Revert "SERVER-31209 Persist targetVersion in FCV document to indicate an upgrade/downgrade in progress."

This reverts commit b0b99866781302ba8b16de033ff2681f20483c14.

9 files changed, 123 insertions, 260 deletions

diff --git a/src/mongo/db/catalog/database_impl.cpp b/src/mongo/db/catalog/database_impl.cpp
index 2891b7977bc..5f6f7d61748 100644
--- a/src/mongo/db/catalog/database_impl.cpp
+++ b/src/mongo/db/catalog/database_impl.cpp
@@ -763,7 +763,7 @@ Collection* DatabaseImpl::createCollection(OperationContext* opCtx,
 
     CollectionOptions optionsWithUUID = options;
     if (enableCollectionUUIDs && !optionsWithUUID.uuid &&
-        serverGlobalParams.featureCompatibility.isSchemaVersion36()) {
+        serverGlobalParams.featureCompatibility.isSchemaVersion36.load() == true) {
         optionsWithUUID.uuid.emplace(CollectionUUID::gen());
     }
 
diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index c35d1eb993f..f6515bc0cd1 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -55,21 +55,51 @@ constexpr StringData FeatureCompatibilityVersion::kCommandName;
 constexpr StringData FeatureCompatibilityVersion::kDatabase;
 constexpr StringData FeatureCompatibilityVersion::kParameterName;
 constexpr StringData FeatureCompatibilityVersion::kVersionField;
-constexpr StringData FeatureCompatibilityVersion::kTargetVersionField;
 
-StatusWith<FeatureCompatibilityVersionInfo> FeatureCompatibilityVersion::parse(
-    const BSONObj& featureCompatibilityVersionInfo) {
-    FeatureCompatibilityVersionInfo versionInfo;
+namespace {
+BSONObj makeUpdateCommand(StringData newVersion, BSONObj writeConcern) {
+    BSONObjBuilder updateCmd;
 
-    for (auto&& elem : featureCompatibilityVersionInfo) {
+    NamespaceString nss(FeatureCompatibilityVersion::kCollection);
+    updateCmd.append("update", nss.coll());
+    {
+        BSONArrayBuilder updates(updateCmd.subarrayStart("updates"));
+        {
+            BSONObjBuilder updateSpec(updates.subobjStart());
+            {
+                BSONObjBuilder queryFilter(updateSpec.subobjStart("q"));
+                queryFilter.append("_id", FeatureCompatibilityVersion::kParameterName);
+            }
+            {
+                BSONObjBuilder updateMods(updateSpec.subobjStart("u"));
+                updateMods.append(FeatureCompatibilityVersion::kVersionField, newVersion);
+            }
+            updateSpec.appendBool("upsert", true);
+        }
+    }
+
+    if (!writeConcern.isEmpty()) {
+        updateCmd.append(WriteConcernOptions::kWriteConcernField, writeConcern);
+    }
+
+    return updateCmd.obj();
+}
+}  // namespace
+
+StatusWith<ServerGlobalParams::FeatureCompatibility::Version> FeatureCompatibilityVersion::parse(
+    const BSONObj& featureCompatibilityVersionDoc) {
+    bool foundVersionField = false;
+    ServerGlobalParams::FeatureCompatibility::Version version;
+
+    for (auto&& elem : featureCompatibilityVersionDoc) {
         auto fieldName = elem.fieldNameStringData();
         if (fieldName == "_id") {
             continue;
-        } else if (fieldName == FeatureCompatibilityVersion::kVersionField ||
-                   fieldName == FeatureCompatibilityVersion::kTargetVersionField) {
+        } else if (fieldName == FeatureCompatibilityVersion::kVersionField) {
+            foundVersionField = true;
             if (elem.type() != BSONType::String) {
                 return Status(ErrorCodes::TypeMismatch,
-                              str::stream() << fieldName
+                              str::stream() << FeatureCompatibilityVersion::kVersionField
                                             << " must be of type String, but was of type "
                                             << typeName(elem.type())
                                             << ". Contents of "
@@ -77,20 +107,20 @@ StatusWith<FeatureCompatibilityVersionInfo> FeatureCompatibilityVersion::parse(
                                             << " document in "
                                             << FeatureCompatibilityVersion::kCollection
                                             << ": "
-                                            << featureCompatibilityVersionInfo
+                                            << featureCompatibilityVersionDoc
                                             << ". See "
                                             << feature_compatibility_version::kDochubLink
                                             << ".");
             }
-
-            ServerGlobalParams::FeatureCompatibility::Version version;
             if (elem.String() == FeatureCompatibilityVersionCommandParser::kVersion36) {
                 version = ServerGlobalParams::FeatureCompatibility::Version::k36;
             } else if (elem.String() == FeatureCompatibilityVersionCommandParser::kVersion34) {
                 version = ServerGlobalParams::FeatureCompatibility::Version::k34;
             } else {
                 return Status(ErrorCodes::BadValue,
-                              str::stream() << "Invalid value for " << fieldName << ", found "
+                              str::stream() << "Invalid value for "
+                                            << FeatureCompatibilityVersion::kVersionField
+                                            << ", found "
                                             << elem.String()
                                             << ", expected '"
                                             << FeatureCompatibilityVersionCommandParser::kVersion36
@@ -101,32 +131,27 @@ StatusWith<FeatureCompatibilityVersionInfo> FeatureCompatibilityVersion::parse(
                                             << " document in "
                                             << FeatureCompatibilityVersion::kCollection
                                             << ": "
-                                            << featureCompatibilityVersionInfo
+                                            << featureCompatibilityVersionDoc
                                             << ". See "
                                             << feature_compatibility_version::kDochubLink
                                             << ".");
             }
-
-            if (fieldName == FeatureCompatibilityVersion::kVersionField) {
-                versionInfo.version = version;
-            } else if (fieldName == FeatureCompatibilityVersion::kTargetVersionField) {
-                versionInfo.targetVersion = version;
-            }
         } else {
             return Status(ErrorCodes::BadValue,
-                          str::stream() << "Unrecognized field '" << fieldName << "'. Contents of "
+                          str::stream() << "Unrecognized field '" << elem.fieldName()
+                                        << "''. Contents of "
                                         << FeatureCompatibilityVersion::kParameterName
                                         << " document in "
                                         << FeatureCompatibilityVersion::kCollection
                                         << ": "
-                                        << featureCompatibilityVersionInfo
+                                        << featureCompatibilityVersionDoc
                                         << ". See "
                                         << feature_compatibility_version::kDochubLink
                                         << ".");
         }
     }
 
-    if (versionInfo.version == ServerGlobalParams::FeatureCompatibility::Version::kUnset) {
+    if (!foundVersionField) {
         return Status(ErrorCodes::BadValue,
                       str::stream() << "Missing required field '"
                                     << FeatureCompatibilityVersion::kVersionField
@@ -135,45 +160,45 @@ StatusWith<FeatureCompatibilityVersionInfo> FeatureCompatibilityVersion::parse(
                                     << " document in "
                                     << FeatureCompatibilityVersion::kCollection
                                     << ": "
-                                    << featureCompatibilityVersionInfo
+                                    << featureCompatibilityVersionDoc
                                     << ". See "
                                     << feature_compatibility_version::kDochubLink
                                     << ".");
     }
 
-    return versionInfo;
+    return version;
 }
 
 void FeatureCompatibilityVersion::set(OperationContext* opCtx, StringData version) {
-    // Upgrades to a version, which sets the 'version' field only.
-    _runUpdateCommand(opCtx, version, [version](auto updateMods) {
-        BSONObjBuilder setOp(updateMods.subobjStart("$set"));
-        setOp.append(FeatureCompatibilityVersion::kVersionField, version);
-    });
-}
+    uassert(40284,
+            str::stream() << "featureCompatibilityVersion must be '"
+                          << FeatureCompatibilityVersionCommandParser::kVersion36
+                          << "' or '"
+                          << FeatureCompatibilityVersionCommandParser::kVersion34
+                          << "'. See "
+                          << feature_compatibility_version::kDochubLink
+                          << ".",
+            version == FeatureCompatibilityVersionCommandParser::kVersion36 ||
+                version == FeatureCompatibilityVersionCommandParser::kVersion34);
 
-void FeatureCompatibilityVersion::setTargetUpgrade(OperationContext* opCtx, StringData version) {
-    // Only set 'targetVersion' field.
-    _runUpdateCommand(opCtx, version, [version](auto updateMods) {
-        BSONObjBuilder setOp(updateMods.subobjStart("$set"));
-        setOp.append(FeatureCompatibilityVersion::kTargetVersionField, version);
-    });
-}
+    DBDirectClient client(opCtx);
+    NamespaceString nss(FeatureCompatibilityVersion::kCollection);
 
-void FeatureCompatibilityVersion::setTargetDowngrade(OperationContext* opCtx, StringData version) {
-    // Set both 'version' and 'targetVersion' fields.
-    _runUpdateCommand(opCtx, version, [version](auto updateMods) {
-        updateMods.append(FeatureCompatibilityVersion::kVersionField, version);
-        updateMods.append(FeatureCompatibilityVersion::kTargetVersionField, version);
-    });
-}
+    // Update the featureCompatibilityVersion document stored in the "admin.system.version"
+    // collection.
+    BSONObj updateResult;
+    client.runCommand(nss.db().toString(),
+                      makeUpdateCommand(version, WriteConcernOptions::Majority),
+                      updateResult);
+    uassertStatusOK(getStatusFromCommandResult(updateResult));
+    uassertStatusOK(getWriteConcernStatusFromCommandResult(updateResult));
 
-void FeatureCompatibilityVersion::unsetTargetUpgradeOrDowngrade(OperationContext* opCtx,
-                                                                StringData version) {
-    // Updates 'version' field, while also unsetting the 'targetVersion' field.
-    _runUpdateCommand(opCtx, version, [version](auto updateMods) {
-        updateMods.append(FeatureCompatibilityVersion::kVersionField, version);
-    });
+    // Close all internal connections to versions lower than 3.6.
+    if (version == FeatureCompatibilityVersionCommandParser::kVersion36) {
+        opCtx->getServiceContext()->getServiceEntryPoint()->endAllSessions(
+            transport::Session::kLatestVersionInternalClientKeepOpen |
+            transport::Session::kExternalClientKeepOpen);
+    }
 }
 
 void FeatureCompatibilityVersion::setIfCleanStartup(OperationContext* opCtx,
@@ -200,10 +225,9 @@ void FeatureCompatibilityVersion::setIfCleanStartup(OperationContext* opCtx,
             // the "admin.system.version" collection.
             invariant(autoDB.justCreated());
 
-            // We update the value of the version server parameter so that the admin.system.version
-            // collection gets a UUID.
-            serverGlobalParams.featureCompatibility.version.store(
-                ServerGlobalParams::FeatureCompatibility::Version::k36);
+            // We update the value of the isSchemaVersion36 server parameter so the
+            // admin.system.version collection gets a UUID.
+            serverGlobalParams.featureCompatibility.isSchemaVersion36.store(true);
 
             uassertStatusOK(storageInterface->createCollection(opCtx, nss, {}));
         }
@@ -229,27 +253,13 @@ void FeatureCompatibilityVersion::onInsertOrUpdate(OperationContext* opCtx, cons
         idElement.String() != FeatureCompatibilityVersion::kParameterName) {
         return;
     }
-    auto versionInfo = uassertStatusOK(FeatureCompatibilityVersion::parse(doc));
-
-    // Indicate if an upgrade or downgrade is in progress.
-    if (versionInfo.targetVersion != ServerGlobalParams::FeatureCompatibility::Version::kUnset) {
-        log() << "targeting featureCompatibilityVersion " << toString(versionInfo.targetVersion);
-    }
-
-    // To avoid extra log messages when the targetVersion is set/unset, only log when the version
-    // changes.
-    auto oldVersion = serverGlobalParams.featureCompatibility.version.load();
-    auto newVersion = versionInfo.version;
-    if (oldVersion != newVersion) {
-        log() << "setting featureCompatibilityVersion to " << toString(newVersion);
-    }
-
-    // On commit, update the server parameters, and close any incoming connections with a wire
-    // version that is below the minimum.
-    opCtx->recoveryUnit()->onCommit([opCtx, versionInfo]() {
-        serverGlobalParams.featureCompatibility.version.store(versionInfo.version);
-        serverGlobalParams.featureCompatibility.targetVersion.store(versionInfo.targetVersion);
-        _closeConnectionsBelowVersion(opCtx, versionInfo);
+    auto newVersion = uassertStatusOK(FeatureCompatibilityVersion::parse(doc));
+    log() << "setting featureCompatibilityVersion to " << toString(newVersion);
+    opCtx->recoveryUnit()->onCommit([newVersion]() {
+        serverGlobalParams.featureCompatibility.version.store(newVersion);
+        serverGlobalParams.featureCompatibility.isSchemaVersion36.store(
+            serverGlobalParams.featureCompatibility.version.load() ==
+            ServerGlobalParams::FeatureCompatibility::Version::k36);
     });
 }
 
@@ -264,8 +274,7 @@ void FeatureCompatibilityVersion::onDelete(OperationContext* opCtx, const BSONOb
     opCtx->recoveryUnit()->onCommit([]() {
         serverGlobalParams.featureCompatibility.version.store(
             ServerGlobalParams::FeatureCompatibility::Version::k34);
-        serverGlobalParams.featureCompatibility.targetVersion.store(
-            ServerGlobalParams::FeatureCompatibility::Version::kUnset);
+        serverGlobalParams.featureCompatibility.isSchemaVersion36.store(false);
     });
 }
 
@@ -275,70 +284,10 @@ void FeatureCompatibilityVersion::onDropCollection(OperationContext* opCtx) {
     opCtx->recoveryUnit()->onCommit([]() {
         serverGlobalParams.featureCompatibility.version.store(
             ServerGlobalParams::FeatureCompatibility::Version::k34);
-        serverGlobalParams.featureCompatibility.targetVersion.store(
-            ServerGlobalParams::FeatureCompatibility::Version::kUnset);
+        serverGlobalParams.featureCompatibility.isSchemaVersion36.store(false);
     });
 }
 
-void FeatureCompatibilityVersion::_validateVersion(StringData version) {
-    uassert(40284,
-            str::stream() << "featureCompatibilityVersion must be '"
-                          << FeatureCompatibilityVersionCommandParser::kVersion36
-                          << "' or '"
-                          << FeatureCompatibilityVersionCommandParser::kVersion34
-                          << "'. See "
-                          << feature_compatibility_version::kDochubLink
-                          << ".",
-            version == FeatureCompatibilityVersionCommandParser::kVersion36 ||
-                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) {
-    _validateVersion(version);
-
-    DBDirectClient client(opCtx);
-    NamespaceString nss(FeatureCompatibilityVersion::kCollection);
-
-    BSONObjBuilder updateCmd;
-    updateCmd.append("update", nss.coll());
-    {
-        BSONArrayBuilder updates(updateCmd.subarrayStart("updates"));
-        {
-            BSONObjBuilder updateSpec(updates.subobjStart());
-            {
-                BSONObjBuilder queryFilter(updateSpec.subobjStart("q"));
-                queryFilter.append("_id", FeatureCompatibilityVersion::kParameterName);
-            }
-            {
-                BSONObjBuilder updateMods(updateSpec.subobjStart("u"));
-                builder(std::move(updateMods));
-            }
-            updateSpec.appendBool("upsert", true);
-        }
-    }
-    updateCmd.append(WriteConcernOptions::kWriteConcernField, WriteConcernOptions::Majority);
-
-    // Update the featureCompatibilityVersion document stored in the "admin.system.version"
-    // collection.
-    BSONObj updateResult;
-    client.runCommand(nss.db().toString(), updateCmd.obj(), updateResult);
-    uassertStatusOK(getStatusFromCommandResult(updateResult));
-    uassertStatusOK(getWriteConcernStatusFromCommandResult(updateResult));
-}
 /**
  * Read-only server parameter for featureCompatibilityVersion.
  */
diff --git a/src/mongo/db/commands/feature_compatibility_version.h b/src/mongo/db/commands/feature_compatibility_version.h
index abb48b82101..d89b2e996d5 100644
--- a/src/mongo/db/commands/feature_compatibility_version.h
+++ b/src/mongo/db/commands/feature_compatibility_version.h
@@ -41,18 +41,6 @@ class BSONObj;
 class OperationContext;
 
 /**
- * Store state of featureCompatibilityVersion document.
- **/
-struct FeatureCompatibilityVersionInfo {
-    ServerGlobalParams::FeatureCompatibility::Version version;
-    ServerGlobalParams::FeatureCompatibility::Version targetVersion;
-
-    FeatureCompatibilityVersionInfo()
-        : version(ServerGlobalParams::FeatureCompatibility::Version::kUnset),
-          targetVersion(ServerGlobalParams::FeatureCompatibility::Version::kUnset) {}
-};
-
-/**
  * Startup parameter to ignore featureCompatibilityVersion checks. This parameter cannot be set if
  * the node is started with --replSet, --master, or --slave. This should never be set by end users.
  */
@@ -65,13 +53,12 @@ public:
     static constexpr StringData kDatabase = "admin"_sd;
     static constexpr StringData kParameterName = "featureCompatibilityVersion"_sd;
     static constexpr StringData kVersionField = "version"_sd;
-    static constexpr StringData kTargetVersionField = "targetVersion"_sd;
 
     /**
      * Parses the featureCompatibilityVersion document from admin.system.version, and returns the
      * version.
      */
-    static StatusWith<FeatureCompatibilityVersionInfo> parse(
+    static StatusWith<ServerGlobalParams::FeatureCompatibility::Version> parse(
         const BSONObj& featureCompatibilityVersionDoc);
 
     static StringData toString(ServerGlobalParams::FeatureCompatibility::Version version) {
@@ -80,8 +67,6 @@ public:
                 return FeatureCompatibilityVersionCommandParser::kVersion36;
             case ServerGlobalParams::FeatureCompatibility::Version::k34:
                 return FeatureCompatibilityVersionCommandParser::kVersion34;
-            case ServerGlobalParams::FeatureCompatibility::Version::kUnset:
-                return FeatureCompatibilityVersionCommandParser::kVersionUnset;
             default:
                 MONGO_UNREACHABLE;
         }
@@ -95,27 +80,6 @@ public:
     static void set(OperationContext* opCtx, StringData version);
 
     /**
-     * Indicate intent to perform an upgrade. Should be set before schemas are modified.
-     * This sets the 'targetVersion' field only.
-     * Use unsetTargetUpgradeOrDowngrade to indicate that the schemas have completed the upgrading.
-     */
-    static void setTargetUpgrade(OperationContext* opCtx, StringData version);
-
-    /**
-     * Indicate intent to perform a downgrade. Should be set before schemas are modified.
-     * This atomically updates both the 'version' and 'targetVersion' fields.
-     * Use unsetTargetUpgradeOrDowngrade to indicate that the schemas have completed downgrading.
-     */
-    static void setTargetDowngrade(OperationContext* opCtx, StringData version);
-
-    /**
-     * Indicate the completion of an upgrade or downgrade. Should be set only when schemas are
-     * done being upgraded or modified.
-     * Unsets the 'targetVersion' field and updates the 'version' field.
-     */
-    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.
      */
@@ -140,28 +104,6 @@ public:
      * Resets the server parameter to its default value on commit.
      */
     static void onDropCollection(OperationContext* opCtx);
-
-private:
-    /**
-     * Validate version. Uasserts if invalid.
-     */
-    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.
-     */
-    typedef stdx::function<void(BSONObjBuilder)> UpdateBuilder;
-    static void _runUpdateCommand(OperationContext* opCtx,
-                                  StringData version,
-                                  UpdateBuilder callback);
 };
 
-
 }  // namespace mongo
diff --git a/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp b/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
index 1859bc13f9e..ae781c8137f 100644
--- a/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version_command_parser.cpp
@@ -43,7 +43,6 @@ constexpr StringData kVersion32 = "3.2"_sd;
 
 constexpr StringData FeatureCompatibilityVersionCommandParser::kVersion34;
 constexpr StringData FeatureCompatibilityVersionCommandParser::kVersion36;
-constexpr StringData FeatureCompatibilityVersionCommandParser::kVersionUnset;
 
 StatusWith<std::string> FeatureCompatibilityVersionCommandParser::extractVersionFromCommand(
     StringData commandName, const BSONObj& cmdObj) {
diff --git a/src/mongo/db/commands/feature_compatibility_version_command_parser.h b/src/mongo/db/commands/feature_compatibility_version_command_parser.h
index 498c9c8261d..c4544fdbff1 100644
--- a/src/mongo/db/commands/feature_compatibility_version_command_parser.h
+++ b/src/mongo/db/commands/feature_compatibility_version_command_parser.h
@@ -49,7 +49,6 @@ public:
      */
     static constexpr StringData kVersion34 = "3.4"_sd;
     static constexpr StringData kVersion36 = "3.6"_sd;
-    static constexpr StringData kVersionUnset = "Unset"_sd;
 
     /**
      * Interprets the specified BSON as a command and extracts the desired compatibility version
diff --git a/src/mongo/db/commands/mr.cpp b/src/mongo/db/commands/mr.cpp
index b9ecf5b4006..84ebb935f39 100644
--- a/src/mongo/db/commands/mr.cpp
+++ b/src/mongo/db/commands/mr.cpp
@@ -421,7 +421,7 @@ void State::prepTempCollection() {
             options.setNoIdIndex();
             options.temp = true;
             if (enableCollectionUUIDs &&
-                serverGlobalParams.featureCompatibility.isSchemaVersion36()) {
+                serverGlobalParams.featureCompatibility.isSchemaVersion36.load() == true) {
                 options.uuid.emplace(UUID::gen());
             }
             incColl = incCtx.db()->createCollection(_opCtx, _config.incLong.ns(), options);
@@ -505,7 +505,8 @@ void State::prepTempCollection() {
 
         CollectionOptions options = finalOptions;
         options.temp = true;
-        if (enableCollectionUUIDs && serverGlobalParams.featureCompatibility.isSchemaVersion36()) {
+        if (enableCollectionUUIDs &&
+            serverGlobalParams.featureCompatibility.isSchemaVersion36.load() == true) {
             // If a UUID for the final output collection was sent by mongos (i.e., the final output
             // collection is sharded), use the UUID mongos sent when creating the temp collection.
             // When the temp collection is renamed to the final output collection, the UUID will be
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index 92d8ce3e0e3..77bfd2bb8d9 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -128,33 +128,30 @@ public:
                 opCtx, writeConcern);
         }
 
-        // Perform an upgrade
         if (version != existingVersion && isFCVUpgrade(version)) {
-            // Set targetVersion to indicate that an upgrade is in progress, then unset when done.
-            FeatureCompatibilityVersion::setTargetUpgrade(opCtx, version);
+            serverGlobalParams.featureCompatibility.isSchemaVersion36.store(true);
             updateUUIDSchemaVersion(opCtx, /*upgrade*/ true);
+            existingVersion = version;
+        }
+
+        // For reproducing failure after adding UUIDs and before upgrade FCV document.
+        if (MONGO_FAIL_POINT(featureCompatibilityUpgrade)) {
+            exitCleanly(EXIT_CLEAN);
+        }
 
-            // For reproducing failure after adding UUIDs and before upgrade FCV document.
-            if (MONGO_FAIL_POINT(featureCompatibilityUpgrade)) {
-                exitCleanly(EXIT_CLEAN);
-            }
-            // Unsets 'targetVersion' and sets 'version' to new version.
-            FeatureCompatibilityVersion::unsetTargetUpgradeOrDowngrade(opCtx, version);
-
-            // Always perform a downgrade when setting the FCV to 3.4, in case of a crash after
-            // writing the document and before completing the upgrade.
-        } else if (version == FeatureCompatibilityVersionCommandParser::kVersion34) {
-            // Set targetVersion to indicate that a downgrade is in progress, then unset when
-            // complete.
-            FeatureCompatibilityVersion::setTargetDowngrade(opCtx, version);
-
-            // For reproducing failure after downgrading FCV document, and before removing UUIDs.
-            if (MONGO_FAIL_POINT(featureCompatibilityDowngrade)) {
-                exitCleanly(EXIT_CLEAN);
-            }
+        FeatureCompatibilityVersion::set(opCtx, version);
+
+        // For reproducing failure after downgrading FCV document, and before removing UUIDs.
+        if (MONGO_FAIL_POINT(featureCompatibilityDowngrade)) {
+            serverGlobalParams.featureCompatibility.isSchemaVersion36.store(false);
+            exitCleanly(EXIT_CLEAN);
+        }
 
+        // Always perform a downgrade when setting the FCV to 3.4, in case of a crash after writing
+        // the document and before completing the upgrade.
+        if (version == FeatureCompatibilityVersionCommandParser::kVersion34) {
+            serverGlobalParams.featureCompatibility.isSchemaVersion36.store(false);
             updateUUIDSchemaVersion(opCtx, /*upgrade*/ false);
-            FeatureCompatibilityVersion::unsetTargetUpgradeOrDowngrade(opCtx, version);
         }
 
         // Ensure we try reading the keys for signing clusterTime immediately on upgrade to 3.6.
diff --git a/src/mongo/db/db.cpp b/src/mongo/db/db.cpp
index ed3ba14e852..8f29f0605bc 100644
--- a/src/mongo/db/db.cpp
+++ b/src/mongo/db/db.cpp
@@ -405,33 +405,17 @@ void repairDatabasesAndCheckVersion(OperationContext* opCtx) {
                                      versionColl,
                                      BSON("_id" << FeatureCompatibilityVersion::kParameterName),
                                      featureCompatibilityVersion)) {
-                    auto swVersionInfo =
-                        FeatureCompatibilityVersion::parse(featureCompatibilityVersion);
-                    if (!swVersionInfo.isOK()) {
-                        severe() << swVersionInfo.getStatus();
+                    auto version = FeatureCompatibilityVersion::parse(featureCompatibilityVersion);
+                    if (!version.isOK()) {
+                        severe() << version.getStatus();
                         fassertFailedNoTrace(40283);
                     }
-                    auto versionInfo = swVersionInfo.getValue();
-                    serverGlobalParams.featureCompatibility.version.store(versionInfo.version);
-                    serverGlobalParams.featureCompatibility.targetVersion.store(
-                        versionInfo.targetVersion);
-
-                    // On startup, if the targetVersion field exists, then an upgrade/downgrade
-                    // did not complete successfully.
-                    if (versionInfo.targetVersion !=
-                        ServerGlobalParams::FeatureCompatibility::Version::kUnset) {
-                        log() << "** WARNING: A featureCompatibilityVersion upgrade or downgrade "
-                                 "did not complete. "
-                              << startupWarningsLog;
-                        log() << "**          The current featureCompatibilityVersion is "
-                              << FeatureCompatibilityVersion::toString(versionInfo.version)
-                              << " and the targeted version is "
-                              << FeatureCompatibilityVersion::toString(versionInfo.targetVersion)
-                              << "." << startupWarningsLog;
-                        log() << "**          To fix this, use the setFeatureCompatibilityVersion "
-                              << "command to resume upgrade to 3.6 or downgrade to 3.4."
-                              << startupWarningsLog;
-                    }
+                    serverGlobalParams.featureCompatibility.version.store(version.getValue());
+
+                    // Update schemaVersion parameter.
+                    serverGlobalParams.featureCompatibility.isSchemaVersion36.store(
+                        serverGlobalParams.featureCompatibility.version.load() ==
+                        ServerGlobalParams::FeatureCompatibility::Version::k36);
                 }
             }
         }
diff --git a/src/mongo/db/server_options.h b/src/mongo/db/server_options.h
index 23f8bddff4f..a82d20acc79 100644
--- a/src/mongo/db/server_options.h
+++ b/src/mongo/db/server_options.h
@@ -158,22 +158,14 @@ struct ServerGlobalParams {
              * 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
         };
 
         // 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};
-
-        // This determines whether to give Collections UUIDs upon creation.
-        bool isSchemaVersion36() {
-            return (version.load() == Version::k36 || targetVersion.load() == Version::k36);
-        }
+        // Read-only global isSchemaVersion36. This determines whether to give Collections UUIDs
+        // upon creation.
+        AtomicWord<bool> isSchemaVersion36{false};
 
         // Feature validation differs depending on the role of a mongod in a replica set or
         // master/slave configuration. Masters/primaries can accept user-initiated writes and