SERVER-30242 SERVER-31513 SERVER-31235 Make method to determine if fCV has been initialized in memory. Make fCV state one atomic variable. Initialize fCV on arbiters once a primary is elected.

1 files changed, 36 insertions, 24 deletions

diff --git a/src/mongo/db/server_options.h b/src/mongo/db/server_options.h
index 12d8018c180..8e77dc8fd41 100644
--- a/src/mongo/db/server_options.h
+++ b/src/mongo/db/server_options.h
@@ -147,61 +147,76 @@ struct ServerGlobalParams {
 
     struct FeatureCompatibility {
         /**
-         * The combination of the version and targetVersion determine this node's behavior.
+         * The combination of the fields in the admin.system.version document in the format
+         * (version, targetVersion) are represented by this enum and determine this node's behavior.
          *
-         * The legal (version, targetVersion) states are:
+         * The legal enum (and featureCompatiblityVersion document) states are:
          *
-         * (3.4, Unset) aka fully 3.4: only 3.4 features are available, and new and existing storage
+         * k34
+         * (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
+         * kUpgradingTo36
+         * (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
          *
+         * k36
          * (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
+         * kDowngradingTo34
+         * (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
+         *
+         * kUnset
+         * (Unset, Unset) aka uninitialized: This is the case on startup before the fCV document is
+         *                                   loaded into memory. isVersionInitialized() will return
+         *                                   false, and getVersion() will return the default (k34).
+         *
          */
-        enum class Version { k34, k36, kUnset };
+        enum class Version { k34, kUpgradingTo36, k36, kDowngradingTo34, kUnset };
 
+        /**
+         * On startup, the featureCompatibilityVersion may not have been explicitly set yet. This
+         * exposes the actual state of the featureCompatibilityVersion if it is uninitialized.
+         */
+        const bool isVersionInitialized() const {
+            return _version.load() != Version::kUnset;
+        }
+
+        /**
+         * This safe getter for the featureCompatibilityVersion returns a default value when the
+         * version has not yet been set.
+         */
         const Version getVersion() const {
-            return _version.load();
+            Version v = _version.load();
+            return (v == Version::kUnset) ? Version::k34 : v;
         }
 
         void reset() {
             _version.store(Version::k34);
-            _targetVersion.store(Version::kUnset);
         }
 
         void setVersion(Version version) {
             return _version.store(version);
         }
 
-        const Version getTargetVersion() const {
-            return _targetVersion.load();
-        }
-
-        void setTargetVersion(Version version) {
-            return _targetVersion.store(version);
-        }
-
         const bool isFullyUpgradedTo36() {
-            return (_version.load() == Version::k36 && _targetVersion.load() == Version::kUnset);
+            return (getVersion() == Version::k36);
         }
 
         const bool isUpgradingTo36() {
-            return (_version.load() == Version::k34 && _targetVersion.load() == Version::k36);
+            return (getVersion() == Version::kUpgradingTo36);
         }
 
         const bool isFullyDowngradedTo34() {
-            return (_version.load() == Version::k34 && _targetVersion.load() == Version::kUnset);
+            return (getVersion() == Version::k34);
         }
 
         const bool isDowngradingTo34() {
-            return (_version.load() == Version::k34 && _targetVersion.load() == Version::k34);
+            return (getVersion() == Version::kDowngradingTo34);
         }
 
         // This determines whether to give Collections UUIDs upon creation.
@@ -217,10 +232,7 @@ struct ServerGlobalParams {
         AtomicWord<bool> validateFeaturesAsMaster{true};
 
     private:
-        AtomicWord<Version> _version{Version::k34};
-
-        // If set, an upgrade or downgrade is in progress to the set version.
-        AtomicWord<Version> _targetVersion{Version::kUnset};
+        AtomicWord<Version> _version{Version::kUnset};
 
     } featureCompatibility;