SERVER-68651 Put isDowngrading to upgraded path under feature flag

8 files changed, 74 insertions, 28 deletions

diff --git a/jstests/libs/feature_flag_util.js b/jstests/libs/feature_flag_util.js
index 92fb58a5288..4ebb5ef23be 100644
--- a/jstests/libs/feature_flag_util.js
+++ b/jstests/libs/feature_flag_util.js
@@ -7,9 +7,13 @@ load("jstests/libs/fixture_helpers.js");
  */
 var FeatureFlagUtil = class {
     /**
-     * Returns true if feature flag is enabled, false otherwise.
+     * If 'ignoreFCV' is true, return whether or not the given feature flag is enabled,
+     * regardless of the current FCV version (this is used when a feature flag needs to be enabled
+     * in downgraded FCV versions).
+     * If 'ignoreFCV' is false or null, we only return true if the flag is enabled and this FCV
+     * version is greater or equal to the required version for the flag.
      */
-    static isEnabled(db, featureFlag, user) {
+    static isEnabled(db, featureFlag, user, ignoreFCV) {
         // In order to get an accurate answer for whether a feature flag is enabled, we need to ask
         // a mongod. If db represents a connection to mongos, or some other configuration, we need
         // to obtain the correct connection to a mongod.
@@ -46,9 +50,11 @@ var FeatureFlagUtil = class {
             return false;
         }
 
-        return flagDoc.hasOwnProperty(fullFlagName) && flagDoc[fullFlagName].value &&
-            (!fcvDoc.hasOwnProperty("featureCompatibilityVersion") ||
-             MongoRunner.compareBinVersions(fcvDoc.featureCompatibilityVersion.version,
-                                            flagDoc[fullFlagName].version) >= 0);
+        const flagIsEnabled = flagDoc.hasOwnProperty(fullFlagName) && flagDoc[fullFlagName].value;
+        const flagVersionIsValid = !fcvDoc.hasOwnProperty("featureCompatibilityVersion") ||
+            MongoRunner.compareBinVersions(fcvDoc.featureCompatibilityVersion.version,
+                                           flagDoc[fullFlagName].version) >= 0;
+
+        return flagIsEnabled && (ignoreFCV || flagVersionIsValid);
     }
 };
diff --git a/jstests/multiVersion/genericSetFCVUsage/set_feature_compatibility_version.js b/jstests/multiVersion/genericSetFCVUsage/set_feature_compatibility_version.js
index ebd7ce28a8b..08c2dd2e6ce 100644
--- a/jstests/multiVersion/genericSetFCVUsage/set_feature_compatibility_version.js
+++ b/jstests/multiVersion/genericSetFCVUsage/set_feature_compatibility_version.js
@@ -15,6 +15,7 @@ TestData.skipCheckDBHashes = true;
 load("jstests/libs/index_catalog_helpers.js");
 load("jstests/libs/write_concern_util.js");
 load("jstests/replsets/rslib.js");
+load("jstests/libs/feature_flag_util.js");
 
 let dbpath = MongoRunner.dataPath + "feature_compatibility_version";
 resetDbpath(dbpath);
@@ -337,6 +338,16 @@ function runReplicaSetTest(downgradeVersion) {
     assert.commandFailedWithCode(res, ErrorCodes.WriteConcernFailed);
     restartServerReplication(secondary);
 
+    // If downgrading->upgrading feature is not enabled,
+    // upgrading the FCV should fail if a previous downgrade has not yet completed.
+    if (!FeatureFlagUtil.isEnabled(primaryAdminDB,
+                                   "DowngradingToUpgrading",
+                                   null /* user not specified */,
+                                   true /* ignores FCV */)) {
+        assert.commandFailedWithCode(
+            primary.adminCommand({setFeatureCompatibilityVersion: latestFCV}), 5147403);
+    }
+
     if (lastContinuousFCV !== lastLTSFCV) {
         // We will fail if we have not yet completed a downgrade and attempt to downgrade to a
         // different target version.
diff --git a/jstests/multiVersion/genericSetFCVUsage/test_replica_set_startup_in_downgrading_state.js b/jstests/multiVersion/genericSetFCVUsage/test_replica_set_startup_in_downgrading_state.js
index 49baaf22540..16c85aa1cc2 100644
--- a/jstests/multiVersion/genericSetFCVUsage/test_replica_set_startup_in_downgrading_state.js
+++ b/jstests/multiVersion/genericSetFCVUsage/test_replica_set_startup_in_downgrading_state.js
@@ -1,14 +1,17 @@
 /*
  * Tests startup with a node in downgrading state.
  * Starts a replica set with 2 nodes.
+ *
+ * @tags: [featureFlagDowngradingToUpgrading]
  */
 
-load('jstests/multiVersion/libs/verify_versions.js');
-load('jstests/libs/fail_point_util.js');
-
 (function() {
 "use strict";
 
+load('jstests/multiVersion/libs/verify_versions.js');
+load('jstests/libs/fail_point_util.js');
+load("jstests/libs/feature_flag_util.js");
+
 function runReplicaSet() {
     let fcvDoc;
 
diff --git a/jstests/multiVersion/genericSetFCVUsage/test_sharding_startup_in_downgrading_state.js b/jstests/multiVersion/genericSetFCVUsage/test_sharding_startup_in_downgrading_state.js
index 8d48e87b102..02d0590d956 100644
--- a/jstests/multiVersion/genericSetFCVUsage/test_sharding_startup_in_downgrading_state.js
+++ b/jstests/multiVersion/genericSetFCVUsage/test_sharding_startup_in_downgrading_state.js
@@ -1,14 +1,17 @@
 /*
  * Tests startup with a node in downgrading state.
  * Starts a sharded cluster with 2 shards, each with 2 nodes.
+ *
+ *  @tags: [featureFlagDowngradingToUpgrading]
  */
 
-load('jstests/multiVersion/libs/verify_versions.js');
-load('jstests/libs/fail_point_util.js');
-
 (function() {
 "use strict";
 
+load('jstests/multiVersion/libs/verify_versions.js');
+load('jstests/libs/fail_point_util.js');
+load("jstests/libs/feature_flag_util.js");
+
 function runSharding() {
     let fcvDoc;
     let shard0PrimaryAdminDB;
diff --git a/jstests/replsets/rollback_set_fcv.js b/jstests/replsets/rollback_set_fcv.js
index 4f15a81f3b8..4a82ed33857 100644
--- a/jstests/replsets/rollback_set_fcv.js
+++ b/jstests/replsets/rollback_set_fcv.js
@@ -16,6 +16,7 @@ load("jstests/replsets/libs/rollback_test.js");
 load('jstests/libs/parallel_shell_helpers.js');
 load("jstests/libs/fail_point_util.js");
 load("jstests/replsets/rslib.js");
+load("jstests/libs/feature_flag_util.js");
 
 function setFCV(fcv) {
     assert.commandFailedWithCode(db.adminCommand({setFeatureCompatibilityVersion: fcv}),
@@ -119,11 +120,16 @@ function rollbackFCVFromDowngradedOrUpgraded(fromFCV, toFCV, failPoint) {
     checkFCV(secondaryAdminDB, lastLTSFCV, fromFCV);
 
     let newPrimary = rollbackTest.getPrimary();
+    const newPrimaryAdminDB = newPrimary.getDB('admin');
     // As a rule, we forbid downgrading a node while a node is still in the upgrading state and
-    // vice versa (except for the added path kDowngradingFromLatestToLastLTS ->
-    // kUpgradingFromLastLTSToLatest -> kLatest). Ensure that the in-memory and on-disk FCV are
-    // consistent by checking that this rule is upheld after rollback.
-    if (fromFCV === lastLTSFCV && toFCV === latestFCV) {
+    // vice versa (except for the added path from downgrading to upgrading).
+    // Ensure that the in-memory and on-disk FCV are consistent by checking that this rule is
+    // upheld after rollback.
+    if (fromFCV === lastLTSFCV && toFCV === latestFCV &&
+        FeatureFlagUtil.isEnabled(newPrimaryAdminDB,
+                                  "DowngradingToUpgrading",
+                                  null /* user not specified */,
+                                  true /* ignores FCV */)) {
         assert.commandWorked(newPrimary.adminCommand({setFeatureCompatibilityVersion: toFCV}));
     } else {
         assert.commandFailedWithCode(
diff --git a/src/mongo/db/commands/SConscript b/src/mongo/db/commands/SConscript
index a97e065d719..d058ade3878 100644
--- a/src/mongo/db/commands/SConscript
+++ b/src/mongo/db/commands/SConscript
@@ -249,6 +249,7 @@ env.Library(
         '$BUILD_DIR/mongo/db/commands',
         '$BUILD_DIR/mongo/db/dbdirectclient',
         '$BUILD_DIR/mongo/db/repl/repl_coordinator_interface',
+        '$BUILD_DIR/mongo/db/repl/repl_server_parameters',
         '$BUILD_DIR/mongo/db/repl/repl_settings',
         '$BUILD_DIR/mongo/idl/server_parameter',
     ],
diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index 8ad7cae43f6..3b8e8a4654d 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -43,6 +43,7 @@
 #include "mongo/db/namespace_string.h"
 #include "mongo/db/operation_context.h"
 #include "mongo/db/repl/optime.h"
+#include "mongo/db/repl/repl_server_parameters_gen.h"
 #include "mongo/db/repl/replication_coordinator.h"
 #include "mongo/db/repl/replication_process.h"
 #include "mongo/db/repl/storage_interface.h"
@@ -74,7 +75,8 @@ namespace {
  * Utility class for recording permitted transitions between feature compatibility versions and
  * their on-disk representation as FeatureCompatibilityVersionDocument objects.
  */
-const class FCVTransitions {
+// TODO SERVER-65269: Add back 'const' qualifier to FCVTransitions class declaration
+class FCVTransitions {
 public:
     FCVTransitions() {
         auto makeFCVDoc = [](
@@ -109,10 +111,6 @@ public:
                 // lastLTS then the second loop iteration just overwrites the first.
                 _transitions[{from, to, isFromConfigServer}] = upgrading;
                 _transitions[{upgrading, to, isFromConfigServer}] = to;
-                // allow downgrading->upgrading->latest path
-                _transitions[{GenericFCV::kDowngradingFromLatestToLastLTS,
-                              GenericFCV::kLatest,
-                              isFromConfigServer}] = GenericFCV::kUpgradingFromLastLTSToLatest;
             }
             _fcvDocuments[upgrading] = makeFCVDoc(from /* effective */, to /* target */);
         }
@@ -146,6 +144,20 @@ public:
     }
 
     /**
+     * If feature flag gDowngradingToUpgrading is enabled,
+     * we add the new downgrading->upgrading->latest path.
+     */
+    void featureFlaggedAddNewTransitionState() {
+        if (repl::feature_flags::gDowngradingToUpgrading.isEnabledAndIgnoreFCV()) {
+            for (auto isFromConfigServer : std::vector{false, true}) {
+                _transitions[{GenericFCV::kDowngradingFromLatestToLastLTS,
+                              GenericFCV::kLatest,
+                              isFromConfigServer}] = GenericFCV::kUpgradingFromLastLTSToLatest;
+            }
+        }
+    }
+
+    /**
      * True if a server in multiversion::FeatureCompatibilityVersion "fromVersion" can
      * transition to "newVersion". Different rules apply if the request is from a config server.
      */
@@ -327,14 +339,13 @@ void FeatureCompatibilityVersion::updateFeatureCompatibilityVersionDocument(
     // We may have just stepped down, in which case we should not proceed.
     opCtx->checkForInterrupt();
 
-    // Only transition to fully upgraded or downgraded states when we
-    // have completed all required upgrade/downgrade behavior.
-    // If kDowngradingFromLatestToLastLTS->kLatest, we want to get the transitional version
-    // i.e. kUpgradingFromLastLTSToLatest
+    // Only transition to fully upgraded or downgraded states when we have completed all required
+    // upgrade/downgrade behavior, unless it is the newly added downgrading to upgrading path.
     auto transitioningVersion = setTargetVersion &&
             serverGlobalParams.featureCompatibility.isUpgradingOrDowngrading(fromVersion) &&
-            !(fromVersion == GenericFCV::kDowngradingFromLatestToLastLTS &&
-              newVersion == GenericFCV::kLatest)
+            !(repl::feature_flags::gDowngradingToUpgrading.isEnabledAndIgnoreFCV() &&
+              (fromVersion == GenericFCV::kDowngradingFromLatestToLastLTS &&
+               newVersion == GenericFCV::kLatest))
         ? fromVersion
         : fcvTransitions.getTransitionalVersion(fromVersion, newVersion, isFromConfigServer);
     FeatureCompatibilityVersionDocument fcvDoc =
@@ -487,6 +498,10 @@ void FeatureCompatibilityVersion::fassertInitializedAfterStartup(OperationContex
 
     auto fcvDocument = findFeatureCompatibilityVersionDocument(opCtx);
 
+    // TODO SERVER-65269: Move downgrading->upgrading transition back to FCVTransitions constructor.
+    // Adding the new fcv downgrading -> upgrading path
+    fcvTransitions.featureFlaggedAddNewTransitionState();
+
     auto const storageEngine = opCtx->getServiceContext()->getStorageEngine();
     auto dbNames = storageEngine->listDatabases();
     bool nonLocalDatabases = std::any_of(dbNames.begin(), dbNames.end(), [](auto dbName) {
diff --git a/src/mongo/db/repl/repl_server_parameters.idl b/src/mongo/db/repl/repl_server_parameters.idl
index d823889e66e..16ae22f7a2a 100644
--- a/src/mongo/db/repl/repl_server_parameters.idl
+++ b/src/mongo/db/repl/repl_server_parameters.idl
@@ -677,4 +677,5 @@ feature_flags:
         description: When enabled, 
                      allow kDowngradingFromLatestToLastLTS -> kUpgrading -> kUpgraded path.
         cpp_varname: feature_flags::gDowngradingToUpgrading
-        default: false
\ No newline at end of file
+        default: false
+        
\ No newline at end of file