SERVER-31634 FeatureCompatibilityVersion::setIfCleanStartup() should store FCV=3.4 for shardsvrs

3 files changed, 47 insertions, 41 deletions

diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index d88cbdf46fc..cef9f6d123a 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -202,49 +202,59 @@ void FeatureCompatibilityVersion::unsetTargetUpgradeOrDowngrade(OperationContext
 
 void FeatureCompatibilityVersion::setIfCleanStartup(OperationContext* opCtx,
                                                     repl::StorageInterface* storageInterface) {
-    if (serverGlobalParams.clusterRole != ClusterRole::ShardServer) {
-        std::vector<std::string> dbNames;
-        StorageEngine* storageEngine = getGlobalServiceContext()->getGlobalStorageEngine();
-        storageEngine->listDatabases(&dbNames);
-
-        for (auto&& dbName : dbNames) {
-            if (dbName != "local") {
-                return;
-            }
+    // A clean startup means there are no databases on disk besides the local database.
+    std::vector<std::string> dbNames;
+    StorageEngine* storageEngine = getGlobalServiceContext()->getGlobalStorageEngine();
+    storageEngine->listDatabases(&dbNames);
+
+    for (auto&& dbName : dbNames) {
+        if (dbName != "local") {
+            return;
         }
+    }
 
-        UnreplicatedWritesBlock unreplicatedWritesBlock(opCtx);
-        NamespaceString nss(FeatureCompatibilityVersion::kCollection);
+    // If the server was not started with --shardsvr, the default featureCompatibilityVersion on
+    // clean startup is the upgrade version. If it was started with --shardsvr, the default
+    // featureCompatibilityVersion is the downgrade version, so that it can be safely added to a
+    // downgrade version cluster. The config server will run setFeatureCompatibilityVersion as part
+    // of addShard.
+    const bool storeUpgradeVersion = serverGlobalParams.clusterRole != ClusterRole::ShardServer;
 
-        {
-            AutoGetOrCreateDb autoDB(opCtx, nss.db(), MODE_X);
+    UnreplicatedWritesBlock unreplicatedWritesBlock(opCtx);
+    NamespaceString nss(FeatureCompatibilityVersion::kCollection);
 
-            // We reached this point because the only database that exists on the server is "local"
-            // and we have just created an empty "admin" database. Therefore, it is safe to create
-            // the "admin.system.version" collection.
-            invariant(autoDB.justCreated());
+    {
+        AutoGetOrCreateDb autoDB(opCtx, nss.db(), MODE_X);
 
+        // We reached this point because the only database that exists on the server is "local"
+        // and we have just created an empty "admin" database. Therefore, it is safe to create
+        // the "admin.system.version" collection.
+        invariant(autoDB.justCreated());
+
+        if (storeUpgradeVersion) {
             // We update the value of the version server parameter so that the admin.system.version
             // collection gets a UUID.
             serverGlobalParams.featureCompatibility.setVersion(
                 ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo36);
-
-            uassertStatusOK(storageInterface->createCollection(opCtx, nss, {}));
         }
 
-        // We then insert the featureCompatibilityVersion document into the "admin.system.version"
-        // collection. The server parameter will be updated on commit by the op observer.
-        uassertStatusOK(storageInterface->insertDocument(
-            opCtx,
-            nss,
-            repl::TimestampedBSONObj{BSON("_id"
-                                          << FeatureCompatibilityVersion::kParameterName
-                                          << FeatureCompatibilityVersion::kVersionField
-                                          << FeatureCompatibilityVersionCommandParser::kVersion36),
-                                     SnapshotName()},
-            repl::OpTime::kUninitializedTerm));  // No timestamp or term because this write is not
-                                                 // replicated.
+        uassertStatusOK(storageInterface->createCollection(opCtx, nss, {}));
     }
+
+    // We then insert the featureCompatibilityVersion document into the "admin.system.version"
+    // collection. The server parameter will be updated on commit by the op observer.
+    uassertStatusOK(storageInterface->insertDocument(
+        opCtx,
+        nss,
+        repl::TimestampedBSONObj{
+            BSON("_id" << FeatureCompatibilityVersion::kParameterName
+                       << FeatureCompatibilityVersion::kVersionField
+                       << (storeUpgradeVersion
+                               ? FeatureCompatibilityVersionCommandParser::kVersion36
+                               : FeatureCompatibilityVersionCommandParser::kVersion34)),
+            SnapshotName()},
+        repl::OpTime::kUninitializedTerm));  // No timestamp or term because this write is not
+                                             // replicated.
 }
 
 namespace {
diff --git a/src/mongo/db/commands/feature_compatibility_version.h b/src/mongo/db/commands/feature_compatibility_version.h
index d1fa7fc742c..dd7db37d5b5 100644
--- a/src/mongo/db/commands/feature_compatibility_version.h
+++ b/src/mongo/db/commands/feature_compatibility_version.h
@@ -118,8 +118,9 @@ public:
     static void unsetTargetUpgradeOrDowngrade(OperationContext* opCtx, StringData version);
 
     /**
-     * If there are no non-local databases and we are not running with --shardsvr, set
-     * featureCompatibilityVersion to the latest value.
+     * If there are no non-local databases, store the featureCompatibilityVersion document. If we
+     * are not running with --shardsvr, set the version to be the upgrade value. If we are running
+     * with --shardsvr, set the version to be the downgrade value.
      */
     static void setIfCleanStartup(OperationContext* opCtx,
                                   repl::StorageInterface* storageInterface);
diff --git a/src/mongo/db/db.cpp b/src/mongo/db/db.cpp
index af021dcf0ed..420428087ac 100644
--- a/src/mongo/db/db.cpp
+++ b/src/mongo/db/db.cpp
@@ -787,15 +787,10 @@ ExitCode _initAndListen(int listenPort) {
     bool nonLocalDatabases = repairDatabasesAndCheckVersion(startupOpCtx.get());
 
     // Assert that the in-memory featureCompatibilityVersion parameter has been explicitly set. If
-    // we are part of a sharded cluster and are started up with no data files, we do not set the
-    // featureCompatibilityVersion until addShard is called. If we are part of a non-shard replica
-    // set and are also started up with no data files, we do not set the featureCompatibilityVersion
-    // until a primary is chosen. For these cases, we expect the in-memory
+    // we are part of a replica set and are started up with no data files, we do not set the
+    // featureCompatibilityVersion until a primary is chosen. For this case, we expect the in-memory
     // featureCompatibilityVersion parameter to still be uninitialized until after startup.
-    if (canCallFCVSetIfCleanStartup &&
-        ((!replSettings.usingReplSets() &&
-          serverGlobalParams.clusterRole != ClusterRole::ShardServer) ||
-         nonLocalDatabases)) {
+    if (canCallFCVSetIfCleanStartup && (!replSettings.usingReplSets() || nonLocalDatabases)) {
         invariant(serverGlobalParams.featureCompatibility.isVersionInitialized());
     }