SERVER-31315 expose the four distinct upgrade/downgrade states in serverGlobalParams.featureCompatibility

Author: EshaMaharishi

src/mongo/db/commands/feature_compatibility_version.cpp

@@ -249,7 +249,14 @@ void FeatureCompatibilityVersion::onInsertOrUpdate(OperationContext* opCtx, cons
     opCtx->recoveryUnit()->onCommit([opCtx, versionInfo]() {
         serverGlobalParams.featureCompatibility.version.store(versionInfo.version);
         serverGlobalParams.featureCompatibility.targetVersion.store(versionInfo.targetVersion);
-        _closeConnectionsBelowVersion(opCtx, versionInfo);
+
+        // Close all connections from internal clients with binary versions lower than 3.6.
+        if (versionInfo.version == ServerGlobalParams::FeatureCompatibility::Version::k36 ||
+            versionInfo.targetVersion == ServerGlobalParams::FeatureCompatibility::Version::k36) {
+            opCtx->getServiceContext()->getServiceEntryPoint()->endAllSessions(
+                transport::Session::kLatestVersionInternalClientKeepOpen |
+                transport::Session::kExternalClientKeepOpen);
+        }
     });
 }
 
@@ -293,18 +300,6 @@ void FeatureCompatibilityVersion::_validateVersion(StringData version) {
                 version == FeatureCompatibilityVersionCommandParser::kVersion34);
 }
 
-void FeatureCompatibilityVersion::_closeConnectionsBelowVersion(
-    OperationContext* opCtx, FeatureCompatibilityVersionInfo versionInfo) {
-
-    // Close all internal connections to versions lower than 3.6.
-    if (versionInfo.version == ServerGlobalParams::FeatureCompatibility::Version::k36 ||
-        versionInfo.targetVersion == ServerGlobalParams::FeatureCompatibility::Version::k36) {
-        opCtx->getServiceContext()->getServiceEntryPoint()->endAllSessions(
-            transport::Session::kLatestVersionInternalClientKeepOpen |
-            transport::Session::kExternalClientKeepOpen);
-    }
-}
-
 void FeatureCompatibilityVersion::_runUpdateCommand(OperationContext* opCtx,
                                                     StringData version,
                                                     UpdateBuilder builder) {

src/mongo/db/commands/feature_compatibility_version.h

@@ -147,13 +147,6 @@ class FeatureCompatibilityVersion {
      */
     static void _validateVersion(StringData version);
 
-    /**
-     * Close incoming connections from interal clients who cannot speak our highest wire protocol
-     * version.
-     */
-    static void _closeConnectionsBelowVersion(OperationContext* opCtx,
-                                              FeatureCompatibilityVersionInfo versionInfo);
-
     /**
      * Build update command.
      */

src/mongo/db/server_options.h

@@ -146,33 +146,51 @@ struct ServerGlobalParams {
     BSONObj overrideShardIdentity;
 
     struct FeatureCompatibility {
-        enum class Version {
-            /**
-             * In this mode, the cluster will expose a 3.4-like API. Attempts by a client to use new
-             * features in 3.6 will be rejected.
-             */
-            k34,
-
-            /**
-             * In this mode, new features in 3.6 are allowed. The system should guarantee that no
-             * 3.4 node can participate in a cluster whose feature compatibility version is 3.6.
-             */
-            k36,
-            /**
-             * This is only used for targetVersion to indicate that no upgrade is in progress.
-             */
-            kUnset
-        };
+        /**
+         * The combination of the version and targetVersion determine this node's behavior.
+         *
+         * The legal (version, targetVersion) states are:
+         *
+         * (3.4, Unset) aka fully 3.4: only 3.4 features are available, and new and existing storage
+         *                             engine entries use the 3.4 format
+         *
+         * (3.4, 3.6) aka upgrading: only 3.4 features are available, but new storage engine entries
+         *                           use the 3.6 format, and existing entries may have either the
+         *                           3.4 or 3.6 format
+         *
+         * (3.6, Unset) aka fully 3.6: 3.6 features are available, and new and existing storage
+         *                             engine entries use the 3.6 format
+         *
+         * (3.4, 3.4) aka downgrading: only 3.4 features are available and new storage engine
+         *                             entries use the 3.4 format, but existing entries may have
+         *                             either the 3.4 or 3.6 format
+         */
+        enum class Version { k34, k36, kUnset };
 
-        // Read-only parameter featureCompatibilityVersion.
         AtomicWord<Version> version{Version::k34};
 
         // If set, an upgrade or downgrade is in progress to the set version.
         AtomicWord<Version> targetVersion{Version::kUnset};
 
+        const bool isFullyUpgradedTo36() {
+            return (version.load() == Version::k36 && targetVersion.load() == Version::kUnset);
+        }
+
+        const bool isUpgradingTo36() {
+            return (version.load() == Version::k34 && targetVersion.load() == Version::k36);
+        }
+
+        const bool isFullyDowngradedTo34() {
+            return (version.load() == Version::k34 && targetVersion.load() == Version::kUnset);
+        }
+
+        const bool isDowngradingTo34() {
+            return (version.load() == Version::k34 && targetVersion.load() == Version::k34);
+        }
+
         // This determines whether to give Collections UUIDs upon creation.
-        bool isSchemaVersion36() {
-            return (version.load() == Version::k36 || targetVersion.load() == Version::k36);
+        const bool isSchemaVersion36() {
+            return (isFullyUpgradedTo36() || isUpgradingTo36());
         }
 
         // Feature validation differs depending on the role of a mongod in a replica set or