName"
                                         << "myOtherSet"
                                         << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     future.timed_get(kLongFutureTimeout);
@@ -753,7 +725,7 @@ TEST_F(AddShardTest, ShardIsCSRSConfigServer) {
                                         << "configsvr"
                                         << true
                                         << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     future.timed_get(kLongFutureTimeout);
@@ -788,7 +760,7 @@ TEST_F(AddShardTest, ReplicaSetMissingHostsProvidedInSeedList) {
                                         << "hosts"
                                         << hosts.arr()
                                         << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     future.timed_get(kLongFutureTimeout);
@@ -824,7 +796,7 @@ TEST_F(AddShardTest, AddShardWithNameConfigFails) {
                                         << "hosts"
                                         << hosts.arr()
                                         << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     future.timed_get(kLongFutureTimeout);
@@ -873,7 +845,7 @@ TEST_F(AddShardTest, ShardContainsExistingDatabase) {
                                         << "hosts"
                                         << hosts.arr()
                                         << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     expectListDatabases(shardTarget, {BSON("name" << existingDB.getName())});
@@ -920,7 +892,7 @@ TEST_F(AddShardTest, SuccessfullyAddReplicaSet) {
                                         << "hosts"
                                         << hosts.arr()
                                         << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     // Get databases list from new shard
@@ -987,7 +959,7 @@ TEST_F(AddShardTest, ReplicaSetExtraHostsDiscovered) {
                                         << "hosts"
                                         << hosts.arr()
                                         << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     // Get databases list from new shard
@@ -1063,7 +1035,7 @@ TEST_F(AddShardTest, AddShardSucceedsEvenIfAddingDBsFromNewShardFails) {
     });
 
     BSONObj commandResponse = BSON("ok" << 1 << "ismaster" << true << "maxWireVersion"
-                                        << WireVersion::COMMANDS_ACCEPT_WRITE_CONCERN);
+                                        << WireVersion::LATEST_WIRE_VERSION);
     expectIsMaster(shardTarget, commandResponse);
 
     // Get databases list from new shard
diff --git a/src/mongo/s/catalog/sharding_catalog_manager_shard_operations.cpp b/src/mongo/s/catalog/sharding_catalog_manager_shard_operations.cpp
index bb208c19063..ac28fb3b82c 100644
--- a/src/mongo/s/catalog/sharding_catalog_manager_shard_operations.cpp
+++ b/src/mongo/s/catalog/sharding_catalog_manager_shard_operations.cpp
@@ -306,7 +306,12 @@ StatusWith<ShardType> ShardingCatalogManager::_validateHostAsShard(
     const ConnectionString& connectionString) {
     auto swCommandResponse =
         _runCommandForAddShard(opCtx, targeter.get(), "admin", BSON("isMaster" << 1));
-    if (!swCommandResponse.isOK()) {
+    if (swCommandResponse.getStatus() == ErrorCodes::IncompatibleServerVersion) {
+        return {swCommandResponse.getStatus().code(),
+                str::stream() << "Cannot add " << connectionString.toString()
+                              << " as a shard because its binary version is not compatible with "
+                                 "the cluster's featureCompatibilityVersion."};
+    } else if (!swCommandResponse.isOK()) {
         return swCommandResponse.getStatus();
     }
 
@@ -328,8 +333,11 @@ StatusWith<ShardType> ShardingCatalogManager::_validateHostAsShard(
         return {ErrorCodes::IllegalOperation, "cannot add a mongos as a shard"};
     }
 
-    // Fail if the node being added's binary version is lower than the cluster's
-    // featureCompatibilityVersion.
+    // Extract the maxWireVersion so we can verify that the node being added has a binary version
+    // greater than or equal to the cluster's featureCompatibilityVersion. We expect an incompatible
+    // binary node to be unable to communicate, returning an IncompatibleServerVersion error,
+    // because of our internal wire version protocol. So we can safely invariant here that the node
+    // is compatible.
     long long maxWireVersion;
     Status status = bsonExtractIntegerField(resIsMaster, "maxWireVersion", &maxWireVersion);
     if (!status.isOK()) {
@@ -340,16 +348,11 @@ StatusWith<ShardType> ShardingCatalogManager::_validateHostAsShard(
                                     << " as a shard: "
                                     << status.reason());
     }
-    if (((serverGlobalParams.featureCompatibility.getVersion() ==
-          ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36) &&
-         maxWireVersion < WireVersion::LATEST_WIRE_VERSION) ||
-        ((serverGlobalParams.featureCompatibility.getVersion() !=
-          ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36) &&
-         maxWireVersion < WireVersion::LATEST_WIRE_VERSION - 1)) {
-        return {ErrorCodes::IncompatibleServerVersion,
-                str::stream() << "Cannot add " << connectionString.toString()
-                              << " as a shard because its binary version is not compatible with "
-                                 "the cluster's featureCompatibilityVersion."};
+    if (serverGlobalParams.featureCompatibility.getVersion() ==
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo34) {
+        invariant(maxWireVersion >= WireVersion::LATEST_WIRE_VERSION - 1);
+    } else {
+        invariant(maxWireVersion >= WireVersion::LATEST_WIRE_VERSION);
     }
 
     // Check whether there is a master. If there isn't, the replica set may not have been
diff --git a/src/mongo/s/sharding_mongod_test_fixture.cpp b/src/mongo/s/sharding_mongod_test_fixture.cpp
index a211fa3e4f9..df8dcd88f48 100644
--- a/src/mongo/s/sharding_mongod_test_fixture.cpp
+++ b/src/mongo/s/sharding_mongod_test_fixture.cpp
@@ -145,6 +145,11 @@ void ShardingMongodTestFixture::setUp() {
     service->setOpObserver(stdx::make_unique<OpObserverImpl>());
     repl::setOplogCollectionName();
     repl::createOplog(_opCtx.get());
+
+    // Set the highest FCV because otherwise it defaults to the lower FCV. This way we default to
+    // testing this release's code, not backwards compatibility code.
+    serverGlobalParams.featureCompatibility.setVersion(
+        ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo36);
 }
 
 std::unique_ptr<ReplicationCoordinatorMock> ShardingMongodTestFixture::makeReplicationCoordinator(